Introduction:

Algae are usually categorized on the basis of the given features:

1) Nature and properties of the pigment.

2) Chemistry of the reserve food products or assimilatory products of photosynthesis

3) Type and number, insertion (that is, point of attachment) and morphology of flagella

4) Chemistry and physiological characteristics of cell walls.

5) Morphological features of cells and thalli.

6) Life history, reproductive structures and process of reproduction.

Rhodophycophyta:

The Rhodophycophyta or simply red algae are marine kinds found in the warmer seas and oceans; however some grow in colder water and also in fresh-water. Most of the red algae grow in the sub tidal (that is, submerged) zone, just a few being capable to survive desiccation or exposure. A few species deposit on their surfaces lime from sea-water; finally this outcomes in deposition of lime in the ocean and plays a portion in the formation of algal reefs. They range from unicellular to multicellular. They are photosynthetic and have chlorophyll A. Their chloroplast is deficient in chlorophyll B and contains phycobiliproteins. The main light-harvesting pigments of the Cyanobacteria, the reddish color of the red algae outcomes from phycoerythrin, an accessory pigment which masks the green color of chlorophyll. Illustration is Gelidium from which agar is made.

Xanthophycophyta: The Yellow-Green Algae:

These yellow-green algae were once categorized by the green algae. Though, their pale green or yellow-green coloration points out that they encompass a unique group of pigments. They are found more often in temperate areas in fresh-water and marine habitats, and also on and in soil. Xanthophyta exist as single cell colonies and as both branched and unbranched filaments. The motile genera are not common; however, a few reproduce asexually by motile reproductive cells (that is, zoospores). Flagella are of irregular lengths.

The xanthophyte walls are usually of pectin and cellulose. The cellular storage product is oil or a branched glucan chrysolaminarin.

Vaucheria, the water felt, is a famous member of this division and is broadly distributed on moist soil and in both quiet and fast flowing water. Both fresh-water and marine species are recognized. Zoospores are mainly formed singly in terminal sporangia in the asexual reproduction.

Chrysophycophyta - The Golden Algae:

Species of Chrysophycophyta are mainly flagellates; some of them are amoeboid, having pseudopodia extensions of the protoplasm. The naked amoeboid makes can ingest particulate food having the pseudopodia. Non-motile coccoid and filamentous forms are as well comprised in the division.

The chrysophycophyta distinct from the green algae in the nature of their pigments, in storing reserved food as oil or chrysolaminarin instead of as starch, and in their common incorporation of silica. Most forms are unicellular, however some form colonies. Their feature color is due to the marking of their chlorophyll by brown pigments. Reproduction is generally asexual (that is, binary fission) however occasionally isogamous. An illustration is Ochromonus.

Phaeoophycophyta - The Brown Algae:

These algae are multicellular and have a brown pigment that gives them their feature color and common name of brown algae, or brown seaweeds. Almost all are marine dwelling and most often, found in the cool ocean waters. They are structurally quite complex and a few - the kelps - are big, the individual plants reaching a length of several hundred feet. Most of them encompass holdfasts; and several encompass air bladders, which give them buoyancy. They reproduce asexually through zoospores and sexually by heterogamy and isogamy. This group comprises algae employed in commerce, such as the numerous varieties of kelp. They are employed as food for humans, fish and other animals.

Bacillariophycophyta - The Diatoms:

Members of the group are diatoms, they are mainly found in both salt and fresh water and in moist soil. They are rich in cold waters. Diatoms are the most abundant form of plankton in the Arctic. The thousands of species diatoms give an ever present and plentiful food supply for aquatic animals. Diatoms are unicellular, colonial or filamentous and take place in a broad variety of shapes. Each and every cell consists of a single prominent nucleus that is massive and ribbon-like or smaller lens-like, plastids. They produce shells (that is, cell walls) having silica, some of which are much beautiful. Shells of diatoms are termed as frustules. Deposits of such shells resultant from centuries of growth are termed as diatomite or diatomaceous earth.

Euglenophycophyta - The Euglenoids:

Euglenophycophyta are unicellular organisms and they are actively motile through flagella. They reproduce via cell division. Of particular interest is the genus Euglena, that is representative of a group designated as animals by several zoologists however as plants by many botanists. Euglena is broadly distributed and takes place in soil and also in water, where it frequently forms a diversity film or bloom.

Chlorophycophyta - The Green Algae:

Members of this big and diverse group of organisms, termed as green algae, are principally fresh-water species. They are as well found in seawater and most of them are terrestrial. The cells of the Chlorophycophyta have a well-defined nucleus and generally, a cell wall, and the chlorophyll pigments are in chloroplasts, as in the higher plants. The greater part of green algae contains one chloroplast per cell. It might be laminate, cup-shaped and reticulate. The chloroplasts as well often include dense areas termed as pyrenoid, on which surface starch granules are formed. Food reserves are stored as starch, a product of photosynthesis.      

They bear chloroplasts having chlorophyll A and B giving them their feature green color. 

There are numerous single-celled forms and most of the colonial kinds of green algae. Most of the unicellular green algae are motile by flagella action. Colonial types take place as spheres, filaments or plates. A few species have special structures termed as holdfasts, which anchor them to submerged objects or aquatic plants. An illustration is Chlamydomonas.

Chlamydomonas is considered as typical green algae. It is a typical motile, unicellular, green alga and is broadly distributed in soils and freshwater. It differs from 3 to 29 μm in common forms and is motile apart from during cell division. Motility is by means of two flagella. Each and every cell consists of one nucleus and a single large chloroplast which in most species is cup-shaped, however in some, it might be star-shaped or layered. The cell wall includes cellulose; in numerous species, an external gelatinous layer is as well present. There is some evidence which the red eyespot or stigma in the chloroplast is the site of light perception.

Moreover to motile unicellular algae similar to Chlamydomonas, other, non-motile unicellular green algae are broadly distributed. One of the most significant of these is Chlorella, which has served beneficially as a tool in numerous investigations on photosynthesis and supplemental food supply.

Volvox is colonial green algae which might form water blooms. Its colonies are visible to the bare eye. Each and every colony comprises from 500 to thousands of cells arranged at the surface of a watery colloidal matrix. The individual cells are biflagellate and are morphologically identical to that of Chlamydomonas.

Desmids are one of the most fascinating green algae found in a broad variety of attractive shapes and designs. Each and every cell is made up of two symmetrical halves having one or more chloroplasts Ulethrix which is a filamentous form found in the flowing streams, joined to wings or stones by holdfasts at the bottom of the filament.

A much common green alga is Spirogyra, a filamentous form seen in the scums which cover ponds and slow-moving water. It is of interest because of its general occurrence and its possession of unusual chloroplasts that are arranged spirally.

Cryptophycophyta-The Cryptomonads:

The Cryptomonads are a small group of biflagellate organisms.  They contain two unequal flagella that arise from the base of a groove; both are of the tinsel kind having stiff hairs. The cells are slipper-shaped, flattened to a dorsal-ventral plane and take place singly. Some of the forms encompass a cellulose wall whereas others are naked, being surrounded only by a plasmalemma having a thin granular material on the outside. There are one or two plastids, with or without pyrenoids, per cell. The food reserve is stored as true starch and also oil. Asexual reproduction is either by means of longitudinal cell division or by the formation of zoospores or cysts. An illustration is the genus Cryptomonas.

Pyrrophycophyta-The Dinoflagellates:

This division comprises the dinoflagellates, a diverse group of biflagellate unicellular organisms. The dinoflagellates are so named due to their twirling motion instead of their morphology. These organisms comprise a significant component of marine, brackish and fresh bodies of water. This is the other group that consists of both plant-like and animal-like features. The cells are usually flattened and encompass a transverse constriction, the girdle, generally around the cell equator. Distinguishing characteristic of dinoflagellates is that the flagella are inserted in the girdle and that the flagella are arranged having one is encircling the cell and one trailing. Hairs project from the flagellar surface. Most of the dinoflagellates are covered merely by a plasmalemma.  In some forms, there is a wall made up of cellulose. Still, others encompass a sequence of cellulose plates having the plasmalemma. These are known as thecal plates and dinoflagellates with them are stated to be armored. Dinoflagellates are significant components of planktons. They are best termed as the organisms which produce 'red tides', or blooms in which the concentration of cells might be so great as to color big areas of the ocean red, brown or yellow. Such an organism is Gonyaulax. The other marine dinoflagellates like Noctiluca are luminescent. Asexual reproduction occurs through the division of cell.

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