What are heterocyst

Heterocyst are specialized nitrogen-fixing cells formed during nitrogen starvation by some filamentous cyanobacteria, such as Nostoc punctiforme, Cylindrospermum stagnale, and Anabaena sphaerica. They fix nitrogen from dinitrogen (N₂) in the air using the enzyme nitrogenase, in order to provide the cells in the filament with nitrogen for biosynthesis. Nitrogenase is inactivated by oxygen, so the heterocyst must create a microanaerobic environment. The heterocysts' unique structure and physiology require a global change in gene expression.

For example heterocysts:

Produce three additional cell walls, including one of glycolipid that forms a hydrophobic barrier to oxygen

Produce nitrogenase and other proteins involved in nitrogen fixation

De-grade photosystem II, which Produces oxygen .

Up-regulate glycolytic enzymes.

Produce proteins that scavenge any remaining oxygen.

Contain polar plugs composed of cyanophycin which slows down cell-to-cell diffusion.