Algae (/ ˈ æ l dʒ iː, ˈ æ l ɡ iː /; singular alga / ˈ æ l ɡ ə /) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms.It is a polyphyletic grouping that includes species from multiple distinct clades.Included organisms range from unicellular microalgae, such as Chlorella, Prototheca and the diatoms, to multicellular forms, such as the giant kelp