Cyanobacteria strands. Coloured scanning electron micrograph (SEM) of the cyanobacterium Nostoc sp. criss-crossing the surface of a small rock. Nostoc is a filamentous organism that has the ability to fix atmospheric nitrogen, thereby playing a major role in soil fertility. It has a wide distribution and is commonly found in soil, on mud, rocks or other plants. It also forms the partner of a number of lichens. Cyanobacteria were among the first forms of life.