Nostoc, a blue-green algae (Cyanophyta) lying on the moist floor of a disused carboniferous limestone quarry, western Ireland. Nostoc is a filamentous algae, which 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 or other plants. It also forms the algal partner of a number of lichens. Nostoc is often found, as here, in a gelatinous mass (several centimetres in diametre), lying on the surface of the soil. Blue- green algae are considered to be exceedingly primitive organisms, dating back to pre-Cambrian times