Nostoc Cyanobacteria with heterocyst in their filament was observed in the water sample collected from the pond near Trimbakeshwar, Nashik, Maharashtra.

Nostoc is the free-living (nonsymbiotic) cyanobacteria capable of transforming atmospheric nitrogen into fixed nitrogen.

Particularly efficient nitrogen fixers are found among the filamentous species that have specialized cells called heterocysts. The heterocysts are thick-walled cell inclusions that are impermeable to oxygen; they provide the anaerobic (oxygen-free) environment necessary for the operation of the nitrogen-fixing enzymes.