zoochlorella, small green alga (often Chlorella) or, sometimes, flagellate protozoan (e.g., Tetraselmis, Carteria) that lives within the bodies of various freshwater protozoans and invertebrates. Zoochlorellae often colour their hosts green (e.g., green hydra and green Paramecium bursaria). As symbionts, zoochlorellae use carbon dioxide and nitrogenous and phosphorous wastes and, in illuminated conditions, provide oxygen and useful nutrients to their hosts. Sometimes zoochlorellae are digested by the host. They may be passed from one generation to another in host germ cells. Some can be grown as pure cultures away from their usual hosts.