Anabaena
About 110 species are found world-wide in freshwater or marine habitats, including mostly freshwater pools and ponds and saline lakes.
The filaments may be solitary or clustered and are usually planktonic. Some researchers place the planktonic species within the subgenus Dolichospermum. The filaments may also grow as periphytic mats on sediments, or on or within the tissues of aquatic plants. Anabaena often forms surface nuisance blooms when nutrients are abundant, and may produce toxins.   Anabaena azollae is a well known symbiont that lives within the water fern Azolla, which is used as a natural fertilizer and source of nitrogen for rice crops throughout the world.