"Alga" redirects here. For places called Alga, see Alga (disambiguation). For other uses, see Algae (disambiguation).Algae (/ˈældʒiː/ or /ˈælɡiː/; singular alga /ˈælɡə/) is an informal term for a large, diverse group of eukaryotes that are not necessarily closely related and are thus polyphyletic. Included organisms range from unicellular genera, such as Chlorella and the diatoms, to multicellular forms, such as the giant kelp, a large brown alga that may grow up to 50 meters in length.