heterocystous

Heterocystous refers to organisms that form heterocysts, specialized cells for nitrogen fixation, typically filamentous cyanobacteria. In response to nitrogen limitation, certain cyanobacteria such as Anabaena, Nostoc, and Nostoc punctiforme differentiate some vegetative cells into heterocysts at regular intervals along the filament. This differentiation enables the organism to fix atmospheric nitrogen (N2) while maintaining photosynthetic activity in other cells.

Structure and function: Heterocysts develop thickened cell walls enriched with glycolipids and a specialized envelope that limits oxygen diffusion. They largely shut down photosystem II to reduce oxygen evolution, while neighboring vegetative cells provide energy through photosystem I. Nitrogenase inside heterocysts converts N2 to ammonia, which is then shared with adjacent cells via intercellular connections across the septa.

Ecology and significance: Heterocystous cyanobacteria contribute substantial nitrogen input in freshwater and terrestrial ecosystems, enabling growth in low-nitrogen environments. They form symbiotic associations with plants, notably Nostoc in the coralloid roots of cycads and with the aquatic fern Azolla, providing fixed nitrogen to hosts.