Light micrograph of Nostoc sp. , a filamentous blue-green alga or cyanobacterium. The strands of alga are in fact linear colonies of primitive, single- celled, prokaryotic organisms, more closely related to bacteria than to algae. Nostoc has the ability to fix atmospheric nitrogen, thereby playing a major role in soil fertility. It has a wide distribution & is commonly found in soil & on mud, rocks & other plants. It also forms the algal partner of a number of lichens. Blue green algae are considered to be exceedingly primitive organisms, dating back to pre-Cambrian times. Magnificaton: x250 at 35mm size.