Darkfield and fluorescence contrast light micrograph of a mass of Nostoc sp. filamentous cyanobacteria (blue-green algae) found on the surface of soil. Nostoc algae have the ability to fix atmospheric nitrogen, thereby playing a major role in soil fertility. They have a wide distribution and are commonly found in soil and on mud, rocks and plants. Magnification: x160 when printed 10 centimetres wide.