"The waters and the ice of the South Polar Ocean were alike found to abound with microscopic vegetables belonging to the order Diatomaceæ. Though much too small to be discernible by the naked eye, they occurred in such countless myriads as to stain the berg and the pack ice wherever they were washed by the swell of the sea”
- Joseph Dalton Hooker in “Botany of the Antarctic Voyage”, a paper he read before the British Association in 1847 (quoted in Huxley, 1894)
Vegetative phase of Navicula reinhardtii showing reduction in size. Image courtesy of David G. Mann.
Diatoms have fascinated scientists and non-scientists alike for hundreds of years, even inspiring small-scale artworks (see Kemp, 2013; Whitty, 2012) and amateur, enthusiast publications (The Amateur Diatomist, 2013).
Diatoms are large group of heterokont phytoplankton. They are unicelluar primary producers, although some will form colonies. They are found in a range of sizes, though commonly between 20-200μm in length (University College London Micropalaeontology Unit, 2013).Therefore, some are classified as large autotrophs and while others are small autotrophs. Thanks to their large abundance and distribution, they may account for up to 20% of the world’s carbon fixation from photosynthesis, which is more than all the world’s tropical rainforests (Mann, 1999).
Diatoms are very distinctive and possess a silicified cell wall, called a frustule, that is made up of two main parts, or valves, and a series narrow bands that form the ‘girdle’. This gives them a roughly cylindrical shape, though cross-sections can range from circular to needle-like and elliptical.
Frustules are dotted with pores, called areolae, which allow the passage of water and dissolved molecules in and out of the cell (Mann, 2010). Due to the particular combination of photosynthetic pigments, they usually appear yellow-brown, green-brown or orange in colour.
The silicified cell walls are very robust and diatoms have been preserved in the fossil record dating back at least to the Cretaceous period (University College London Micropalaeontology Unit, 2013). However, silica will dissolve once the organic coating has been lost, which releases it back into the oceanic silicon cycle.
Distribution, abundance and diet
As one of the most widespread and common taxa of phytoplankton they play an important role as primary producers for many of aquatic communities. They are autotrophic, that is they produce their own food through photosynthesis. Sunlight is harvested in organelles called chloroplasts and triggers the synthesis of sugars from carbon dioxide. Therefore growth and reproduction of diatoms can be restricted if light is limited. This particularly true for polar diatoms, such as those found in Southern Ocean and Antarctica, which have to be able to tolerate the very limited light conditions of the austral winter.
Their silicate frustules mean that diatoms are sensitive to changes in silicate concentrations and form part of the ocean’s silicon cycle. Due to the great abundances of diatoms, their frustules constitute a large proportion of the biogenic silica found in the photic layer at the surface of the ocean. Some of the is recycled at the surface, while some of it sinks to the deep ocean. Some of the silica dissolves out as it sinks, but some make it to the sea floor and contribute to siliceous marine sediment. For a detailed discussion of the global silicon cycle see Tréguer & De La Rocha (2013).
The strong seasonality in the habitat of Southern Ocean phytoplankton leads to dramatic, seasonal changes population size. Large blooms occur in spring and summer, which support the species in higher trophic levels. Zooplankton species such as krill rely on phytoplankton during blooms and they, in turn, support animals like some penguins, seals, and whales. Many of these blooms are dominated by diatoms, though the exact species composition and relative dominance varies spatially and temporally (Beans et al., 2008).
Phytoplankton blooms in the Southern Ocean occur in spring and summer when increased light availability increases the photosynthetic rate. In addition, the melting sea ice is thought to ‘seed’ these blooms, which follows the retreating ice edge, with nutrients and viable phytoplankton cells that were frozen in the ice (Arrigo & Dijken, 2003).
Waters around Antarctica are generally characterised as being high in nutrients and low chlorophyll (HNLC), meaning that the primary production is less than would be expected. However, this description does not factor in the low iron concentrations and in these waters diatoms growth is largely limited by iron availability (El-Sayed, 2005). There have been a number of experimental studies showing evidence that fertilising such areas with iron could trigger blooming events (Boyd et al., 2007). There is also evidence that these blooms increase the amount of carbon drawn down out of the atmosphere by diatoms (Smetacek et al., 2012).
Diatoms can be found in a wide range of habitats, as long at it is moist enough and there is sufficient light for photosynthesis. They are found commonly in the euphotic zones of oceans and freshwater, and form an important part of sea ice communities in the Southern Ocean. However, they can also be found in some unexpected places like soil, moss, rock faces and in feathers of diving birds (Mann, 2010).
Some diatoms are capable of exhibiting a jerky, gliding motion when connected to a substratum. Only diatoms that have a raphe slit, a groove in the valve of pennate diatoms, are capable of this locomotion, and only when it is adjacent to the substrate. This suggests that the raphe slit is connected to motility (Edgar, 1982). Pelagic diatoms, on the other hand, move only at the whim of ocean currents and mixing forces. In particular, Langmuir circulation could push them in or out of the euphotic zone at times.
Around Antarctica, phytoplankton blooms tend to follow the retreating ice edge in spring and summer as more nutrients become available to support the rapid growth and reproduction.
Life history (growth, reproduction, mortality)
The growth and reproduction of diatoms is extremely dependent on factors such as light and nutrient availability and therefore high seasonality exists in the life history of southern ocean diatoms. Diatoms are capable of both sexual reproduction and asexual divisions. When dividing asexually, frustules are unable to keep growing. Therefore long periods of asexual division leads to smaller and smaller diatoms. It can take a number of years for diatoms to reach their smallest size (Mann, 2010).
Arrigo, K. R., & Dijken, G. L. Van. (2003). Phytoplankton dynamics within 37 Antarctic coastal polynya systems. Journal of Geophysical Research, 108(C8), 3271. doi:10.1029/2002JC001739
Beans, C., Hecq, J. H., Koubbi, P., Vallet, C., Wright, S., & Goffart, a. (2008). A study of the diatom-dominated microplankton summer assemblages in coastal waters from Terre Adélie to the Mertz Glacier, East Antarctica (139°E–145°E). Polar Biology, 31(9), 1101–1117. doi:10.1007/s00300-008-0452-x
Boyd, P. W., Jickells, T., Law, C. S., Blain, S., Boyle, E. a, Buesseler, K. O., Coale, K. H., et al. (2007). Mesoscale iron enrichment experiments 1993-2005: synthesis and future directions. Science (New York, N.Y.), 315(5812), 612–7. doi:10.1126/science.1131669
Edgar, L. (1982). Diatom locomotion: a consideration of movement in a highly viscous situation. British Phycological Journal, 17(3), 37–41. Retrieved from http://www.tandfonline.com/doi/full/10.1080/00071618200650261
El-Sayed, S. Z. (2005). History and evolution of primary productivity studies of the Southern Ocean. Polar Biology, 28(6), 423–438. doi:10.1007/s00300-004-0685-2
Guiry, M. D., & Guiry, G. M. (2013). AlgaeBase. National University of Ireland, Galway. Retrieved May 31, 2013, from http://www.algaebase.org
Huxley, T. H. (1894). ON SOME OF THE RESULTS OF THE EXPEDITION OF H.M.S. “CHALLENGER” . In I. Ghory, S. Goodman, R. Prairie, & PG Distributed Proofreaders (Eds.), Discourses: Biological and Geological Essays (EBook.). Project Gutenberg. Retrieved from http://www.gutenberg.org/cache/epub/10060/pg10060.html
Kemp, K. D. (2013). Microlife Services. Retrieved May 31, 2013, from http://www.diatoms.co.uk/
Mann, D. G. (1999). The species concept in diatoms. Phycologia, 38, 437–495.
Mann, D. G. (2010). Diatoms. Tree of Life Web Project. Retrieved May 31, 3013, from http://tolweb.org/Diatoms/21810/2010.02.07
Round, F. E., Crawford, R. M., & Mann, D. G. (1990). The Diatoms: The Biology and Morphology of the Genera. Cambridge: Cambridge University Press.
Scott, F. J., & Thomas, D. P. (2005). 02 Diatoms.pdf. In F. J. Scott & H. J. Marchant (Eds.), Antarctic Marine Protists (pp. 13–201). Canberra: Australian Biological Resources Study and Australian Antarctic Division.
Smetacek, V., Klaas, C., Strass, V. H., Assmy, P., Montresor, M., Cisewski, B., Savoye, N., et al. (2012). Deep carbon export from a Southern Ocean iron-fertilized diatom bloom. Nature, 487(7407), 313–9. doi:10.1038/nature11229
The Amateur Diatomist. (2013). The Amateur Diatomist. Retrieved May 31, 2013, from http://www.diatoms.co.uk/ad/index.htm
Tréguer, P. J., & De La Rocha, C. L. (2013). The world ocean silica cycle. Annual review of marine science, 5, 477–501. doi:10.1146/annurev-marine-121211-172346
University College London Micropalaeontology Unit. (2013). Diatoms. Retrieved May 31, 2013, from http://www.ucl.ac.uk/GeolSci/micropal/diatom.html
Whitty, J. (2012). Psychadelic Diatoms. Deep Blue Home. Retrieved May 31, 2013, from http://deepbluehome.blogspot.com.au/2011/01/psychedelic-diatoms.html
CitationPlease cite this page as:
SOKI Wiki (2014) Friday 6 Jun 2014.
There phylogenetic position of diatoms, and the classifications within the group, is unsettled and has varied with new techniques and research. However, they are traditionally classified in two orders:
- radially symmetric Centric diatoms (Centrales)
- bilaterally symmetric Pennate diatoms (Pennales)
A classification in 1990 suggested the formation of three classes, Coscinodiscophyceae, Fragilariophyceae and Bacillariophyceae (Round, Crawford, & Mann, 1990), however it has since been recognised that although they describe morphological groups, these classifications do not reflect phylogeny (Mann, 2010). Indeed further alternative phylogenies are also given in a more recent taxonomic database (Guiry & Guiry, 2013).
The following is a list of families commonly found in Antarctic waters compiled in the diatoms chapter of Antarctic Marine Protists (Scott & Thomas, 2005)
In this essay we will discuss about Algae. After reading this essay you will learn about:- 1. Meaning of Algae 2. Life Cycle Patterns in Algae 3. Sexuality 4. Classification 5. Cellular Organization 6. General Characteristics 7. Septa 8. Nucleus 9. Chloroplast, Pyrenoids and Pigments 10. Nutrition 11. Storage of Food 12. Other Structures 13. Plant Body Types 14. Origin of Sex and Others.
- Essay on the Meaning of Algae
- Essay on the Life Cycle Patterns in Algae
- Essay on the Sexuality in Algae
- Essay on the Classification of Algae
- Essay on the Cellular Organization of Algae
- Essay on the General Characteristics of Algae
- Essay on the Septa of Algae
- Essay on the Nucleus of Algae
- Essay on the Chloroplast, Pyrenoids and Pigments of Algae
- Essay on the Nutrition of Algae
- Essay on the Storage of Food in Algae
- Essay on the Other Structures of Algae
- Essay on the Plant Body Types of Algae
- Essay on the Origin of Sex in the Algae
- Essay on the Alternation of Generations in the Algae
- Essay on the Origin and Evolution in the Algae
- Essay on the Fossil Algae
- Essay on the Algae Causing Biological Disturbances
- Essay on the Reproduction in Algae
- Essay on the Review of Algological Studies (Algae) in Abroad
- Essay on the Review of Algological Studies (Algae) in India
Essay # 1. Meaning of Algae:
Although Linnaeus (1753) first introduced the name Algae, but by Algae he meant the Hepaticae. The actual delimitation of a group of plants from their macroscopical features and naming as Algae was done by A. L. De Jussieu (1789).
The term algae, (sing, alga) is applied to a vast assemblage of organisms which are highly diverse with respect to habitat, size, organization, physiology, biochemistry and reproduction. In these organisms are found some of the oldest and most primitive types of plant life. The study of Algae is phycology and phycologists are those who study Algae.
What are Algae? The Algae are chlorophyll bearing lower plants which possess markedly different patterns of unicellular to multicellular organization, both prokaryotic and eukaryotic. Their reproduction is vegetative, by formation of spores, and gametic; gametes produced in unicellular to multicellular gametangia, with some exceptions where the entire unicellular body behaves as a gamete.
During the latter part of the nineteenth century and the early decades of the twentieth, the term Algae was used both as a common name and as a formal taxonomic designation of a group of organisms.
At the present time, however, the differences among these organisms are so strongly emphasized that the formal, collective taxon ‘Algae’ has been abandoned, and a polyphyletic treatment of these organisms is widely accepted.
Accordingly, the old Subdivision Algae of the Division Thallophyta has been replaced by taxa of divisional rank: Cyanophyta, Chlorophyta, Phaeophyta, Rhodophyta, and so on.
Algae range from minute unicellular solitary or colonial organisms represented by some of the common pond scums which may or may not be motile and are entirely invisible as individuals to the naked eye, to large, diffuse and comparatively complex plants, such as seaweeds, usually growing submerged in water or in moist situations.
They are largely aquatic, being common in both fresh and sea water although some are terrestrial, being found in moist soil as an important part of the soil flora essential in soil productivity, and on bark of trees.
Generally speaking, the large and more complex forms are marine; the fresh-water and terrestrial representatives are both smaller and simpler. Even in apparently looking large and complex forms, however, the plant body shows very little differentiation.
Some of the large seaweeds which show considerable tissue differentiation with organs analogous to the roots,-stem, and leaf of the higher plants, may still be termed a thallus, for being structurally simple. Algae are of various colours—blue-green, green, brown, red, etc. but majority of them contain the green chlorophyll pigments and hence are autotrophic.
Although the mode of nutrition resembles that of higher plants, yet the method of absorption differs, as they do not possess specialized absorptive organs comparable with root hairs, but food material diffuses in, in solution through the cell wall over the whole surface. In some algae the chlorophyll is frequently masked by other pigments.
These are mainly phycocyanin, a blue pigment; fucoxanthin, a yellowish-brown pigment; phycoerythrin, a red pigment. The principal food reserve is carbohydrate of some form or the other, in addition to which there are also fats and oils.
Algae are very important as a source of food and serve as an early step in the food chain of larger aquatic animals especially fish. They are also of great use to human beings being the source of food especially rich in vitamins A and E. Many marine algae are important sources of iodine, potassium (potash), and other minerals.
There are certain algae which play a great role in enriching the soil fertility by fixing atmospheric nitrogen. In the more primitive members reproduction is often entirely asexual, while in advanced forms it is sexual of varied kinds. Formerly, the classification of Algae was largely based on the basis of pigmentation.
But the present tendency is to use a combination of other characters along with the nature of pigmentation of cells.
Essay # 2. Life Cycle Patterns in Algae:
Members of the sexually reproducing algae exhibit various life cycle patterns which may be outlined as:
I. An alternation of predominantly gametophytic generation or haploid phase with a very short and insignificant sporophytic generation or diploid phase which is the diploid zygote. This type of life cycle pattern is known as haplontic or mono- genetic life cycle.
II. In the second type of life cycle the haploid generation is very insignificant and is represented by the gametes only. It occurs alternating with a very predominant diploid generation. Life cycle of such pattern is called diplontic life cycle.
III. In the third kind of life cycle both the gametophytic and sporophytic generations are equally predominant, and is enumerated as haplodiplontic or digenetic life cycle. In such life cycle pattern, again, both the haploid and the diploid phases may be morphologically similar or dissimilar, that is, isomorphic or heteromorphic respectively.
IV. In the fourth one both the gametophytic as well as the sporophytic generations are elaborately developed. The difference being, the diploid phase is long drawn, part of which is dependent on the gemetophytic generation and later on gives rise to an independent diploid phase. Such a life cycle pattern is enumerated as diplohaplontic with trigenetic life cycle.
Alternation of Generations in Algae:
Sexually reproducing algae, in general, pass through two phases of development to complete their life cycle.
(i) the spore producing phase, the sporophyte or sporophytic generation or diploid generation (2n), whose nucleus bears two chromosome complements (2n); and
(ii) the gamete producing phase, the gametophyte or gametophytic generation or haploid generation (n) having only one chromosome complement (n) in each nucleus.
These two phases or generations’ as they are commonly known, follow each other in regular sequence in the life cycle of algae. They are distinct from each other both functionally as well as for having the number of chromosome complements different.
These two ‘phases or generations which occur in regular succession, one alternating with the other is known as alternation of generations. The process of alternation of generations was discovered by Hofmeister in 1851.
As early as 1894, Strasburger made the generalization that, for plants showing alternation of generations, the generations differ in nuclear constitution. This difference lies in the fact that the nuclei of all cells of the sporphytic generation contain twice the number of chromosomes present in the nuclei of the cells of the gametophytic generation.
The sporophytic generation is initiated by the fusion of two gametic nuclei, each possessing one chromosome complement (n).
The fusing nuclei may be present in morphologically similar or dissimilar gametes. The two nuclei fuse to form a diploid nucleus having two chromosome complements (2n). Sooner or later, meiosis takes place whereby the chromosome number is reduced by half during the formation of spores.
This marks the end of the sporophytic generation and the beginning of the gametophytic generation. The spore is thus the first cell of the gametophyte. The gametophyte produces gametes, which, on fusion, result in the formation of a new sporophyte.
A plant arising either from a zygote or from a spore may give rise to an independent plant exactly like itself. Each member of such a series in a very natural use of the word constitutes a generation. Hence, the generation extending from zygote to a spore or from spore to gametes may consist of a series of generations of subordinate order.
Furthermore, each generation, in either of these senses, may include hundreds or thousands of cell generations.
A more adequate expression, however, is lacking for the two alternating arcs of the life cycle, one diploid and closing with meiosis; the other haploid and ending in the production and union of gametes. In what follows, each of these major arcs will be called a generation, however many successive independent individuals it may include.
The advantages resulting from alternation of generations are: (i) it provides for the production of many spores, so increasing the possibilities of reproduction; and (ii) meiosis of a zygote nucleus can give rise in consequence of new groupings of chromosomes and chromosome parts to at most four genetically different lines.
The two generations also differ in their relative conspicuousness in structure and duration. Majority of algae have a conspicuous gametophyte and the sporophyte is relatively inconspicuous, short in duration, and more or less dependent for all food upon the gametophytic thallus. In such cases the diploid nucleus divides meiotically almost immediately after its formation.
As such, there is alternation of an elaborate haploid or gametophytic generation and a very short single-celled diploid or sporophytic generation. A reverse condition is also encountered, where a well-developed diploid generation alternates with a very short Laploid condition being represented by the gametes only and meiosis takes place during the formation of gametes.
Such conditions indicate that there is no. clear alternation of phases having different functions and only alternation of chromosome numbers takes place in such life cycle.
Again, others possess the two alternating generations as independent plant? that are often evenly balanced as to size and conspicuousness. In them, the zygote resulting from the gametic union directly develops into an elaborate diploid individual, the sporophyte.
Meiosis takes place during the formation of spores. In such cases both the generations may be morphologically similar or dissimilar. An alternation of two morphologically similar generations is known as isomorphic alternation of generations and when the two alternating generations are morphologically dissimilar it is called heteromorphic alternation of generations.
There are some other algae which exhibit alternation of generations in their life cycle with little deviation from the typical one. Here the post-fertilization diploid, condition is very much long drawn. The earlier part of the diploid condition is entirely dependent on the gametophyte, but subsequently an independent sporophyte is developed which bears sporangia where spores are formed as a result of meiosis.
The spores thus produced give rise to the gametophytic, generations. This is known as triphasic alternation of generations.
Essay # 3. Sexuality in Algae:
In Algae, sexual reproduction generally takes place after the plant has had a certain amount of vegetative growth, and a certain amount of food reserve has been built up. It involves two distinct processes, a fusion of protoplasts, plasmogamy, and a fusion of two nuclei, karyogamy.
The occurrence of plasmogamy and karyogamy during the union of gametes to form a zygote is known as syngamy. But where sexual reproduction includes only karyogamy, that means union of two nuclei both of which are usually derived from the same parent cell the process is called autogamy.
Syngamy is of general occurrence among Chlorophyta, Euglenophyta, Chrysophyta, sparingly in Pyrrophyta, Phaeophyta, and Rhodophyta. Autogamy has been reported in some Pyrrophyta and Euglenophyta. Whereas, both syngamy and autogamy are lacking in Cyanophyta.
The gametes taking part in the sexual process are of varied structure and in most cases their fusion takes place only when they are liberated out from the mother plant. But among the oogamous forms along with the enclosing of the ovum there is a distinct tendency for the retention of the fertilization product within the oogonium.
This condition of advancement may very well be considered a case of incipient stage of embryo development. The most agreed kind of evolution is, isogamy —most primitive, with anisogamy—a step in advance of isogamy, and oogamy—being the most advanced type of sexual reproduction.
Essay # 4. Classification of Algae:
The classification of Algae has been continually modified since 1753 when Linnaeus in Species Plantarum included Algae in the class Cryptogamia along with other organisms which are now recognized as Mosses, Vascular Cryptogams and Fungi.
Unger in 1838 grouped Algae, Lichens and Fungi under the taxonomic category Thallophyta which arrangement in spite of serious drawbacks is still being widely favoured.
Although Unger separated Thallophyta from other plants because of their lack of differentiation into stem, leaves, and roots, this has created confusion, particularly for those algae whose body is differentiated into stem-like, leaf-like, and root like fixing organs.
Again to accept Thallophyta as a natural division of the plant kingdom means acceptance of the view that all the algae are more or less closely related to one another. Although limited in number, the members of the group Algae are of varied type and are sufficient to show that evolution has paralleled within the group.
Although it has become a customary practice to include Algae under the division Thallophyta, along with Fungi, in the late nineteenth century and first three decades of the twentieth, such classification has been abandoned in modern polyphyletic treatments of the plant kingdom, since algae and fungi are not phylogenetically closely related though these organisms possess many common attributes with respect to organization and reproduction.
Viewing the living world based on the micro-anatomy of protoplasmic structures may apparently appear to some very unrealistic. But this biological concept has already become acceptable to a large section of biologist. Christensen (1962) gave due consideration to this aspect and outlined a scheme of classification of major groups of Algae.
He introduced the terminologies Procaryata and Eucaryota for algae possessing procaryotic and eucaryotic cells respectively. Christensen also combined other principal characters like, flagella present or lacking, and chlorophyll b present or lacking along with procaryotic and eucaryotic cell character. His classification may be summarized below.
It has been an age-old practice to classify Algae largely on the basis of the colour of pigments and to subdivide into major classes: Cyanophyceae, Chlorophyceae, Phaeophyceae and Rhodophyceae. But with the increase in knowledge, it has been realized that beside pigments, reserve food, flagellation, and comparative chemistry of the several pigments should be considered in the classification of Algae.
Modern discussion of the relationships between the various classes of Algae holds that certain classes are so distinct from all others that each should be placed in separate division.
In accordance with the International Rules of Botanical Nomenclature, the Algae are classified into eight divisions by adding the suffix-phyta to each division:
Cyanophyta, Chlorophyta, Euglenophyta, Xanthophyta, Bacillariophyta, Pyrrophyta, Phaeophyta and Rhodophyta.
Objection has, however, been raised to this practice because these names do not indicate the algal nature of the groups thus named. It has been proposed by some phycologists to name the divisions as: Cyanophycophyta, Chlorophycophyta, Phaeophycophyta, Rhodophycophyta. etc.
The introduction of these cumbersome names is not really very essential since the shorter names do show the algae nature of the divisions even though they do not have the root -phyco.
Algae may thus be subdivided into the following divisions:
i. Cyanophyta (Myxophyta) or (Blue-Green Algae):
The internal structure of the cell is extremely simple, true nucleus and chromatophores lacking. Cells are procaryotic. The protoplast is differentiated into peripheral pigmented region together with oil drops and cyanophycean starch and non-pigmented central body. The pigmentation is chracterized by chlorophyll a, β- carotene, myxoxanthin, myxoxanthophyll, c-phycocyanin, and ophycoerythrin.
There is complete absence of motile cells. Reproduction is chiefly by accidental breaking and in some forms by the production of specialized cells. Inefficient sexual system has recently been reported.
ii. Chlorophyta (Green Algae):
Cells are eucaryotic. Pigments are the same as those present in the higher plants and are in same proportions being located in definite plastids of variable nature. Starch is the customary form of storage food. The majority of the representatives exhibit sexuality ranging from isogamy, anisogamy to oogamy. Motile cells may be bi-, quadri- or multiflagellate and are extremely variable in size.
They possess a definite nucleus and the pigmentation is chlorophyll associated with carotenoids localized in definite plastids. Cells are eucaryotic. The reserve food is paramylum (an insoluble carbohydrate related to starch) and fats.
Almost all are naked, unicellular flagellates having one, two or rarely three flagella. Reproduction is generally by longitudinal cell division with the exception of one genus which reproduces sexually.
iv. Xanthophyta (Yellow-Green Algae):
This group of algae is mainly distributed in fresh-water with few marine forms. The vegetative body may be unicellular or multicellular. Cells are eucaryotic. The cell wall is composed chiefly of pectic compounds with very rare occurrence of cellulose. Cells contain numerous discoid chromatophores.
The chromatophores are yellow-green in colour owing to the presence of excess yellow xanthophyll. Pyrenoids are usually absent. The food reserves are fats or leucosin. The formation of starch is completely absent. Motile cells bear two flagella of unequal length. Asexual reproduction may be by the formation of flagellate or non- flagellate spores.
v. Bacillariophyta (Diatoms):
Members of this taxon of algae are commonly known as the diatoms. They occur in various habitats ranging from fresh and salt waters and in damp places including soil, bark of trees, rocky cliffs, etc. They have single-celled, usually diploid, vegetative body which may be solitary or colonial.
The shape of diatom cells may be varied being composed of two nicely fitted halves. Cells are eukaryotic. The cell wall of pectic material with particles of silica impregnated on it is richly ornamented. The shape of the cells and their wall markings are very important characters of taxonomic interest.
The rich deposit of diatom shells that is formed at the bottom of the water after the death of the diatom cells is known as diatomaceous earth which has great economic importance. The diatom cells are uninucleate. The pigments are located in the chromatophores whose shape and number are very variable. A special golden-brown pigment—diatomin masks the chlorophyll and other associated pigments.
Fats are the main food reserves, besides which there may be present a special insoluble food of uncertain chemical composition—volutin. Reproduction by cell division is rather very common. But when the cells become smaller and smaller with each succeeding division the original size is restored by the formation of rejuvenescent cells or renewal scores or auxospores.
The formation of auxospores is associated with either sexual reproduction or parthenogenesis.
Members of this division constitute some of the most abundant kind of life on earth, being common components of phytoplankton. They cover a wide range of habitats, but are chiefly fresh water. There is a range of vegetative body from the smallest flagellates to filamentous, and colonial forms.
But vegetative body chiefly consists of unicellular forms, being uniflagellate and biflagellate (heterokont) having both flagella of the acronematic type or with a longer pantonematic type and the other of the acronematic type.
The photosynthetic pigments include chlorophylls a and β-carotene, fucoxanthin and diadinoxanthin. The main reserve products are oil and chrysolaminarin (leucosin). The most common method of reproduction is by longitudinal division of the cell.
A characteristic feature of the Chrysophyta is the formation of silica impregnated thick-walled cysts (statospores) from the vegetative cells. The contents of the cyst either give rise to a new vegetative cell or divide to produce 2-4 zoospores. Sexual reproduction may be isogamy (between two motile gametes), hologamy (fusion of protoplasts of two vegetative cells), or autogamy (-fusion of two nuclei only within the cyst).
vii. Pyrrophyta (Fire Algae):
Most of the orgnisms are unicellular bearing two usually unequal flagella. A few are unicellular and colonial alga-like in form. Cells are eukaryotic. Yellowish-green to golden-brown chromatophores are due to the presence of dinoxanthin, diadinoxanthin and peridinin.
The reserve foods are starch or starch-like compounds or oils. Common mode of reproduction is by cell division, although some produce zoospores, while sexuality has been reported in a few.
viii. Cryptophyta (Cryptomonads):
The vegetative cells are naked, unicellular and motile with two equal (sometimes slightly unequal) usually anteriorly (in kidney- shaped cells laterally) located flagella. Members of this division occur in both fresh water and marine habitats.
Cells may be red, blue, olive-yellow, brown or green having photosynthetic pigments chlorophylls a and c, β-carotene, some distinctive xanthophylls and biloproteins which are different from those of blue-green and red algae.
Nature of pigmentation suggests Cryptophyta as an intermediate stage in the evolution of eucaryotic forms from procaryotic algae. Most forms are photosynthetic, but saprophytic nutrition also occurs. Again some reside in the tissues of host invertebrates. Reproduction is by longitudinal division of vegetative cells, thick-walled cyst formation has also been reported in some genera.
ix. Phaeophyta (Brown Algae):
Cells of the brown algae are eukaryotic having definite nuclei, cytoplasm, and chloroplasts. The chloioplasts contain photosynthetic pigments masked by golden-brown pigments, called fucoxanthin. Food reserves are stored as simple sugars, the alcohol mannitol and complex polysaccharides such as laminarin or there may be fats in different forms.
The motile reproductive cells are pear-shaped with two laterally inserted flagella. Reproduction is isogamy to oogamy.
x. Rhodophyta (Red Algae):
Cell structure of red algae is eucaryotic being of high organization with a single nucleus although some cells are multinucleate. One or several plastids floating in the cytoplasm possess chlorophyll associated with a red pigment—phycoerythrin, and often a blue pigment—phycocyanin.
Food reserves may occur as alcohols, but are chiefly stored in the form of a polysaccharide as small granules free in the cytoplasm rather than in plastids and are known as floridean starch. Growth of the thallus may be apical as well as intercalary.
One of the striking features of red algae is the conspicuous cytoplasmic connection from cell to cell. Another outstanding characteristic of this taxon is a total absence of any flagellate reproductive cells. Oogamous sexual reproduction is of general occurrence in red algae.
Essay # 5. Cellular Organization of Algae:
It will not be wrong to generalize the algal characteristic of possessing a definite cell wall being made of cellulose, of course, this does not lose sight of many of the naked flagellate forms.
Algal cells are of two kinds prokaryotic and eukaryotic. In a prokaryotic cell there is absence of internal membranes which separate the resting nucleus from the cytoplasm, and which isolate the enzymatic machinery of photosynthesis and respiration in specific organelles and as such the cell lacks organized nucleus, chromatophores and mitochondria.
Nuclear division does not take place by mitosis. Whereas, a eukaryotic cell is characterized by the presence of well-organized nucleus, chromatophores and mitochondria. Nuclear division is by mitosis in a vegetative cell.
Generally, where a cell wall is present the surface layer of the protoplast is constituted by a plasma-membrane, but in numerous naked types it is developed as a more or less rigid periplast.
The mucilage envelopes and mucilage accumulations found in some forms may be in part modifications of the cell wall. But in a number of cases the products of secretion of protoplast may perform diverse functions, like, holding the individuals of colonial forms together, as a protection against desiccation in many terrestrial algae, as a means of movement, etc.
When a wall is present, its chemical constituents vary from one group to another and are sometimes important indications of the taxonomic position of a particular alga. In general, algal cell wall is composed of a variety of carbohydrate materials which may be either soluble or insoluble in boiling water.
The insoluble materials are usually considered as cell wall materials and the soluble carbohydrates as accessory matrix or sheaths which are outside the cell wall.
The wall materials are composed of polymers of hexose and pentose sugars and of sugar acids, often a mixture of these. The major water-insoluble carbohydrates of algal cell wall include cellulose I, mannan, xylan and alginic acid. Cellulose I is widespread among the green algae and occurs also in certain red and brown algae; its presence in blue-green algae has not been confirmed.
Mannan occurs in the cell wall of certain red algae and in a number of green algae, while water-insoluble xylans have been reported in the wall of certain marine green algae. Characteristic components of the cell wall include the polyuronic acid, alginic acid, fucinic acid are found in the wall of brown algae; and a characteristic mucopeptide component is present in the cell wall of prokaryotic cell of blue-green algae.
Alginic acid, a polymer of D-mannuronic and L-guluronic acids, may comprise up to 25 per cent, of the dry weight of some brown algae.
The extract of alginic acid in the form of sodium alginate is usually called algin. Certain algae, particularly the diatoms have a silicified wall. Water-soluble polysaccharides comprise the amorphous sheaths and matrix of algae.
These include, among others, agar, carrageenin and gelans (xylan) of red algae; pectin and ulvin of the green algae; the fucoidin of brown algae; the mucilaginous compounds of blue-green algae and those of diatoms.
Essay # 6. General Characteristics of Algae:
1. The word algae (sing, alga) is derived from a Latin word ‘alga’ which means ‘sea weeds’. The science that deals with their study is called ‘algology’.The Greek word for algae is ‘phykos and, therefore, their study is also called phycology (Gr. phykos, sea weeds; logos, study or discourse).
2. In different systems of classification of plants, the algae have been differently ranked as an order, a class, a division, or a group. They may, however, be defined as “an assemblage of chlorophyll – bearing autotrophic Thallophytes, bounded by a cell wall, made up of pure or mixed carbohydrates”.
3. According to the latest estimates of B.N. Prasad (1984), “out of a world total of 2475 genera and 28,305 species of algae, 666 genera and 5136 species are represented in India. This amounts to a proportion of 28.3% of global generic representation and 18.1% of world species”.
4. Algae are of universal occurrence because of their presence in nearly all types of habitats. They are found in fresh water (e.g., Spirogyra, Oedogonium), sea water (marine algae, e.g., Sargassum, Laminaria), on soil (terrestrial algae, e.g., Fritschiella, Vaucheria), on rocks and stones (lithophytic algae, e.g., Batrachospermum, Enteromorpha), in highly concentrated salty water (halophytic algae, e.g., Dunaliella), on sand (psammophytic algae, e.g., Phormidium, Vaucheria), in very hot waters (thermal algae, e.g., Onconema, Synechococcus), on ice or snow (cryophytic algae, e.g., Chlamydomonasnivalis, Raphidonema), on other plants (epiphytic algae, e.g., Oedogonium, Bulbochaete), inside other plants (endophytic algae, e.g., Nostoc inside the thallus of Anthoceros, Anabaena in the coralloid roots of Cycas), on animals (epizoic algae, e.g., Cladophoracrispata on the shells of molluscs), inside the animals (endozoic algae, e.g., Stigeoclonium in the nose of fishes), as parasites (e.g., Cephaleuros on leaves of tea plants) and also in combination with fungi in the form of lichens.
5. Thallus organization in algae also varies greatly and shows a clear range. They may be motile colonial (e.g., Volvox), palmelloid (e.g., Tetraspora), dendroid or tree-like (e.g., Ecballocystis), coccoid or non-motile (e.g., Chlorella), filamentous (e.g., Oedogonium, Cladophora), heterotrichous (e.g., Ectocarpus), siphon-like or siphonaceous (e.g., Vaucheria), uniaxial (e.g., Batrachospermum), multiaxial (e.g., Polysiphonia) or evfenparenchymatous (e.g., Sargassum).
6. The size of algal members is also highly variable. It varies between 0.5 µ (e.g., Dunaliella, Chlamydomonas) to as much as 30 metres or more (e.g., Macrocystis). There is an “unsubstantiated record of a plant of Macrocystispyrifera reaching up to 700 feet (i.e., 213 metres)-the longest plant in the world” (Prescott, 1969).
7. The cell wall is mostly made of cellulose but in some genera it also contains some other substances such as pectin, chitin, algin and fucoidin.
8. Cytoplasm contains structures such as contractile vacuoles, mitochondria, eyespot, chloroplast, pyrenoids, Golgi bodies, pigments and nucleus. However, in Myxophyceae, the membrane-bound structures such as mitochondria, Golgi bodies, endoplasmic reticulum and a well-defined nucleus are all absent.
9. Chief pigments are chlorophyli-a, chlorophyll-b, β-carotene and xanthophylls.
10. Reserve food material in majority of algae is starch. However, it is in the form of fats and oils in Xanthophyceae and Bacillariophyceae; laminarin and mannitol in Phaeophyceae; floridean starch in Rhodophyceae; and myxophycean starch in Myxophyceae.
11. Vegetative reproduction takes place by fragmentation, fission (desmids), akinetes (Oedogonium), tubers (Chara) or hormogones (Myxophyceae).
12. Chief modes of asexual reproduction are zoospores (Chlamydomonas), synzoospores (Vaucheria), aplanospores (Vaucheria), hypnos-pores (Vaucheria), autospores (Chlorella) and tetraspores (Dictyota).
13. Sexual reproduction takes place by isogamy (Chlamydomonas), anisogamy (Ectocarpus) or oogamy (Oedogonium).
14. Life cycles may be haplontic (Chlamydomonas), diplontic (Sargassum), diplohaplontic showing isomorphic type of alternation of generations (Cladophora) or heteromorphic type of alternation of generations (Laminaria), haplobiontic or diphasic (Batrachospermum) and haplodiplo- biontic or triphasic (Polysiphonia).
Essay # 7. Septa of Algae:
The septa may be complete or incomplete. The former are mainly of five types:
(iv) Colligate, and
The plane type has a homogeneous contour as in Ulothrix. The replicate septa are common in some species of Spirogyra where middle lamella shows circular in-folding’s. A septum with two infolds in opposite directions is called replicate septum. When there is only a single infold in alternate positions, it is known as semi-replicate septum.
The colligate septum is an H-shaped cross wall, as in Microspora. In the un-duliseptum the middle lamella is much expanded and has a wavy undulating margin, found in Spirogyra undulisepta. The incomplete septum found in members of the Rhodophyta and some Cyanophyta, has a central opening known as pit through which the cytoplasm of adjacent cells remains connected.
Essay # 8. Nucleus of Algae:
In majority of algae where the cells are eukaryotic, the cells are uninucleate (Fig. 6A). But algae with multinucleate cells are also found in considerable number. The structure of nucleus in algae does not differ in any appreciate respect from that of higher plants.
Again in algae (Cyanophyta) with prokaryotic cells where membrane-bounded nucleus is absent, the chromatin material of nucleus is located towards the centre of the cell and consists of DNA fibrils which are not associated with basic proteins (histones) so that no organized chromosomes are found (Fig. 6B).
Algal nuclear division may or may not be followed by wall formation. The chromosomes are small and chromosome number varies from 5 to 48.
Essay # 9. Chloroplast, Pyrenoids and Pigments of Algae:
The photosynthetic pigment—chlorophyll is lodged either in a well-organized photosynthetic apparatus, known as chloroplast which is composed of stroma and grana surrounded by a membrane; or in an ill-organized photosynthetic apparatus, called chromatophore.
The structure, number, and position of chloroplast in an algal cell is extremely variable depending on the species of algae. The simpler forms of algae lack chloroplast. It has long been accepted that, in them, the pigment remains diffused in the peripheral region of the cytoplasm which is differentiated from a central colourless region having structural details resembling a nucleus.
But recent researches have confirmatively indicated that in those algae, the pigment is located in lamellar structures (thylakoids) (Fig. 18) distributed in the cytoplasm, visible only under electron microscope.
Apart from the nature of pigmentation which constitutes one of the basis of classification of algae, the distinctive features exhibited by the chloroplast are often of great value in generic diagnosis. Associated with the chloroplast are bodies spoken of as pyrenoids which consist of colourless masses of protein surrounded by starch plates (Fig. 6G to I).
Again, there are certain algae whose chloroplasts do not bear any pyrenoids at all. Cells of most algae contain only one chloroplast each with the exception of a few species whose cells have more than one chloroplasts.
Besides this, almost all chloroplasts bear one or more pyrenoids. There is a wide range of variation in the shape of the chloroplasts. Depending on species, the chloroplasts may be: Cup-shaped, Parietal, Discoid, Lobed-discoid, Star-shaped, Spiral, Barrel- or Girdle-shaped with incised margins, and Reticulate (Figs. 6 and 7).
The chloroplasts are genetically semi-autonomous systems containing his- tone-free DNA, messenger RNA (ribonucleic acid), transfer RNA and ribosomes. New chloroplasts arise from pre-existing ones.
The photosynthetic pigment chlorophyll may be of different kinds—Chlorophyll a, Chlorophyll b, Chlorophyll c, Chlorophyll d, and Chlorophyll e. Chlorophyll a is present in all algae. Chlorophyll b is found only in the Chlorophyta and Eugleno-phyta. It is indispensible for the assimilation of true starch. Due to the presence of chlorophyll b assimilation product in the Chlorophyta and Euglenophyta is true starch.
Chlorophyll c is present in Bacillariophyta, Chrysophyta, Pyrrophyta, Cryptophyta, and Phaeophyta—a feature which seems to link them together. Chlorophyll d is present in the Rhodophyta. Chlorophyll e has been identified only in two genera: Tribonenia and in the zoospores of Vaucheria, belonging to the Xanthophyta.
Besides Chlorophyll, two kinds of special pigments —Carotene and Xanthophylls, collectively called Carotenoids are present in algae.
Carotene is fairly well distributed in algae except in the Siphonales. There are different Xanthophylls in algae which are important diagnostic features: peridin in the Pyrrophyta, myxoxanthin in the Cyanophyta, taraxanthm in the Rhodophyta and antheraxanthin in the Euglenophyta. Besides these, there are various other pigments.
Some of them are: Phycobilins, Fucoxanthin, Dia- toxanthin, Diadinoxanthin, and Haematochrome.
The name of pigments Phycobilins has been changed to Biloproteins. Biloproteins are present in the Cyanophyta, Rhodophyta and Cryptophyta. Phycobilins include red coloured phycoery-thrins and blue coloured phycocyanins.
R. A. Lewin and N. W. Withers (1975) collected Synechocystis didemni, a unicellular marine alga with a procaryotic cellular organization like that of a blue-green alga which possesses chlorophylls a and b, β-carotene, at least three xanthophylls and no demonstrable water-soluble phycobilin pigment.
It has been observed that certain algae completely lack pigments. Some of them are: colourless diatoms, colourless dinoflagellates, colourless Cryptophyceae, colourless green algae (as in certain Volvocales), and colourless red algae that parasitize closely related hosts.
Some colourless algae lack not only the pigments but also the chloroplast structure as well. Except few, almost all colourless algae lead a saprobic existence. They are often collectively known as leucophytes.
Essay # 10. Nutrition of Algae:
The algae, broadly speaking, like other chlorophyllous plants, require G, H, O, P, K, N, S, Ca, Fe and Mg and also traces of Mn, Bo, Zn, Cu, and Go. For certain organisms additional elements are required such as Si for diatoms and Mo for nitrate reduction in Scenedesmus. Algae do not differ markedly from other chlorophyllous plants with respect to the major and trace elements required for growth.
With respect to the major elements they require, algae are able to use a wide range of inorganic and organic sources and probably do so in natural habitats. For example, the element nitrogen may be used by many algae in such alternate forms as nitrate, nitrite, ammonia, urea, certain amino acids, purines, and even as casein and gelatine.
They are also able to use a wide variety of inorganic compounds as sources of the required major and trace elements. Those algae which can grow in an entirely inorganic medium in the presence of light are known as photoautotrophic. They synthesize protoplasm from exclusively inorganic sources using light energy.
Again algae which require in addition certain vitamins, usually B12, thiamine or biotin; such algae are said to be photoauxotrophic. In contrast to photoautotrophic algae a number of algae are heterotrophic, either facultative or obligate so. Several algae, among them a species of Ochromonas and certain dinoflagellates may digest solid particles of food; such organisms are phagotrophic.
Essay # 11. Storage of Food in Algae:
The storage food in algae which is dependent on the nature of pigmentation, is extremely variable. In green algae it is starch, in other algae it may be carbohydrate of various forms. Some of the algal food reserves are: Cyanophycean starch, Floridean starch, Laminarin, Mannitol, Diatomin, Leucosin, Oils, Fats, Paramylum, etc.
Whatever their number, the structure of algal flagella resembles that of cilia and flagella of other organisms (except bacteria) in showing the typical ‘9+2’ pattern of component fibrils enclosed within a flagellar sheath, the latter continuous with the plasma membrane. Two fibrils forming the central core are surrounded by nine. Each fibril is again composed of sub-fibrils.
The flagella may be equal (isokontan) or unequal (heterokontan) in length. For a long time algal flagella were thought to be of two kinds acronematic or whiplash type being smooth and whip-like with stiff base and a flexible upper portion; and pantonematic or tinsel type is feathery in appearance having lateral rows of fine hairs arranged along the axis of the flagellum.
Recent work with the electron microscope has revealed one other kind of flagella in which the flagellar surface is covered by minute hairs (different from those on the pantonematic flagella) and scales. Algae with such flagella are included in a new; lass, the Prasinophyceae of uncertain systematic position. Flagella of algae, irrespective of kind, may be apical or lateral or all around the flagellate cell.
The motile cells of algae, like those of fungi, may be flagellate and/or amoeboid. Associated with motility are structures like contractile vacuoles, flagella, stigmata (sing, stigma) and eye spots. Contractile vacuoles range in number from one, two, four to many. They lie near the cell surface, often in close proximity to the flagellar bases.
They apparently regulate osmotic balance in the cells. Contractile vacuoles are present in both flagellate and amoeboid cells, and in non-motile cells as well.
Essay # 12. Other Structures of Algae:
Both mitochondria and Golgi bodies are present in algae possessing eukaryotic cells. A system of tubules and vesicles traversing the cytoplasm known as endoplasmic reticulum exists in algal cells except in the blue-green algae performing the function of synthesizing proteins or enzymes.
Except in the blue-green algae, the mature cells of algae possess one or more vacuoles bound by distinct membranes. Their function is osmoregulation of cell fluid. But the healthy cells of many planktonic blue-green algae contain numerous small bodies of irregular shapes known as pseudo-vacuoles or gas vacuoles.
Essay # 13. Plant Body Types of Algae:
Algae are plants of simple structure, simplest of which consists of a non-motile single-celled to filamentous forms with no well-organized nucleus and plastids. There exist many unicellular motile forms which may be solitary or colonial. Some are filamentous.
Of the filamentous forms again some are un-branched with very little differentiation of base and apex having all the cells almost alike and every cell is capable of growth and division and formation of reproductive units.
Whereas, others show diversified organization of plant body. Particularly, certain brown algae possessing .long flexuous stem-like structure and an expanded blade portion, have holdfasts attaching them to rocks. Some of these plants have considerable differentiation of tissue very similar to higher plants. But they lack vascular tissues.
On the other hand, in the terrestrial forms there are distinct adaptations in the plant body to survive under constant inadequate supply of moisture.
Again, there are heterotrichous forms where the plant body is highly developed being differentiated into prostrate and erect portions resembling prototype of simplest plant body of Bryophyte level with very clear- division of labour.
In general, the varied forms of algae may be outlined as follows:
i. Unicellular Motile Form:
Single-celled plant body being spherical, oval or pear-shaped bearing two flagella in the anterior region (Fig. 8A).
ii. Unicellular Non-Motile (sedentary) Form:
The cells are commonly small and spherical without any flagella and do not exhibit any movement. Some are solitary, others in groups being embedded in a gelatinous material (Fig. 8B). Also there are slightly elongated forms which can be differentiated into base and apex (Fig. 10E).
iii. Motile Coenobial Form:
Definite number of motile cells are embedded in a gelatinous matrix with their flagella protruded out, or are held together by cytoplasmic connection. The cells may be compact or loosely arranged. Thus a colony is formed of definite number of cells arranged in a specific manner forming what is called a coenobium (pi. coenobia) (Fig. 8C).
iv. Non-Motile Coenobial Form:
Coenobia may be composed of non-motile cells arranged in a single layer being closely ad-pressed to each other along the long axis (Fig. 8D) or may be attached end to end forming a network, the meshes of the being pentagonal or hexagonal (Fig. 8I). The coenobium may also be star-shaped in appearance with a single central cell surrounded by peripheral cells of the coenobium (Fig. 1D).
v. Filamentous Form:
Filamentous thallus may be of indefinite length. Cell division in filamentous forms occurs in one plane so that a single row of cells is formed.
The cells of the filament are usually joined firmly end to end either in a single file or having a tendency towards a multiseriate arrangement. The filament may or may not be differentiated into base and apex. All the cells of a filament may or may not be alike. The filamentous form may be of various kinds—un-branched, branched, and having false branches (Fig. 8E to G), (Fig. 9B & D) and (Fig. 8H).
vi. Thalloid Form:
A parenchymatous thallus is resulted by the division of cells in more than one plane (Fig. 9F). Growth of parenchymatous thalli may be diffuse (all cells capable of division), intercalary (well-defined dividing regions not located terminally), trichothallic (a specialized intercalary meristem at the base of a terminal hair) or apical (one or more well-defined apical cells dividing to give remainder of the thallus).
vii. Siphonaceous Form:
The thallus is non-septate multinucleate, a coenocyte. Depending on the organism, a coenocyte may be simple branched or very elaborately developed with clear division of labour being differentiated into aerial and subterranean and in some cases into sub-aerial portions (Fig. 9A and Fig. 2G).
viii. Heterotrichous Form:
This is a highly advanced type of thallus which is characterized by the differentiation of the vegetative body into a prostrate system growing along the substratum and a projecting system developing away from the substratum. In some algae both the systems are equally well developed, whereas in others one system is highly developed at the cost of the other.
For example, the prostrate system may be very elaborately developed and the aerial portion being rudimentary (Fig. 9E), and in others, reverse is the case.
ix. Special Forms:
The thallus in some algae is highly complex being differentiated into a perennial portion, an annual portion, and a meristematic region in between. The meristematic region produces the annual portion every year (Fig. 9G). In others, there is extreme degeneration of heterotrichous condition.
The aerial portion is very elaborately developed being represented by a repeatedly branched erect system and the anchorage is performed by very poorly developed prostrate system. Again in others, the aerial portion of the thallus is highly developed into a multiseriate— polysiphonous branched condition with clearly visible cytoplasmic connections from cell to cell (Fig. 9C).
Essay # 14. Origin of Sex in the Algae:
Sexual reproduction is one of the means of perpetuation of species in algae with ultimate increase in the population of the individuals. The process involves the fusion of two gametes or sex cells which may originate from the same thallus (monoecious) or from different thalli (dioecious). The gametes may he flagellate or non-flagellate. In algae, the mode of sexual reproduction ranges from isogamy, anisogamy to oogamy.
In general, sexual reproduction takes place after the alga had attained a certain amount of vegetative growth and having certain amount of reserve food built up. In algae, an isogamous fusion of two zoogametes is the most primitive type of sexual reproduction. The evolutionary tendency is isogamy, anisogamy to oogamy. Anisogamy is an advanced condition than isogamy.
Among anisogamy again there are two conditions prevailing:
(i) both the fusing pair of gametes may be flagellate, and
(ii) the larger one is non-flagellate as against the smaller which is flagellate.
It is evident that the development of a large-sized gamete permits the greater accumulation of reserve food for the use of the zygote during the post-sexual stages. At the same time increase in size means greater surface area which facilitates the chance of meeting of gametes.
Hence, anisogamy has distinct advantage over isogamy. Within oogamy also there is wide range of variations including incipient oogamy to highly advanced stage where the female gamete is permanently retained within the female gametangium. A progressive evolution from isogamy to oogamy has taken place along independent phyletic lines in the Chlorophyta and the Phaeophyta.
The development and differentiation of gamete producing structures and the gametes are extremely variable. The simplest condition is exhibited by filamentous algae Spirogyra and Zygnema where all the cells of the thallus have the potentiality to behave as gamete producing structures (gametangia) and can bear gametes.
The vegetative cell of the filament and the gametangium resemble each other morphologically except in the fact that in the former there is no change in the protoplasm of the cell, whereas, in the latter the entire protoplasm of the cell is metamorphosed into a gamete. As such no specialized gametangia are developed and any vegetative cell is a potential gametangium.
But in filamentous algae Ulothrix any cell of the filament except the apical and basal one has the potentiality to behave as a gametangium without undergoing any morphological change except the protoplasm which divides and redivides to develop into gametes. Some of the gametes are morphologically very similar to the zoospores, but difference is only in their behaviour.
In unicellular alga Chlamydomonas also the vegetative cell divides and redivides to form either daughter vegetative cells or gametes depending on the conditions of the habitat. In both Ulothrix and Chlamydomonas difference in the gamete or zoospore formation lies in the number of division of the protoplasm and the behaviour of the daughter cells.
The number of division during the formation of gametes is much larger than that of the formation of daughter vegetative cells.
In Oedogonium, Vaucheria, Ectocarpus there is distinguishable gametangia development but the gametangia may be formed anywhere in the thallus without having any specificity for any particular region of the thallus. It is much more interesting in Ectocarpus, where, the plurilocular structures may behave as gametangia as well as sporangia.
In some species the controlling factors are the conditions of the substratum in which the alga is growing.
In Volvox, Coleochaete, Chara, Nitella, Laminaria, Fucus, Polysiphonia, there are particular regions or branches or particular cell or cells where after a certain period of vegetative growth there is development of specialized gamete producing structures of varied names.
Usually when the sexual fusion between the gametes fails to occur the gametes degenerate. But there are exceptions to this process. Particular mention may be made in case of Spirogyra and Zygnema where, in absence of sexual fusion the gametes behave as spores.
What induces the formation of gametes? What factor or factors control the sexual reaction? It is broadly agreed that conditions of the substratum play very important role in the entire process. In case of certain marine algae the formation of gametes is very much pronounced during the warmer months of the year.
It has also been experimentally demonstrated that the gametes secrete certain substances in the surrounding medium. The secretion products may have some effect to envigourate the sexual reaction. The sexual reaction inducing substances in some cases have the capacity to change zoospores into gametes. It is also believed that certain algae reproduce vegetatively under favourable conditions.
Whereas, the same algae will start reproducing sexually by the development of gametes when the prevailing conditions of the substratum are unfavourable, which suggests that the origin of sexual reaction is directly correlated with the environmental conditions of the surrounding medium. Hence the origin of sex is dependent upon the microclimate which again accelerates the division of the protoplasm.
In case of gametes behaving as spores after the failure of sexual act, there is a possibility that with the failure of sexual act the sexual reaction inducing substance of the gametes are destroyed due to which the gametes behave as spores. This situation suggests that a gamete has basically the property to behave as a spore. But this quality is suppressed by the development of so-called sexual reaction inducing substance.
Similar interpretation is tenable in case of Ulothrix where the development of gametes or zoospores is dependent on the number of division of protoplasm. With more division of protoplasm the result will be development of gametes. It is quite possible that there is some relation between the number of divisions of protoplasm and the development of sexual reaction inducing substance.
That gametes secrete sexual substances into the surrounding water was demonstrated by Jollos (1926), Geitler (1931), Pascher (1931), Moewus (1933, 1940), and many others. These substances can be utilized for increasing sexual intensities of weak gametes.
Moewus was able to extract substances from fertile sexual plants of the members of the Ulvaceae and. species of Chlamydomonas and with these substances he made zoospores function as gametes.
It has been ascertained by spectroscopic analysis that these sexual substances are carotinoids in nature and the first-formed substance is protocrocin which breaks down into two molecules of picrocrocin and one of crocin. Each molecule of carotinoid picrocrocin breaks down into a molecule of glucose and molecule of carotinoid safranol.
Each molecule of carotinoid crocin again breaks down into two molecules of sugar gentiobiose and one molecule of crocetin dimethyl ester and ultimately becomes trans crocetin dimethyl ester. Each step in the degradation of protocrocin is brought about by different genes.
Crocin is the chief substance that induces motility in gametes, and cis and trans crocetin dimethyl esters are primarily concerned with the mutual attraction of gametes. The carotinoids picrocrocin and safranol are sex determining substances.
Recent researches indicate that although the sexual process is a manifestation of profound physiological changes within organisms, it may be evoked or modified by a manipulation of certain external factors such as duration of illumination, temperature, the amount of nitrogen available, etc.
Essay # 15. Alternation of Generations in the Algae:
Alternation of generations in algae has been manifested in the manner indicated below.
An elaborately developed gametophyte followed by a very short sporophyte represented by the zygote nucleus only, is exhibited by Chlamydomonas, Volvox, Ulothrix, Spirogyra, Zygnema, Cosmarium, Oedogonium, and Vaucheria. In them meiosis is zygotic. There is no organized sporophytic phase with a different function. In the life cycle of these algae there is alternation of a well-developed gametophyte with a single-celled sporophyte.
This results in definite alternation of chromosome numbers from haploid to diploid and back to haploid stage. There is, however, no alternation of individuals with different functions. Such a condition is the starting point of the origin of the diploid body or the sporophyte in the life cycle.
A deviation from above is found in Coleochaete. Where, as elaborately developed gametophyte followed by a post-zygotic spore-producing phase is present. It thus indicates a tendency to develop an elaborate post-zygotic phase. But could not go further due to zygotic meiosis.
If the zygote were to divide without meiosis, the natural consequence would be a transfer of the post-zygotic phase from haplont to diplont. Meiosis would be delayed until spore formation. The two generations would be unlike, as they are throughout the bryophytes.
The development of an organized sporophytic phase is encountered in Viva, Enteromorpha, Cladophora, and Chaetomorpha. These algae exhibit isomorphic alternation of generations in their life cycle. Both the gametophytic and sporophytic plants are independent and are alike except for the reproductive cells they bear.
The delayed meiosis has given rise to the sporophytic plant where meiosis takes place during the development of spores which on germination give rise gametophytic individuals.
A further advancement along this line exists in Ectocarpus and Dictyota. Where evolution of gametangia and sporangia has taken place on an isomorphic alternation of generations.
In Cladophora glomerata there exists only a diploid generation, meiosis results in the production of gametes. Haploidy, except for the gametes, has disappeared in consequence of a mutation. There are indications of a similar history in certain races of Ectocarpus. This condition of diploidy except for the gametes appears also in the Bacillariophyta.
Steps in the gradual dominance of diplont over haplont are illustrated by Laminaria, Fucus, and Sargassum. In Laminaria the diplont is large, often perennial; the haplont is a distinct but very small plant and thus exhibits heteromorphic alternation of generations. In Fucus and Sargassum the haplont, no longer independent, has almost disappeared, being represented by the gametes only.
Similar condition of reduction of the haplont appears in Acetabularia, Bryopsis, and Codium. The haplont has disappeared, gamete nuclei now being produced by meiotic division of the diploid nuclei.
A somewhat different method of alternation of generations is found in red algae. In Batrachospermum a post-zygotic phase (zygotic meiosis) is dependent upon the gametophyte, gives rise to haploid carpospores. It increases the possibilities of multiplication, because through it a single zygote gives rise to many spores.
A condition of delayed meiosis has been exemplified by Ceramium, Callithamnion and Polysiphonia, where an elaborately developed post-zygotic diploid phase results in the development of diploid carposporangium and carpospores. The diploid carpospores on germination produce independent sporophytic individuals which are functionally different from the gametophytic ones.
These sporophytic individuals bear sporangia in which spores are produced by meiosis. The spores again on germination give rise to gametophytic plants exhibiting a distinct alternation of generations.
Since part of the diploid phase is dependent upon the haploid phase and the rest is entirely independent, and both these phases alternate with gametophytic plants, such a condition is also known as triphasic alternation of generations.
Essay # 16. Origin and Evolution in the Algae:
As different authors differ widely in their lines of thinking with regard to the origin and evolution in algae, it is felt necessary to review those possible lines instead of expressing one’s personal opinion in this aspect of algae. This will give everyone a wide scope for independent thinking after giving due consideration of every body’s line of thought.
This discussion is primarily based on living forms of algae. The difficulties associated with the interpretation are mainly, wide variation in the morphology of the thallus and the diversity in the mode of sexual reproduction in all the taxa of algae. Cases of parallelism are very common which need quite a careful consideration.
Distinguishing differences among the major groups of algae are not primarily morphological, but biochemical concerning types of pigments and metabolic processes. Structure and placement of flagella are also significant.
The principal algal groups:
Chlorophyta, Phaeophyta and Rhodophyta have probably originated from a common primitive motile chlamydomonad type of ancestry which followed a single line of evolution (Fig. 132).
The parallelism existing within various groups may be due to the plasticity of primitive organism. The separation of Rhodophyta was possibly by a side line from an intermediate simple filamentous condition and that of Phaeophyta much higher up from the heterotrichous condition.
Considering the morphological features and the mode of reproduction, Coleochaete appears to be also another point from which some members of the Rhodophyta might have evolved either directly or from a point very close to it.
But from the phylogenetic standpoint both Chlorophyta and Rhodophyta follow parallel lines of development as in both the taxa the haploid condition is primitive and the diploid generation is interpolated in the life cycle due to delay in the occurrence of meiosis of the doploid zygotic nucleus.
The evolution of heteromorphic alternation of generations has taken place due to certain modification from the isomorphic alternation of generations which starts earlier. Again there are Chlorophyta which have maintained the simple form of life cycle along with very elaborate morphological differentiation of the thallus forming divergent lines of evolution.
One other interesting feature is the development of completely dominant diploid generation indicating an extreme case of interpolation and elaborate development of doploid generation by the gradual suppression of the haploid generation. As to the unicellular red algae, it is probable that they are reduced forms than being primitive.
There is a possibility, that the Rhodophyta have evolved from the Cyanophyta, as both the classes have similarities in the pigments, but chemically they are different; and complete absence of any motile structure.
The main drawback of this line of interrelationship is the complete absence of sexual reproduction in any form in Cyanophyta, whereas, in most red algae the development of sexual reproductive organs and reproduction by sexual means are quite common.
Cyanophyta and Chlorophyta are considered by some as most primitive. But if the Phaeophyta and Rhodophyta had a flagellate ancestry, in that case the four taxa: Cyanophyta, Chlorophyta, Phaeophyta and Rhodophyta may have originated from the same stalk.
Again, there are workers who are of opinion that all the three taxa:
Chlorophyta, Phaeophyta and Rhodophyta have originated independently from different stalks.
In 1952 Feldmann expressed his opinion in considering algae having isomorphic alternation of generations as primitive. This may be true only in case of Phaeophyta and Rhodophyta and not in Chlorophyta, where the primitive orders have simple life cycle. Elaborately developed thallus, associated with isomorphic alternation of generations is usually encountered only in advanced types of Chlorophyta.
Whereas, according to Feldmann, this is the possible starting point in Phaeophyta and Rhodophyta. Hence it does not seem tenable to have the same characteristic features of primitiveness in all the taxa of algae.
The possible source of origin of the algae has been suggested from an ancestral flagellate Chlorophyta. If the chlorophycean ancestry is accepted of the Phaeophyta then the diploid condition has evolved by the interpolation subsequently in the haploid state. The Phaeophyta have their origin probably among the Chaetophoraceae or Cladophoraceae.
Similarities are in the morphology of the thallus and the nature of alternation of generations.
Besides these, on the basis of pigments it has also been suggested that Phaeophyta have their origin from the Chrysophyta according to the Scherre outlined by Senn (Fig. 133).
Essay # 17. Fossil Algae:
The soft delicate bodies of algae do not ordinarily leave much fossil impressions or petrification’s as after death they decompose very rapidly. Thus, quite often fossil records of most algae are imperfect. In spite of these difficulties quite a good number of fossil representatives of algae have been found in various era of the earth’s crust.
One should not restrict himself only to the study of the present-day living forms of algae, since a large number of fossil algae have also been worked out. Since invertebrates existed in pre-Cambrian times it is quite obvious that algae for long ages, before animals began eating each other, have been required as food of animals were in existence even earlier than animals, may be in Archeozoic era.
It is fairly well accepted that the blue-green algae existed in the early Archeozoic era. Many of them secreted lime or silica during their lifetime. Careful examination of the sedimentary rocks of the Proterozoic era has revealed the presence of fossil algae in great abundance. The blue-green algae may be among the earliest algae to appear, since a considerable number of them have been found in ancient rocks.
The Proterozoic limestone beds were possibly resulted due to accumulation of algae. Besides these, well-preserved fossils of both marine and fresh-water blue-green algae are also known. Girvanella, a cyanophycean fossil form-genus is one of the calcareous algae.
Some other fossil forms of the blue-green algae are: Crypto zoon, Collenia and Neulandia. The vast warm sea that covered much of the earth of the Paleozoic era was very suitable for the growth of algae. Thus, the Paleozoic era has been quite often called the ‘Age of Seaweeds’. Many algae thrived in the earliest period of the Paleozoic era. They are not only of simple pattern, such as blue-green, and green, but also more highly differentiated types similar to our modern brown algae.
Among green algae, the Characeous remains have been found from the Paleozoic era. Kidston and Lang have reported from the Paleozoic era the two form-genera, Lagynophora and Palaeonitella which have very close similarity with the present-day Characeae.
Among the Chrysophyceae, Scyphosphaera and Thorosphaera have been found in early Paleozoic time. Chrysophycean fossil algae are rather abundant in Cretaceous onwards. Several genera of the family Codiaeeae under Siphonales have been found to be common on the Lower Silurian. Ovulites, a representative of Eocene has been considered to be allied to the alga Penicillus.
The form-genus, Microcodium, a member of the Miocene may have some affinities with the members of the Codiaceae under Siphonales. About sixty well-preserved fossil genera of the family Dasycladaceae (Siphonales) have already been discovered from the Ordovician.
They serve link with living members o£ the Dasycladaceae. It is quite possible that the incrustation with carbonate of lime on the various parts of these algae have been advantageous for the preservation of the numerous fossil forms of this family.
Mention may also be made of various other genera which are of wide interest, for example, Mizzia from Permian, Gyroporella from Triassic, Triploporella and Tetraploporella from Jurassic and Cretaceous periods, and Dactylopora from Eocene.
Almost all imperishable siliceous shells of Diatoms are laid down in various habitats to form thick deposits. This large accumulation of Diatom shells is known as diatomaceous earth. The earliest reported diatomaceous deposit is during late Mesozoic (Cretaceous). But immensely thick deposits of diatomaceous earth have been found to be associated with rocks of Tertiary age.
In the Diatoms, the Centrales are older than the Pennales for having only a few living representatives.
The brown algae which probably had their origin in the early Proterozoic can very well be traced in many geological ages. One of the fossil types of Phaeophyta that is most striking of the Silurian and Devonian in Prototaxites.
Though some red algae are traceable in the later Archeozoic actually their fossil forms are found in the early Cambrian. Certain fossil forms of Rhodophyta have resemblance with the living species with regard to external appearance. It is quite probable that highly organized red algae have attained their maximum development in the middle Cambrian.
Walcott named these Cambrian algae as:
Waputika, Dalvia, Wahpia and Bosivorthia and compared them with the corresponding living forms, such as, Dasya, Ceramium and Griffithsia. Limestone rocks consisting largely of Lithothamnion are well known from various parts of the world being predominant in Miocene. Besides this, Solenopora of Ordovician-Jurassic is an allied genus.
Essay # 18. Algae Causing Biological Disturbances:
Nowhere can there be found more striking examples of interactions between and, interdependence of organisms in nature than in the ramifying effects produced by algal populations. The roles which these plants play in aquatic biology and in limnology are clearly recognized but often incompletely understood.
Whereas the place of algae in nature may involve some benefits to man and to other organisms, some of the effects produced are radically offensively disturbing, sometimes costly and even lethal. The putrefying masses of algae in lakes and in water-supply reservoirs are aesthetically disturbing and economically aggravating.
But of greater importance are the indirect and far-reaching effects algae have on other aquatic organisms, including their ability to kill.
It is from populations of algae in standing water, of course, that effects are most obvious and most disturbing because of the concentration of numbers of individuals. It is well known that of the countless species of fresh-water algae, only a few produce disturbances which attract our attention.
This may be exemplified by the common ‘water bloom’. Some of the so-called water bloom producing species of algae are: Microcystis aeruginosa, M. toxica, Oscillatoria rubescens, O, lacustris, Anabaena circinalis, and A. flosaquae.
The disturbances attributable to algae are both directly and indirectly produced. The direct effects are those arising from substances given off by certain objectionable algae, or through their physiology wherein nutrients necessary for the growth of less offensive forms are taken up; clogging of fish gills because of their concentration in shallow water; producing disagreeable tastes and odours in drinking water, etc.
Indirect effects arise as a result of a chain reaction in modification of ecological conditions leading oxygen depletion, and formation of toxic substances.
Most of the disturbances of algae are related directly to their physiology, it may be noted in the instances of cyanophycean trouble-makers especially, that their effects are related to morphology and to their habits. For example, the sticky sheath possessed by blue-green algae makes it possible for plants to adhere to one another, thus forming dense mats.
Many blue-green algae, especially the trouble-makers, possess pseudovacuoles which permit or force the plants to float high in the water. Because of their tremendous numbers, the mat is actually elevated above the water-level. Thus, in intense illumination and in warm water layers, a decaying blanket of blue-green algae forms.
The indirect effects produced by dense algal populations is found in fish deaths and in human poisonings from eating fish. Because blue-green algae especially are high in proteins, when they decompose proteinaceous by-products are formed, some of which are poisonous.
It has been demonstrated that when algal blooms are dense in shallow water, the decomposition products may be concentrated enough to kill fish.
Another indirect effect chargeable to algae is that offish-kills by oxygen depletion. As the oxygen decreases below the threshold necessary for them, various groups of organisms including fish are suffocated to death.
The incompatibility of two algal species and the inhibition of growth of one plant in presence of another are often due to the antibiotic substances that are produced by algae and are liberated out in water in which they survive. Chlorellin from Chlorella vulgaris is a well-known antibiotic which inhibits the growth of other algae.
The antibiotics may even have inhibitory effects on fish. Pandorine from Pandorina morum produces marked effects and abnormal shapes in Scenedesmus. Both Microcystis and Chlorella secrete substances active against Closterium.
A serious disease of sheep is reported from Africa, Canada, and the United States. Death from blue-green algal poisoning had experienced lung haemorrhages and liver lesions. Where horses, cattle, and dogs were killed by drinking water infested with Microcystis aeruginosa and Anabaena lemmermannii.
The blue-green alga Microcystis toxica produces ‘soupy’ blooms in the shallow pans and reservoirs. Cattle drinking these waters died by the thousands in Central South Africa.
Essay # 19. Reproduction in Algae:
All three processes of reproduction—vegetative, asexual, and sexual are encountered in Algae.
The vegetative reproduction takes place by:
i. Simple Cell Division:
This method is usually common in unicellular algae (Fig. 10A). In multicellular forms cell division leads to growth. Growth of individual filament is usually intercalary, but in a few cases there is a definite apical cell growth.
This process is common in the filamentous forms. Fragmentation of the filaments may be: (i) accidental (Fig. 10F); (ii) by the development of a special structure—separation disc (Fig. 21B & C), which facilitates the breaking down of the filaments into small fragments, or (iii) otherwise, resulting in the formation of multicellular fragments which grow into new filaments by the division of cells of the fragments.
The process of splitting of mature colonies into small parts that takes place in many colonial forms may also be included under this category.
A large section of Algae reproduce asexually by the development of various kinds of spores and similar other structures.
They may be:
The vegetative cells of certain filamentous algae become elongated, thick-walled, and develop into structures known as akinetes (Fig. 10B). The akinetes can survive unfavourable conditions. With the return of favourable conditions they germinate into new individuals.
These are thick-walled non-flagellate spores with abundant food reserve. They can stand unfavourable condition.
Fragments of some filamentous algae develop thick sheath, tide over unfavourable conditions, become functional and give rise to new filaments when conditions are favourable. These organs of perennation are also known as hormocysts (Fig. 10G).
In some unicellular algae the entire protoplast of the vegetative cell divides to form internally a number of non-flagellate thin-walled spores known as endospores which develop into new individuals (Fig. 10D).
These are thin-walled non-flagellate spores that are cut out externally from the protoplast of the vegetative cell of certain unicellular algae (Fig. 10E-E’).
Certain vegetative cells of some filamentous algae become larger in size than the neighbouring eel’s, wall somewhat thickened being composed of two layers—outer of pectic substance and inner of cellulose, contents, turn transparent and are transformed into heterocysts.
The heterocysts develop button-like thickenings at the poles known as polar nodules, whose number, depending on the position of the heterocyst, may be one or two (Fig. 10B).
A mature heterocyst is pale yellow due to the absence of photosynthetic pigments. During heterocyst formation a very young vegetative cell which has lost its capacity of division swells and becomes hyaline by the loss of photosynthetic pigments. The inner cellulose layer is added simultaneously keeping gap along the pores at the poles. By the pores the heterocyst retains connection with neighbouring cells.
As the heterocyst matures these pores are plugged off with mucilage-forming polar nodules. Intensive enzymatic, structural, and functional changes take place during heterocyst formation. Functions of heterocysts are not clearly known. They are considered by some as dead cells serving the purpose of fragmentation of the filaments.
Others are of opinion that they are spores. In certain species of algae, the heterocysts germinate into new individuals. Heterocyst’s are also considered as receptacles of enzymes which help formation of akinete. They may also take part in nitrogen fixation.
These are flagellate spores which may be formed either in a specialized structure, zoosporangium or directly from the vegetative cells. The zoospores may be bi- (Fig. 11A to G), quadri- (Fig 11D), or multiflagellate (Fig. 11E to F).
The multi-flagellate zoospores may again be of two kinds:
(i) flagella distributed throughout the entire body (Fig. 11 F)
(ii) flagella arranged in a ring surrounding a beak-like projection (Fig. 11E).
Each zoospore swarms around in water, comes to rest, sheds its flagella and germinates into new individual.
These are non-flagellate spores that are formed in some aquatic algae by the failure of the development of flagella during zoospore formation due to certain unfavourable condition in the substratum (Fig. 10G). The aplanospores may also be formed in certain algae of semiaquatic habitat.
When they appear identical to the parent cell they are referred to as autospores. Aplanospores with thickened walls and abundant food reserve are also called the hypnospores.
The occurrence of sexual reproduction probably marks a relatively advanced stage of evolution among Algae and is very rare in all the types that are regarded as of relatively less advanced. There is a wide range of variations in the nature of the gametes and the mode of sexual reproduction.
Any vegetative cell of the plant body may produce gamete and thus behave as a gametangium or a specialized gametangium may be developed in which gametes are produced.
The gametangia may be morphologically similar, isogametangia or dissimilar, heterogametangia. In hetero- gametangia, the smaller one is the male gametangium or antheridium (pi. antheridia) (Fig. 12M) which produces one or more male gametes which again may be flagellate or non-flagellate.
The flagellate male gametes are the antherozoids or spermatozoids (Fig. 12O). The larger gametangium is the female gametangium or oogonium (pi. oogonia) (Fig. 12N), also known as carpogonium (pi. carpogonia) which produces non-flagellate female gamete. Such a non-flagellate female gamete is known as ovum (pi. ova) or oosphere or egg (Fig. 12O).
The number of female gamete in a female gametangium is usually one, may also be more than one, but restricted in number. In general, gametes may be morphologically similar in appearance, isogamous or dissimilar, anisogamous or heterogamous. They may be flagellate, planogamous or non-flagellate, aplanogamous.
The process of fusion between the two isogametes which may be planogamous (Fig. 12 A-E) or aplanogamous (Fig. 12F-H) is known as isogamy or conjugation and that between the two dissimilar planogametes (Fig. 12 I-L) or the smaller one planogamete and the larger one aplanogamete is anisogamy or heterogamy.
The term conjugation is also applied to the process of fusion between two isogametangia: In heterogamy, when the smaller gamete is flagellate or non-flagellate and the larger one is non-flagellate, enclosed, and retained either permanently or up to maturity within the oogonium, the process of fusion between these two gametes is known as oogamy or fertilization.
During fertilization, the female gamete remains confined within the oogonium either permanently or until maturity or after being liberated out remains attached with some part of the oogonium. The antherozoids, when mature, are liberated out from the antheridium, swim around in water and enter the oogonium. But only one unites with the ovum. The fusion product of the two gametes, in general, is called a zygote.
The zygote bears a nucleus with two chromosome complements (2n), is said to be diploid (Gk. diploos, double), one chromosome complement being contributed by each gametic nucleus. The gametic nucleus which possesses one chromosome complement (n), is designated as haploid (Gk. haploos, simple). The resulting zygote of isogamy is known as zygospore, and that of oogamy is called oospore.
No new individual is produced so long the zygote remains enclosed within the oogonium. Hence, the zygote itself is liberated out of the parent individual and either directly develops into a new individual or the zygotic nucleus undergoes meiosis resulting in the development of spores from which new individuals are developed.
All these three kinds of sexual reproduction—isogamy, anisogamy, and oogamy are encountered in Algae. The details of post-sexual stages exhibited by the members of the Algae are indicated below:
I. The diploid zygotic nucleus divides reductionally immediately after its-formation resulting in the production of four haploid nuclei—zygotic meiosis.
Further development may follow any one of the following ways:
(i) Of the four haploid nuclei, three abort and one survives to develop into a new haploid individual resembling the parent plant (Fig. 13).
(ii) All the four haploid nuclei develop into flagellate spores—zoospores, which germinate into new individuals (Fig. 14).
II. The zygote germinates directly without undergoing any meiotic division of the diploid zygotic nucleus. The subsequent stages may be:
(ii) The zygote directly produces an elaborately developed diploid’ individual which bears gametangia. The haploid stage is very short being represented by the gametes only, which are again formed by meiosis just before the sexual reproduction—gametic meiosis (Fig. 15).
(ii) Due to delay in the reduction division of the diploid nucleus there is development of a new diploid phase—the sporophytic generation or the sporophyte which occurs regularly alternating with the haploid gamete producing phase—the gametophytic generation or the gametophyte.
The sporophyte is the spore producing phase in which spores are produced by meiosis, sporic meiosis (Fig. 16). To complete their life cycle these algae pass through two phases of growth—the gametophyte and the sporophyte occurring alternately in regular sequence. This is known as alternation of generations (Fig. 17).
There are cases where during sexual reproduction gametic union may fail to take place. In such cases the gametes develop directly into spore-like structure, parthenospores, also known as zygospores (Fig. 75 G & H). The process by which the parthenospores are formed is known as parthenogenesis. These parthenospores behave like zygospores without any meiotic division of the parthenospore nucleus.
Essay # 20. Review of Algological Studies (Algae) in Abroad:
The history of Algae is as old as that of botany. The first writings on Algae appeared in the ancient Chinese classics, where references to the collecting, eating, and to the medicinal uses of algae are found. There are also references in Roman and Greek literatures the use of algae for cosmetic purposes.
Algae have been given many designations by different people. The Chinese named them as Tsao for having esthetic significance, whereas the Howaiians called Limn as they used algae as a food. References of several kinds of Tsao are available in the eighth century writings. It is interesting to note that the early century writings about Algae were restricted either to their use or else to their taxonomy.
In the north coast of France, algae were being used for manual purposes as early as twelfth century. This practice spread to Great Britain, as mentioned in the sixteenth century reference. The use of brown seaweeds for fertilizers climaxed in France during the seventeenth century.
The utilization of certain algae for agar making introduced in China, slowly spread in Japan later on turned out to be a great industry which has its foundation in the seventeenth century.