Classification and Different Criteria for Classification of Algae

November 27, 2023 Agric4Profits

Classification means grouping 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 related from the evolutionary point of view (phylogenetically related) and one is justified in grouping them.

Algae could be classified according to their common characters into 8 divisions of’ Kingdom Protista.

Criteria for Classification of Algae

The criteria used by phycologists are quite varied. Generally, several 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, DNA banding, etc.

As new techniques are developed they are used to decide more precisely the relatedness (or absence of it) of organisms that 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. 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.

Classification and Different Criteria for Classification of Algae

1. Prokaryotic Algae

i. 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 the cell wall is similar to the bacterial cell wall. It is, made up of distinctive mucopeptides and muramic acid.

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

The main photosynthetic pigments are chlorophyll a and phycobilins – (phycocyanin and phycoerythrin). Several carotenoids including β carotene are also present some of which are specific to the division. Carbon is reserved in the cells as glycogen granules and nitrogen as cyanophyte granules.

Other granules like polyphosphate granules, and 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 a sexual method of reproduction. Thick-walled cells called ‘akinetes’ or spores are present in some forms for perennation and asexual reproduction.

Cyanobacteria is distributed all over the earth in diverse habitats, freshwater lake ponds, rivers, arctic, and antarctic areas, hot water springs, brine salt pans, desert soils, subaerial surfaces like tree trunks, building terraces, and rock surfaces.

Examples: Anacystis, Microcystis, Nostoc, Anabaena, Oscillatoria, Spirulina, Calothrix, Gleotrichia, and Scytonema.

2. Eukaryotic Algae

As you have learned earlier, the Kingdom Protista includes eight divisions of some phycologists make nine divisions treating Bacillariophyta separate from Chrysophyta.

i. Division CHLOROPHYTA (Green Algae)

This includes unicellular to multicellular forms of green algae. The multicell forms may be in the form of filamentous, branched or unbranched, thalloid, or sheet-like arrangement of cells.

Some of the green algae are colonial in form, cell structure is eukaryotic type as in higher plants with membrane-bound organized nucleus, plastids, mitochondria, and cytoplasmic ribosomes of 80s type.

The cell wall is generally made up of cellulose. Sometimes the cells are also with chitin.

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

The storage products of the cell are mostly starch but in some algae lipids.

Reproduction occurs through asexual and sexual methods. Asexual reproduction is biflagellate or quadri-flagellate zoospores whereas gametes (sexual reproductive biflagellate). The flagella are anterior and of whiplash-type. Sexual reproduction includes isogamy, anisogamy, and oogamy.

Green algae are distributed in freshwater and marine habitats; some may be subaerial on wet soil or the bark of trees.

Examples: Chlorella, Chlamydomonas, Pediastrum, Spirogyra, Cladophora, Acelabularia, Trentephohlia, Micrasterias, and Caulerpa.

ii. Division PHAEOPHYTA (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 may contain algin, fucoidan

Principal photosynthetic pigments are chlorophyll a and c and carotenoids. Fucoxanthin (brown) is present in a large amount that gives alga brown color by masking the green color of chlorophyll. Photosynthetic storage product is mannitol, sometimes laminarin. Rarely, lipid droplets may be found in the cells.

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

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

Examples: Ectocarpus, Fucus, Laminaria, Sargassum, Dictyota, Alaria, Macrocystis, Nereocystis, and Padina.

iii. Division RHODOPHYTA (Red Algae)

Most forms are multicellular and highly branched, a few are haploid and one alga Porphyridium is unicellular. The body may be covered with calcium carbonate incrustations.

Besides cellulose, their cell wall contains pectin, polysulphate, esters, and a large number of polysaccharides on the outside of their surface. These polysaccharides are the source of agar and carrageenans. Certain red algae for example coralline algae secrete calcium carbonate around their cells and form stiff thalli.

Caroline algae are a group of red algae that secrete calcium carbonate around their cells and form still thalli. Caroline algae are important builders of coral reefs in tropical water, contrary to the belief that coral animals alone make up oral reefs.

The main photosynthetic pigments are chlorophyll a, d and phycoerythrin. Some red algae contain phycocyanin also. The algae appear red or pink because of large amounts of phycoerythrin. The food reserve in the cells is Floridian starch.

No motile cells are found at any stage of reproduction. Sexual reproduction is an advanced oogamous type. Male gametes spermatia are passively transported by water movements to 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 in fresh water lakes, rivers, streams and ponds. Some are epiphytic or parasitic in nature.

Examples: Porphyridium (unicellular), Porphyra, Polysiplonia, Gracilaria, Gelidium and Corallina.

iv. Division XANTHOPHYTA (Yellow-green algae)

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

Food reserves include lipid and chrysolaminarin (β-1,3 – linked polymer of 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.

Examples: Vaucheria, Botrydium

v. 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 wall is made of pentin and silica or scales of carbonate. The chloroplasts are deeply lobed.

Principal pigments are chlorophyll a, c, and carotenoids like β-carotene, fucoxanthin, diatoxanthin and neofucoxanthin. Storage products are mostly oil droplets, and true starch is absent but glucan granules or leucosin are present.

Sexual reproduction is rare. Most common features are the formation of resting cysts, resting spore (statospores), 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 coccolithophorides are responsible for the formation of chalk beds on the bottom of the sea.

Examples: Synura, Chromulina, Ochromonus, Mallomonas, and Dinobryon.

vi. Division EUGLENOPHYTA (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.

Members of some algal divisions such as the englenoids, cryplophytes dinolligellates, chrysophytes are predominantly unicellular. Some biologists consider these organisms to be more related to the animal kingdom and classify them under protozoa.

The photosynthetic pigments located in the plastids include chlorophyll a, b and carotenoids including β-carotene. Some euglenoids are also colourless. A form of starch-paramylon is present as distinct granules. Oil droplets and polyphosphate granules 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 polluted ponds and temporary rain water pools.

Examples: Euglena, Trachelomonas, Phacus.

vii. Division DINOPHYTA (Dinollagellates)

Cell wall consists of cellulose plates which are inside the plasma membrane and 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 e.g Noctiluca, are luminescent. They glow in the dark when they are disturbed.

Most of these algae contain chlorophyll, a and c and distinctive carotenoid specific to dinoflagellates. Reserve foods are 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 cysts with or without gametic fusion it found.

Dinoflagellates are mostly found as marine phytoplankton, sometimes as ‘redtide’ blooms. Many occur as symbionts in marine animals like corals (zooxanthellae).

Examples: Noctiluca, Gonyaulax, Peridinium, Ceratium.

viii. 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 colorless and saprophytic in nature.

Cells are without cell wall ovoid and dorsiventrally flattened. The two flagella are apical and unequal in length. The chloroplasts may be single or many in a cell. In some cryptomonads there are two, large parietal choroplasts, 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: Crytomonas, Chroomonas.

ix. Division BACILLAR1OPHYTA (Diatom)

Mostly unicellular forms, some are colonial and filamentous in structure. Cell wall is silicified, consisting of two perforated overlapping plates. It is highly ornamented on the surface.

Chromatophores are brownish in colour due to large amount of carotenoids. Diatoms (cut in half) each cell is made up of two parts. The larger part fitting tighty over the slightly smaller part like a petridish.

Photosynthetic pigments are chlorophyll a and c, fucoxanthin, diatoxanthin and diadinoxanthin. Common storage product is oil and chrysolaminarin. 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 amoehoid 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 rocks, and sand. They may even be epiphytic, epizoid or endozoid. Large deposits of fossil diatom shells known as diatomaceous earth are mined and used in various industries.

Examples: Navicula, Cymbella, Coscinodiscus, Diatoma and Fragilaria.

At the end it has to be pointed out that classification of algae is tentative and can be improved by using new and advanced techniques like DNA fingerprinting which can clarify the genetic relatedness of organisms.

In summary, Algae could be classified according to their common characters into 8 divisions of Kingdom Protista. The relationship among different groups and differences was also discussed.

Algae have been grouped into two major types: prokaryotes and eukaryotes because of the basic differences in the ultrastructure of the cells.

Cyanobacteria or blue-green algae although related to bacteria, are grouped with oilier algae because of the similarity in pigment composition and presence of oxygenic photosynthesis.

Eukaryotic algae can be classified into 9 divisions each sharing a large number of common characters. All photosynthetic algae have chlorophyll a and β-carotene, but other pigments may vary.

Three divisions Cyanophyta, Rhodophyta and Cryptophyta have similar phycobilin pigments blue phycocyanin, and red phycoerythrin, otherwise they are unrelated in any of the other characters.

Green algae (Division Chlorophyta) are unicellular, colonial and filamentous in forms, motile and free floating. The photosynthetic pigments are chlorophyll a, b, β-carotene and xanthophylls. Food is stored as starch. Though euglenoids also contain chlorophyll a and b, but they are different from green algae.

Brown algae (Division Phaeophyta) are mostly marine, large, complex usually multicellular and non-motile. The chlorophylls are masked by brown pigment fucoxanthin. Food is stored as oil and complex carbohydrate-laminarin. The zoospores and gametes are motile.

Red algae (Division Rhodophyta) are marine, multicellular and filamentous. The chlorophylls are masked by phycobilins. Food is stored as floredian starch. There are no motile cells in the life cycle of the algae.

Members of Xanthophyta, Chrysophyta, Dinophyta and Cryptophyta are mostly unicellular. They contain chlorophyll a and c and are collectively called chromophytes.

In Xanthophyta, Chrysophyta, Dinophyta the cell wall is made either of cellulose or is absent. In Euglenophyta and Cryptophyta cell wall is absent.

Read Also: How to Care for a Birch Tree