Algae include a group of chlorophyll containing thalloid plants of the simplest type, having no true roots, stems, leaves or leaf-like organs. In the tree-kingdom classification, they are placed in the division Thallophyta along with fungi. But in the five-kingdom classification, the multicellular algae are included in the kingdom Plantae, unicellular eukaryotics in Protista and prokaryotic blue-greens in Monera.

Algae are of universal existence and they are found in a variety of habitats such as fresh water, sea water, on snow, on rocks and on or within the plant and animal bodies. Of these, aquatic forms are most common. The body of multicellular algae ranges from simple, floating colonial aggregate of cells to filaments or to sheets of cells. Some algae are much complex and their thalli attain a length of 65 metres or more. Their plant body is differentiated into holdfast, stem-like stipe and flattened leaf-like blades. Some of these algae have considerable differentiation of tissue very similar to those of higher plants. But they lack vascular tissues. In the earthly forms, there is distinct adaptation in the plant body to survive under constant inadequate supply of moisture. In heterotrichous forms, the plant body is highly developed and differentiated into a flat and an aerial system with a very clear division of labor. These algae approach the prototype of simplest plant body of bryophyte level with a veiy clear division of labour.

Types of Algae

The three divisions of multicellular algae—red algae (Rhodophyta), brown algae (Phaeophyta) and green algae (Chlorophyta)—are distinguished chiefly on the basis of their photosynthetic pigments. Chlorophyll a is the main photosynthetic pigment of all multicellular algae.

Various other pigments present are used as accessories (to pass light energy to chlorophyll a) and as protection against photochemical damage. Since only certain portions of the light spectrum penetrate fail through water, the pigments used by algae in photosynthesis control the depth of water they can live.

Red Algae

The red algae (DivisionRhodophyta) have chlorophyll a, chlorophyll d and phycobilins (phycocyanin and phycoerythrin) pigments. Phycoerythrin absorbs mostly green light and phycocyanin mostly yellow light. Red algae are especially adapted for deep water, where only blue and green light penetrate. In such an environment, the chlorophylls of red algae absorb blue light and phycoerythrin green light, thus empowering photosynthesis to continue to the near darkness of deep water. Red algae are not always red; their color varies with water depth. In the full sunlight of shallow water, phycoerythrin is broken down by light and the algae lose their red colour, becoming dark green or brown as light is reflected from chlorophyll and carotene. In deeper waters, they develop more phycoerythrin and become redder.

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Brown Algae

The brown algae (Division Phaeophyta) have chlorophyll a and chlorophyll c, which absorb red and blue light, and large amounts of the brown pigment fucoxanthin. Since their chlorophyll absorbs most of the light, brown algae succeed in shallow water, where they take maximum advantage of blue and red light. The presence of fucoxanthin prevents damage from the bright light.

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Green Algae

The green algae (Division Chlorophyta) are biochemically similar to terrestrial plants. Their plastids contain chlorophyll a, chlorophyll b and p-carotene. The chlorophylls of green algae absorb mostly blue and red light, so that, like terrestrial plants, they thrive in foil sunlight; p-carotene mostly prevents cell damage from bright light. The green algae live in shallow water, where they can absorb blue and red light that reaches the earth's surface. It is believed that land plants have arisen from green algae and not from ancestral red or brown algae. All land plants share with green algae the presence of chlorophyll a, chlorophyll b, and P-carotene, starch as the principal storage carbohydrate, and cellulosic cell walls. These biochemical correspondences are further supplemented by a common pattern of cytokinesis. A cell plate is formed at the centre of the dividing cell that ultimately divides the cytoplasm into two compartments. A important characteristic, such as this, is not likely to arise independently in unrelated lineages. Chloroplasts of both green algae and land plants have a stacked grana structure. The green algae are therefore viewed as the parent stock from which ancestors of land plants have ascended.
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