Cyanophyta (Cyanobacteria, formerly blue- green algae)
Cells lack nucleus and other internal structures. Most common in fresh waters but also found in marine waters and in terrestrial habitats, such as soil, tree trunks, or desert rocks. Some survive in extreme environments reaching 73° C (163° F). Reproduce asexually.
Chlorophyta (Green Algae)
Spirogyra, Ulva, Volvox
Cells contain plantlike chlorophyll pigments that give algae their grass-green color. Found in lakes and oceans, and on land in soil and on tree trunks. Reproduce via sexual and asexual reproduction.
Rhodophyta (Red Algae)
Deepest-dwelling algae. Many species contain the red pigment phycoerythrin, which captures light even at great ocean depths. Reproduce via asexual and sexual reproduction. Commercial source of carageenan, a thickener used in ice cream, cosmetics, and medicines; and agar, a gel used in laboratories.
Heterokontophyta (Golden-Brown Algae, Brown Algae, and Diatoms)
Vaucheria, Nereocystis, Fragilaria
Includes some of the largest algae, such as kelp and other seaweeds. Many species are golden-brown from carotenoid pigments. Commercial source of edible foods (kombus and wakame) and algin, a stabilizer used in paints, plastics, and foods. Diatoms form diatomaceous earth, used in aquarium filters, insecticides, and as a polishing agent.
Unicellular with stiff cellulose plates resembling armor. Many species have ornamentation that resemble horns, spines, or wings. Some species are photosynthesizers, others feed on other tiny organisms or are parasites. Reproduce asexually. Some species undergo a population explosion that forms a red tide that may suffocate fish or produce toxins that are lethal to humans who eat contaminated shellfish. Some species produce bioluminescence.