Heterocysts are specialized, pale-yellow, thick-walled cells with disputed function nitrogen-fixing  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
• degrade 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