Nostoc sp., a filamentous blue-green alga or cyanobacterium. These single celled prokaryotic organisms are more closely related to bacteria than to algae. Nostoc sp live in colonies seen here as pink strands, and are found in soil, mud, and on rocks and plants. Nostoc sp. algae plays a major role in soil fertility as it has the ability to fix atmospheric nitrogen. It also forms the symbiotic algal partner of a number of lichens. Blue green algae are considered to be primitive organisms, dating back to the Precambrian period (over 600 million years ago). Magnification: 160x.