Heterocistos

Heterocistos are specialized nitrogen-fixing cells found in some filamentous cyanobacteria. They differentiate from vegetative cells when fixed nitrogen is scarce and typically form at regular intervals along the filament, creating a coordinated system that allows atmospheric nitrogen (N2) to be converted into usable nitrogen compounds for the colony.

Structure and function

Heterocistos have a thick, multi-layered cell envelope rich in glycolipids and other materials that limit the diffusion of oxygen. This creates a microoxic environment inside the cell, which is essential because the enzyme nitrogenase is highly sensitive to oxygen. In heterocistos, photosystem II activity is reduced while photosystem I remains functional, providing energy for the energy-intensive nitrogen fixation process. The fixed nitrogen is usually released as ammonia or amino acids to neighboring vegetative cells, which in turn supply carbon compounds to the heterocyst via photosynthate exchange. The two cell types thus form a metabolic partnership: vegetative cells fix carbon, while heterocysts fix nitrogen.

Development and regulation

Differentiation into heterocistos is triggered by nitrogen limitation. A network of regulatory factors governs the process, including the master regulator HetR and global regulators such as NtcA. Inhibitory signals, notably PatS and HetN, help establish and maintain the regular spacing of heterocysts along the filament. Once formed, heterocistos are typically non-dividing and persist for as long as the filament experiences nitrogen limitation.

Ecology and significance

Heterocistos play a key role in the global nitrogen cycle, particularly in aquatic systems and soil crusts where filamentous cyanobacteria contribute bioavailable nitrogen. They are of interest for biofertilizer development and studies on cellular differentiation and intercellular metabolite exchange.