Heterocysts in Cyanobacteria – Definition, Structure, and Function Explained
Heterocysts (also called heterocytes) are specialized nitrogen-fixing cells produced by certain filamentous cyanobacteria—such as Nostoc, Cylindrospermum, and Anabaena—when environmental nitrogen is limited. They house the enzyme nitrogenase, which converts atmospheric N2 into a usable nitrogen form for the filament. Since nitrogenase is deactivated by oxygen, heterocysts create a microanaerobic interior. Their formation involves extensive changes in gene expression and cell structure. Heterocysts: add three extra cell-wall layers, including a glycolipid layer that restricts oxygen and CO2 entry synthesize nitrogenase and other nitrogen-fixation proteins dismantle photosystem II (the oxygen-evolving complex) upregulate glycolytic enzymes produce proteins that scavenge residual oxygen form polar cyanophycin plugs that slow diffusion between cells Because heterocysts lack the water-splitting photosystem II, they cannot carry out typical photosynthesis; ...