heterocyst

Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, notably Anabaena and Nostoc. They arise in response to nitrogen limitation and enable a filament to fix atmospheric nitrogen for growth when combined nitrogen is scarce.

Structural and metabolic features: Heterocysts differ from neighboring vegetative cells by a thick, modified cell wall containing a glycolipid layer and a laminated envelope that restricts oxygen diffusion. They typically lack photosystem II, reducing oxygen evolution, while maintaining respiration to keep the internal environment low in oxygen. Inside the heterocyst, the enzyme nitrogenase reduces N2 to ammonia. The cell imports carbon compounds from adjacent vegetative cells and, in return, supplies fixed nitrogen to the filament through intercellular connections.

Development and regulation: Heterocyst formation is triggered by nitrogen scarcity and involves a regulatory network centered on the transcription factor HetR, with additional control by hetF, hetN, and the signalling peptide PatS that helps space heterocysts at regular intervals. Once formed, heterocysts are largely terminally differentiated and do not divide, creating a patterned distribution along the filament.

Ecological role: Heterocysts contribute to the global nitrogen cycle by enabling biological nitrogen fixation in aquatic and terrestrial habitats where fixed nitrogen is limiting. They also participate in symbiotic associations with plants and fungi, supplying bioavailable nitrogen in ecological and agricultural contexts.