Algae

Algae are chiefly aquatic, chlorophyll-bearing organisms whose plant body is a thallus -- meaning it is never differentiated into a true root, stem and leaves, however large or complex that thallus may be.

Where they live and how they look. Most algae are found in fresh water or in the sea; a smaller number grow as epiphytes on other plants, or live non-parasitically on the surface of other organisms. Their size spans an enormous range: microscopic single cells, either non-motile (such as Chlorella) or motile with flagella (such as Chlamydomonas); multicellular filaments, either unbranched (such as Spirogyra) or branched (such as Chara); and, at the largest scale, giant marine seaweeds many metres long (such as Sargassum).

Cell wall, stored food and reproduction. The algal cell wall is generally built of polysaccharides -- cellulose and related carbohydrates -- sometimes combined with proteins. Algae reproduce in all three possible ways: vegetatively (by fragmentation), asexually (by spores) and sexually (by the fusion of gametes), and their life cycles show alternation of generations in patterns that vary from group to group.

Basis of classifying algae. Algae are sorted into classes chiefly on the basis of the dominant photosynthetic pigments they carry, together with their stored food, cell-wall composition, and the number and position of flagella on their motile cells. On these criteria, algae fall into three principal classes.

Chlorophyceae (green algae). Named for the dominance of chlorophyll a and b in their chloroplasts, which gives them their green colour. They are mostly freshwater organisms, with only a few marine or brackish-water members. The plant body may be unicellular, colonial or filamentous, and the cell wall is made of cellulose. Chloroplasts vary strikingly in shape between genera -- discoid, cup-shaped, ribbon-shaped or spirally coiled -- and typically carry small proteinaceous bodies called pyrenoids, associated with starch synthesis and storage. Food is stored as true starch. Examples: Chlamydomonas (unicellular, motile), Volvox (colonial), Spirogyra and Ulothrix (unbranched filaments), Chara (branched filament).

Phaeophyceae (brown algae). Their brown colour comes from the accessory pigment fucoxanthin, which masks the green of their chlorophyll a and c. They are almost entirely marine. In the larger members, the body is differentiated into three regions -- a rootlike holdfast that anchors the plant to rock, a stalk-like stipe, and a flattened, leaflike photosynthetic frond. The cell wall contains cellulose together with fucans and algin, and food is stored as mannitol, laminarin and starch. Examples: Ectocarpus, Fucus, Sargassum, Laminaria. …