Algae ( \ / ˈældʒiː \ / or \ / ˈælɡiː \ / ; singular alga \ / ˈælɡə \ / , Latin for `` seaweed '' ) are a very large and diverse group of eukaryotic organisms , ranging 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 .
Algae are a large and diverse group of simple , typically autotrophic organisms , ranging from unicellular to multicellular forms .