Classification of Algae

in this post you will learn classification of this diverse group. Classification means grouping of organisms according to the similarity in their characters. it is not far Fetched but true that organisms showing similar morphology, life cycle, physiology and biochemistry are genetically i-elated from the evolutionary point of view (phylogenetically related) and one is justified in grouping them together.

it was indicated that algae could be classified according to their common characters into 8 divisions of Kingdom Protista. The relationship among different groups ‘vas also discussed. You may recall that blue-green algae have been grouped as Division Cyanobacteria, and clubbed with bacteria tinder the Kingdom Monera. In this unit you are introduced to the characteristics of different divisions of algae.

Criteria for Classification of Algae

the criteria used by phycologists are quite varied. Generally a number of characters are used together ranging from external morphology, ultrastructure, chromosome number and their morphology, pigment composition, nature of cellular storage products, enzymes. isoenzymes, DNA homology, and DNA banding etc. As new techniques are developed they are used to decide more precisely the relatedness (or absence of it) of organisms which seem otherwise related to each other. Given below are the salient characters of each of the divisions of the algae. it is to be noted that each division is again divided into orders, families, genera and species. you will find the classification of all the algae which are included in your study. Please note that they represent certain divisions, orders, and families only. Because of the restriction of time representatives of other divisions are not included in your course, not because they are any less important in the biological world.

Prokaryotic Algae 

Division CYANOPHYTA 

(Cyanobacteria or Blue-green algae) Prokaryotic algae are placed in Division Cyanophyta. Algae of this division may be unicellular, colonial) and filamentous, with or without branches, branching may be ‘true’ or ‘false’ type. Most forms are embedded in mucilaginous or gelatinous sheaths.

The composition of cell wall is similar to bacterial cell wall. It is, made up of distinctive mucopeptide and muramic acid.

The ultrastructure of the cell shows no organised nucleus, mitochondria or chloroplasts, Photosynthetic lamellae and ribosomes of 70s type are present in the cytoplasm of the cells. Some filamentous forms possess specialised cells termed as ‘heterocyst’, which are involved in nitrogen fixation.

The main photosynthetic pigments are chlorophyll a and phycobilins - (phycocyanin and phycoerythrin). A number of carotenoids including 10-carotene are also present, some of which are specific to the division.

Carbon is reserved in the cells as glycogen granules and nitrogen as cyanophycean granules. Other granules like polyphosphate granules, some enzyme aggregates like carboxysomes may also be present.

Reproduction occurs by simple cell division. No motile cells are found in cyanobacteria and they do not have sexual method of reproduction. Thick walled cells called ‘alkinets’ or spores are present in some forms for perennation and asexual reproduction.

Cyanobacteria are distributed all over the earth in diverse habitats, fresh water lakes, ponds, rivers, arctic, Antarctic areas, lint water springs, brine salt pans, desert soils, subaerial surfaces like tree trunks, building terraces and rock surfaces.

Examples: Anacystis, Microcystis, Nostoc, Anabaena, Oscillatoria, Spiral ma, Calothrix, tolypo thrix, Gloeotrichia, Lyngbya, Scytonema. and Stigonerna.

Eukaryotic Algae

As you have learnt earlier, that Kingdom Protista includes eight divisions of algae. Some phycologists make nine divisions treating bacillariophyta separate from Chrysophyta. You may note that we have also taken it as a Separate division. In the following account (hey are described in detail below.

Division CHLOROPHYTA (Green algae) 

This incudes unicellular (o multicellular forms of green algae. The multicellular forms may be in the form of filamentous. branched or unbranched, thailioid. tubular or sheet like arrangement of cells. Some of the green algae are colonial in form. The cell structure is eukaryotic type as its higher plants with membrane bound organelles. Nucleus. Plastids. mitochondria. and cytoplasmic ribosomes of 80s type. The cell wall is generally made up of cellulose. Sometimes the cells arc also covered wills chitin.

The principal photosynthetic pigments are chlorophyll a and b. carotenes and xanthophylls located in the thylakoids.

The storage products of his cell are mostly starch, but in some algae lipids. Reproduction occurs by asexual and sexual methods. Asexual reproduction is by biflagellate or quadri-flagellate zoospores whereas gametes (sexual reproduction) are biflagellate. The flagella are anterior and of whiplash type. Sexual reproduction includes isogamy, anisogamy. And oogamy. Green algae are distributed in fresh water and marine habitats; some nay be subaerial on wet soil or bark of trees. Examples: Chlorella, Chlamydomonas, Pediastrum Spirogyra, cladophora, acetabularia, Trenrepliolilia, Micrasteríos and Caulerpa.

Division PHACOPHYTA (Brown algae) 

Structurally they are most complex in morphology. They range from simple branched filaments to massive bodies.

Cell wall composition is complex. Besides cellulose, it many contain algin, fucoid in Principal photosynthetic pigments are chlorophyll a and e and carotenoids. Fucoxanthin (brown in color) is present in large amount [hat gives alga brown colour by masking Elle green colour of chlorophyll.

Photosynthetic storage product is mannitol. sometimes laminarin. Rarely, lipid droplets may be round in the cells.

Sexual reproduction ranges from isogamy to oogamy. The motile swarmer have two unequal laterally inserted flagella, one of the flagella is larger and anterior and the oilier is smaller and posterior.

Most of the brown algae are seaweed, very large in size, commonly known as kelps. They are the main source of iodine. agar and related products.

Examples: Eciocatpus, Fucus, laminaría. Sargassum. Dictyota, Alrea, macrocystis, Nereocystis and Padina.

Division RHODOPHYTA (Red algae) 

Most forms are multicellular and highly branched a few are thalloid and one alga porphyridium is unicellular. The body may be covered with calcium carbonate incrustation.

Besides cellulose their cell all contains pectin, polysulphate. esters and large amount of polysaccharides on the outside of their surface. These polysaccharides are the source of agar arid carrageenans. Ceilain red algae for example coralline algae secrete calcium carbonate around their cell and form stiff thalli, The main photosynthetic pigments are chlorophyll a and phycoerythrin. Some red algae contain phycocyanin also. The algae appear red or pink in colour because of large amounts of phycoerythrin

The food reserve in the cells is floridian starch.

No motile cells are round at any stage of reproduction. Sexual reproduction is advanced oogamous type. Male gametes — spermatia are passively transported by ‘valer movements lo the tip of trichogyne of the female carpogonium. After fertilisation, special developmental changes occur, that are not found in any other division of the algae.

Most of the red algae are marine in habitat. A few are found iii fresh water lakes, rivers, streams and ponds. Some are epiphytic or parasitic in nature.

Example: Porphyridum (unicellular), Porphyra, Polysiphonia, Gracilaria, Gelidiun, and Corallina.

Division XANTHOPHYTA (Yellow-green algae) 

Some forms are unicellular and motile while others are filamentous, with multinucleate cells. . Photosynthetic pigments are chlorophyll a, e, 13-carotene which is present in large amount, and xanthophylls giving the cells greenish—yellow colour.

Food reserves include lipid and chrysolaminarin (13-13 - linked polymer glucose. also known as leucosin).

Cell wall frequently consists of two overlapping halves, containing pectin, silica and small amount of cellulose.

Sexual reproduction is rare. The motile cells have two unequal flagella present on the anterior end; one is tinsel and the other whiplash type.

Yellow-green algae are widely distributed in aquatic, fresh water habitats. Some are sub-aerial and a few are marine in distribution.

Example: Vancheria Botrydium

Division CHRYSOPHYTA (Golden brown algae) 

Mostly unicellular or colonial, filamentous forms are rare.

Motile cells have two equal or unequal flagella present on the anterior end. The longer one has stiff hairs and the shorter is smooth. The cell ‘vail is made of pectin and silica or scales of carbonate, the chloroplasts are deeply lobed. Principal pigments are chlorophyll a, e. and carotenoids like B-carotene, fucoxanthin, diatoxanthin and fucoxanthin.

Storage products are mostly oil droplets, and true starch is absent but glucan granules or leucosin are present.

Sexual reproduction is raie. Most common features are the formation of resting cysts, resting spore (slatspores), with silica walls. The cysts are formed as a result of asexual or sexual reproduction.

Golden-brown algae are distributed in marine and fresh water habitats, and in fast flowing mountain streams. Marine coccolithophorids are responsible for the formation of chalk beds on tIte bottom of the sea.

Example: synura, Chromlina, Ochromonas, Mallomonas, and Dinobryon.

division EUCLENOPHYTA (Euglenoids) 

Most of the euglenoids are simple unicellular motile flagellates. They have no firm cell wall. and possess characteristics like protozoans. They have a contractile vacuole. Cell surface is pellicle (thin membrane) and has helicaL knob like projections. Cell shape changes constantly (euglenoid-movements). Chloroplasts show variety of shapes such as discoid. Ribbon like or stellate. Cells are biflagellate but only one flagellum emerges anteriorly.

The photosynthetic pigment located in the plastids include chlorophyll a, b and carotenoids including 3-carotene. Sonic euglenoids are also colourless.

A form of starch-paramiylon is present as distinct granules. Oil droplets and polyphosphate gratiu les are also common in the cells. Cells divide by binary fission. Many species produce cysts under adverse conditions. - Sexual reproduction is absent.

Euglenoids occur in fresh water and brackish water and very commonly in polluted ponds and temporary rain water pools. Examples: Euglena, Trachelomonas, Phacus.

Division DINOPHYTA (Dinoflagellates) 

Cell wall consists of cellulose plates which are inside the plasma membrane. A number of plates or body scales may be present on the cell wall. Cell structure is complex. Majority of forms are unicellular and motile. Many dinoflagellates such as noctifuca, are luminescent. They glow iii the dark when they are disturbed. Most of these algae contain chlorophyll. a and c and distinctive carotenoid specific to dinoflagellates.

Reserve foods we mostly in the form of starch and oil.

Asexual method of reproduction is by cell division. Parent cell divides into a number of aplanospores or zoospores or non-motile cells. Sexual reproduction has been recently reported, gametes are smaller than the vegetative cells and the fusion is isogamous. Formation of eysts with or without gamsetic fusion is also found.

Division CRYPTOPHYTA (Crytomonads) 

Unicellular motile organisms, when alive they are brown in colour. Several genera are animal like in morphology and mode of nutrition, some are colourless and saprophytic in nature. Cells are without cell wall ovoid and dorsiventrally flattened. The Iwo flagella are apical and unequal in length. The chloroplasts may be single or many in a cell. in some crytomonads there are two, large parietal chloroplast, or many disc like ones.

Pigments include chlorophyll a, c, phycocyanin, phycoerythrin,and diverse carotenoids.

Reserve photosynthate is starch.

Reproduction is by longitudinal division of the cell. Palmelloid forms may produce zoospores. Sexual reproduction has not been reported so far. Examples : Cryptomonas. Chroonioncrs,

Division BACILLRIOPHYTA (Diatoms) 

Mostly unicellular forms, some are colonial and filamentous in structure. Cell is wall is silicified, consisting of two perforated overlapping plates. It is highly ornamented on the surface. Chromophores are brownish in colour due to large amounts of carotenoids.

Photosynthetic pigments are chlorophyll a and c, fucoxanthin, diatoxanthin and diadinoxanthin. Reproduction occurs by vegetative and sexual methods. Diatom cells unlike other algae are diploid in nature. Sexual Fusion is homothallic, within the individuals of the same clone. Two amoeboid gametes fuse to form a zygote which develops into an auxospore. Fusion may be isogamous, anisogamous or oogamous type.

Diatoms are widely distributed in fresh water and sea as planktons, on mud surfaces, moist locks, and sand. They may even be epiphytic, epizod or endozoid.

Large deposits of fossil diatom shells known as diatomaceous earth are mined and used in various industries. At the end we “mild like to point out that classification of algae is tentative and can he improved by using new and advanced techniques like DNA fingerprinting etc, which can clarify the genetic relatedness of organisms.

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