Different types of algae contain different pigments. Most contain chlorophyll, a green pigment. The presence of other pigments, however, may cause algae to appear brown, blue-green, red, or golden brown.

Algae vary greatly in structure. The single-celled species can be round, oval, or elongate, and they can exist individually or in colonies (loose groupings of similar cells). The colonies have various shapes, such as hollow spheres or slender threads. Single-celled algae called diatoms have cell walls covered with hydrated silica, a glasslike substance that produces a great variety of geometric patterns. (S.) Multicelled algae, such as kelp, sea lettuce, and rock-weed, usually have irregular, complex shapes. color page titled Examples of Ocean Life.)Some multicelled algae have hard shells composed of calcium carbonate.

Many species of single-celled algae are capable of limited forms of self-propulsion. These algae move about by means of flagella, which are long, whiplike growths that project from the main part of the cell.

Single-celled algae typically reproduce asexually by fissionthe division of a cell into two or more new cells. Some single-celled algae reproduce sexually, through either the fusion of two cells or the exchange of genetic material between cells. Most multi-celled algae reproduce by a process called alternation of generations, in which one generation reproduces asexually and the next sexually. In one generation, a cell called a spore develops directly into a new organism. In the next, two cells called gametes, or sex cells, combine their genetic material to produce a new organism. Multicelled algae can also reproduce asexually by fragmentationthe formation of a new individual from a detached portion of another.