Classification of algae, Biology tutorial


Algae are differentiated from fungi, protozoans and animals by their capability to prepare their own food via photosynthesis and are differentiated from plants by their relative simplicity of structure.  All algae encompass the green pigment chlorophyll and the organelles chloroplasts, both of which are necessary for the photosynthesis.

Classification signifies grouping of organisms according to the similarity in their features. It is not farfetched however true that organism exhibiting identical morphology, life cycle, physiology and biochemistry are genetically associated from the evolutionary view-point (phylogenetically associated) and one is validated in grouping them altogether.

Criteria for classification of Algae:

The criteria employed by phycologists are quite diverse. Usually a number of characters are employed altogether ranging from the external morphology, ultra structure, chromosome number and their morphology, nature of cellular storage products, pigment composition, enzymes, DNA homology & DNA banding and so on. As new methods are build up they are utilized to decide more accurately the relatedness (or absence of it) of organisms that seem or else associated to one other.

Algae can be classified as Prokaryotic Algae and Eukaryotic Algae.

1) Prokaryotic Algae (Cyanophyta)

2) Eukaryotic Algae (Chlorophyta, Phaeophyta, Rhodophyta, Xanthophyta, Chrysophyta, Euglenophyta, Dinophyta and Bacillariophyta).


Division Cyanophyta (Cyanobacteria or Blue-green algae):

Prokaryotic algae are positioned in Division Cyanophyta. Algae of this division might be unicellular, colonial and filamentous, with or without branches, branching might be true or false kind. Most of the forms are embedded in the mucilaginous or gelatinous sheaths.

The composition of cell wall is identical to bacterial cell-wall. It is build up of individual mucopeptides and muramic acid.

The ultra structure of the cell exhibits no organized nucleus, mitochondria and chloroplasts. Photosynthetic lamellae and ribosomes of 70s kind are present in the cytoplasm of the cells.

The major photosynthetic pigments are chlorophyll a and phycobilins. A number of carotenoids comprising β-carotene are as well present some of which are particular to the division. Carbon is reserved in the cells as glycogen granules and nitrogen as the cyanophycear granules. Some other granules such as polyphosphate granules and some enzyme such as carboxysomes might as well be present.

Reproduction takes place by means of simple cell division. No motile cells are found in the Cyanobacteria and they don't encompass sexual process of reproduction

Cynobacteria are spread all over the earth in diverse habitats, fresh-water, rivers, ponds, lakes, arctic, Antarctic areas, brine salt pans, hot water springs, desert soils, sub aerial surfaces such as tree trunks, building terraces and rock surfaces.

Examples:  Microcystis, Nostoc, Anacystis, Anabaena, Oscillatoria, Spirulina, Gleotrichia etc.


Division Chlorophyta (Green algae):

This comprises unicellular to multicellular forms of green algae. The multicellular forms might be in the form of filamentous, branched or un-branched, thalloid, or sheet like display of cells. A few of the green algae are colonial in form; cell structure is eukaryotic kind as in the higher plants having membrane bound organized nucleus, plastids, mitochondria and cytoplasmic ribosomes of 80s kind.

The cell wall is usually built up of cellulose. At times the cells are as well with chitin. The main photosynthetic pigments are chlorophyll a and b, carotenes xanthophylls positioned in the thylakoids.

The storage products of the cell are generally starch, however in some algae lipids.

Reproduction takes place through asexual and sexual methods. Asexual reproduction is biflagellate or quadri1flagellate zoospores while gametes is sexual reproductive biflagellate. Sexual reproduction comprises isogamy, anisogamy and oogamy.

The Green algae are spread in fresh-water and marine habitats; some might be sub aerial on wet soil and bark of trees.

Examples:  Chlorella, Chlamydomonas, Spirogyra, Pediastrum, Cladophora and so on.

Division Phaeophyta (Brown algae);

Structurally, they are most complicated in morphology. They range from simple branched filaments to huge bodies.

Main photosynthetic pigments are chlorophyll a and c and carotenoids. Fucoxanthin (brown in color) is there in huge amount which provides alga brown color by masking the green color of chlorophyll. Photosynthetic storage product is mannitol, at times laminarin.

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

Examples:  Fucus, Ectocarpus, Sargassum, Laminaria, Dictyota and so on.

Division Rhodophyta (Red algae):

Most of these forms are multicellular and highly branched; a few are thalloid and one alga Porphyridium is unicellular. The body might be covered by calcium carbonate incrustations.

Besides cellulose their cell-wall includes pectin, polysulphate, esters and big amount of polysaccharides on the exterior of their surface. Such polysaccharides are the source of carageenans and agar.

The main photosynthetic pigments are chlorophyll a, d and phycoerythrin. A few red algae have phycocyanin as well. The algae emerges red or pink in color because of huge amounts of phycoerythrin. The food preserve in the cells is Floridian starch.

No motile cells are obtained at any phase of reproduction. Sexual reproduction is advanced oogamous kind. Male gametes spermatia are inertly transported through water movements to the tip of trichogyne of the female carpogonium. After fertilization, special developmental modifications take place, which are not found in any other division of the algae.

Example:  Porphyridium, Porphyra, Gracilaria, Polysiplonia, Gelidium and so on.

Division Xanthophyta (Yellow-green algae):

Some of the forms are unicellular and motile whereas others are filamentous having multinucleate cells. Photosynthetic pigments are chlorophyll a, c, β-carotene that is present in big amount and xanthophylls provide the cells greenish-yellow color.

Sexual reproduction is exceptional. The motile cells encompass two unequal flagella present on the anterior end; one is tinsel and the other of whiplash kind.

Examples: Vaucheria, Botrydium and so on.

Division Chrysophyta (Golden brown algae):

In this division, algae are mainly unicellular or colonial, filamentous forms are very rare. Motile cells encompass two equivalent or unequal flagella present on the anterior end. The longer one consists of stiff hairs and the shorter is smooth. The cell wall is build up of silica and pentin or scales of carbonate. Main pigments are chlorophyll a, c and carotenoids such as β1carotene, Fucoxanthin, neofucoxanthin and diatoxanthin.

The storage products are mainly oil droplets and true starch is not present however glucan granules or leucosin are present.

Examples: Synura, Ochromonus, Chromulina, Mallomonas and so on.

Division Euglenophyta (Euglenoids):

Most of the Euglenoids are simple, unicellular motile flagellates. They do not comprise firm cell-wall and have characteristics similar to protozoans. They encompass a contractile vacuole. Cell surface is pellicle (that is, thin membrane) and consists of helical; knob-like projections. Cell shape modifies constantly (movements of Euglenoids).

Chloroplasts exhibit variety of shapes like discoid, ribbon like or stellate. Cells are biflagellate however just one flagellum represents anteriorly.

The photosynthetic pigments positioned in the plastids comprise chlorophyll a, b and carotenoids comprising β1carotene.

Cells divide or split by binary fission. Most of the species produce cysts under adverse conditions. Sexual reproduction is not present.

Examples: Euglena, Phacus, Trachelomonas and so on.

Division Dinophyta (Dinoflagellates):

Cell wall comprises of cellulose plates that are in the plasma membrane and a number of plates or body scales might be present on the cell wall. Cell structure is complicated. Majority of forms are motile and unicellular. Most of the Dinoflagellates example: Noctiluca is luminescent.

Most of such algae have chlorophyll, a and c and distinctive carotenoid particular to Dinoflagellates. Reserve foods are mainly in the form of oil and starch.

Asexual process of reproduction is through cell division. Parent cell divides or splits into a number of aplanospores or zoospores or non-motile cells. Sexual reproduction has been newly reported, gametes are smaller than the vegetative cells and the fusion is isogamous.

Examples: Noctiluca, Peridinium, Gonyaulax, Ceratium and so on.

Division Bacillariophyta (Diatom):

In this division, algae are generally of unicellular forms, some are colonial and filamentous in the structure. Cell-wall is silicified, comprising of two perforated overlapping plates. It is very ornamented on the surface. Chromatophores are brownish in color due to big amount of carotenoids. Diatoms (that is, cut in half) each cell is build up of two portions. The bigger part is fitted tightly over the slightly smaller part such as petridish.

The photosynthetic pigments are chlorophyll a and c, diatoxanthin, fucoxanthin and diadinoxanthin. Common storage product is chrysolaminarin and oil.

Reproduction takes place through vegetative and sexual processes. Diatom cells dissimilar other algae are diploid in nature. Sexual fusion is homothallic, in the individuals of the similar clone. Two amoeboid gametes fuse to make a zygote which builds up into an auxospore. Fusion might be isogamous, anisogamous or oogamous kind.

Examples: Cymbella, Navicula, Diatoma, Coscinodiscus and so on.

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