The term algae refers to a large number of photosynthetic organisms that are multi- or unicellular eukaryotes and are not classified with plants. The organisms are plantlike, however, because they contain chloroplasts with chlorophyll. Most algae can be found in the oceans, but freshwater forms are also abundant.
The algae are subdivided into several divisions (rather than phyla, like the protozoa): Rhodophyta, Pyrrophyta, Chrysophyta, Phaeophyta, and Chlorophyta. The divisions are based in part on the types of pigments and colors they have.
Rhodophyta is the division of red algae. These organisms are almost exclusively marine types. Most are unicellular, but some multicellular forms grow anchored to rocks below the level of the low tide. Some are large enough to be seaweeds. Red algae carry on photosynthesis using chlorophyll a. The red pigments are very similar to those in many species of cyanobacteria. A derivative of red algae called agar is commonly used in bacteriological media in the laboratory.
Members of the Pyrrophyta are dinoflagellates. Dinoflagellates are unicellular organisms that are usually surrounded by thick plates that give them an armored appearance. Two flagella move the organism. Many dinoflagellates are luminescent. When affected by sudden movements, they give off light. When optimal conditions exist in the oceans, the dinoflagellates reproduce at explosive rates. Their red pigments cause the water to turn the color of blood. This condition is known as red tide.
Members of the division Chrysophyta are golden algae, most of which are diatoms. Diatoms have cell walls or shells composed of two overlapping halves impregnated with silica. In the oceans, the diatoms carry on photosynthesis. They serve as an important source of food in the oceanic food chains. Diatomaceous earth, a light-colored porous rock composed of the shells of diatoms, is made into a commercial product called diatomite. Diatomite is used as a filler, as an absorbent, and as a filtering agent.
Members of the division Phaeophyta are the brown algae. These organisms, which are multicellular, are found almost exclusively in saltwater, where they are known as rock weeds and kelp. Despite their great size, the tissue organization in these algae is quite simple compared with plants. Often they are used as fertilizers and sources of iodine.
Members of the division Chlorophyta are green algae, some of which are flagellated. The common laboratory specimen Spirogyra is classified here, as are other flagellated species. Many colonial forms are also classified as Chlorophyta. The colonial forms may represent the first evidence of multicellularity in evolution, and possess features that are intermediate between those of single-celled eukaryotes and those of more complex multicellular organisms. Ulva, the common sea lettuce, is classified here.