Nostoc
Over 200 species of Nostoc are common and widespread in a diverse array of habitats. Nostoc is found mostly in terrestrial habitats, such as moist rocks and cliffs, alkaline soils, wet meadows, and the edges of shallow lakes, and is associated with assorted vascular plants, mosses, ferns, and fungi in lichens. The genus is also found in mostly benthic habitats of unpolluted lakes, ponds, streams, and rivers. Nostoc is often abundant in flooded rice paddies where the nitrogen-fixation capabilities of its heterocysts help to fertilize two million hectares. Nostoc is sometimes occupied by aquatic midge larvae and subsequently produces ear-shaped colonies.