'Heterocysts' are specialized N fixing stricture, found in

Heterocysts are specialized N fixing structures found in BGA (Blue green algae).
A heterocyst is a differentiated cyanobacterial cell that carries out nitrogen fixation in aerobic conditions. It is formed during nitrogen starvation by some filamentous cyanobacteria. It works by differentiating highly specialized cells to provide fixed nitrogen to the vegetative cells in a filament.