Animated coloured transmission electron micrograph (TEM) of Cyanobacterium (Nostoc sp.) Gram-negative, oxygenic, photosynthetic, filamentous cyanobacterium (prokaryote). Note the thylakoids (photosynthetic membranes) and polyhedral bodies. Lichens often are associated with Nostoc in a symbiotic relationship. Specialized cells in the Nostoc filament, called heterocysts, fix atmospheric nitrogen gas into amino acids and thus progressively enrich the soil with nitrogen for plant growth.