Representatives of the major division of algae.
Algae form a large, diverse group of simple photosynthetic organisms, all of which are eukaryotes. They occur in aquatic (salt and freshwater) environments or in moist locations on land. The body of an alga may be unicellular, or multicellular in the form of filaments, ribbons, or plates. They range in size from microscopic to more than 45 m (150 ft) long in the case of giant kelps.
Algae can be separated into seven major divisions, primarily on the basis of pigmentation (although there are other ways of categorizing them). So-called blue-green algae are prokaryotes and not true algae at all; instead they are bacteria of a type known as cyanobacteria. Green algae (division Chlorophyta) are found mainly in freshwater and may be single-celled, form long filaments (like Spirogyra) or a flat-leaf mass of cells called a thallus (like the sea-lettuce, Ulva lactuca). Golden-brown algae (division Chrysophyta) also include the diatoms. Brown algae (division Phaeophyta) include the familiar seaweeds found on rocky shores. Red algae (division Rhodophyta) are found mostly in warmer seas and include several species of economic importance. Desmids and dinoflagellates (both in division Pyrrophyta) are single-celled algae and are important constituents of marine plankton. Yellow-green algae and chloromonads (division Xanthophyta) are mainly freshwater forms, mostly unicellular and nonmotile. Motile unicellular algae such as Euglena (division Euglenophyta) are classified by some biologists as protozoan, but most contain chlorophyll and can synthesize their own food.
Algae in both marine and freshwater plankton are important as the basis of food chains. Many of the larger algae are important to man; for example, the red algae Porphyra and Chondrus crispus are used as foodstuffs. Gelidium, another red alga, is a source of agar, and the kelps (such as the giant kelp Macrocystis) produce alginates, one use of which is in the manufacture of ice cream. Other uses of algae are in medicine and as manure.