algae, Members of a group of mostly aquatic photosynthetic organisms (see photosynthesis) that defy precise definition. They range in size from the microscopic flagellate Micromonas to giant kelp that reach 200 ft (60 m) in length. Algae provide much of Earth’s oxygen, serve as the food base for almost all aquatic life, and provide foods and industrial products, including petroleum products. Their photosynthetic pigments are more varied than those of plants, and their cells have features not found among plants and animals. The classification of algae is changing rapidly because new taxonomic information is being discovered. Algae were formerly classified into three major groups—the red, brown, and green seaweeds—based on the pigment molecules in their chloroplasts. Many more than three groups are now recognized, each sharing a common set of pigment types. Algae are not closely related to each other in an evolutionary sense. Specific groups can be distinguished from protozoans and fungi (see fungus) only by the presence of chloroplasts and by their ability to carry out photosynthesis; these specific groups thus have a closer evolutionary relationship with the protozoa or fungi than with other algae. Algae are common on “slimy” rocks in streams (see diatoms) and as green sheens on pools and ponds. Use of algae is perhaps as old as humankind; many species are eaten by coastal societies.
