Anabaena

Anabaena is a genus of filamentous cyanobacteria that live as plankton, forming chain-like filaments in fresh water. Under nitrogen-limited conditions its cells develop specialized structures called heterocysts, within which the enzyme nitrogenase converts atmospheric nitrogen gas into ammonia using ATP and reducing power, letting the organism fix its own nitrogen; some species are used as a natural fertilizer in rice paddies for this reason. Anabaena also forms a symbiosis with the water fern Azolla, supplying it with nitrogen, and some Anabaena partners in that relationship have become so dependent on their host that they have lost or transferred genes to it over evolutionary time.