heterocysten

Heterocysts are specialized nitrogen-fixing cells formed by some filamentous cyanobacteria, notably Anabaena and Nostoc. They develop in response to nitrogen starvation and arrange periodically along a vegetative filament, creating a cooperative unit in which photosynthetic cells supply carbon to the nitrogen-fixing cells.

Heterocysts possess a thickened cell envelope rich in glycolipids and a suberized layer that markedly reduces the diffusion of oxygen into the cell. This creates an anaerobic microenvironment necessary for the enzyme nitrogenase, which catalyzes the reduction of atmospheric N2 to ammonia. To prevent oxygen from inhibiting nitrogenase, heterocysts downregulate photosystem II and rely on respiratory metabolism for energy, while neighboring vegetative cells provide fixed carbon compounds to sustain heterocyst function.

Development is triggered by depletion of combined nitrogen (nitrate, ammonia). In many species, one or two vegetative cells differentiate into a heterocyst at regular intervals along the filament. The process involves regulatory networks (for example, hetR and ntcA in many cyanobacteria) that control cell differentiation and envelope formation. Mature heterocysts display limited exchange of photosynthetic products and enhanced transport of metabolites via intercellular connections known as septal junctions, which connect the cells of the filament.

Ecologically, heterocysts play a key role in the global nitrogen cycle, enabling certain cyanobacteria to grow in nitrogen-poor environments and contributing fixed nitrogen to aquatic and soil ecosystems. They are also found in symbiotic associations, such as those with plants.