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 (N
2) 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