A colony of Nostoc commune growing on a concrete footbridge, Norfolk, UK. N. commune is a photosynthetic nitrogen fixing colonial cyanobacterium with a world-wide distribution. In nutrient deficient soils it can be an important source of fixed nitrogen. The colonies have a remarkable ability to exist in a highly dessicated form, often for years. Upon wetting, rehydration is complete within hours, with the restoration of respiration preceding the ability to fix nitrogen. In its dry form, the organism appears as dry fragments of brittle brown material; the sudden appearance of the extended olive green slimey colony following rain may account for its sometimes being identified with Biblical mannah. N. commune is used as a food item in many countries, including Peru, Japan, Indonesia, The Philippines and China. It is also the photosynthetic partner in many lichens