Nostocs are photosynthesizers which

Nostocs are photosynthesizers which use cytoplasmic photosynthetic pigments rather than chloroplasts in their metabolic process. They are single-celled, and lack a nucleus or other internal membrane systems; their cytoplasm is composed 70%-85% of water. The cells do not possess flagella, but are motile by a swaying motion. Division is by binary fission; some branching may occur. The cells form filamentous structures known as trichomes, which in turn make up colonies encased by a thin sheath; these colonies may be mat-like or spherical and are either micro- or macroscopic--spherical colonies may reach sizes of up to 2.6 kg wet weight.
Ecology

Nostoc environments are diverse and widespread over the globe; isolates have been found in fresh water, soils, and both extremely cold and extremely arid habitats. Their role as a nitrogen fixer in terrestrial ecosystems allow them to maintain symbiotic interactions with organisms including fungi, lichen, mosses, and ferns. They are largely protected from predation by their outer sheath covering and the large size of their colonies, which make them difficult for some algivores to ingest. Some types of Nostoc are edible, and are even considered delicacies in some regions; in China during holidays a black hairlike vegetative species, Nostoc flagelliforme or "fat choy", are consumed. However, these algae can also cause problems for humans by growth on sport turf and buildings, and can lead to unpleasant odors in drinking water. Some phenolic extracts from Nostoc are known as human pathogen inhibitors, and may in the future be valuable to scientists medicinally. Very rarely Nostoc have been found to be symbionts of terrestrial plants, such as species which colonize the root nodules of Hawaiian cycad Gunnera genus.