Cyanobacterial heterocysts: terminal pores proposed as sites of gas exchange

In many filamentous cyanobacteria, oxygenic photosynthesis is restricted to vegetative cells, whereas N-2 fixation is confined to microoxic heterocysts. The heterocyst has an envelope that provides a barrier to gas exchange: N-2 and O-2 diffuse into heterocysts at similar rates, which ensures that concentrations of N-2 are high enough to saturate N-2 fixation while respiration maintains O-2 at concentrations low enough to prevent nitrogenase inactivation. I propose that the main gas-diffusion pathway is through the terminal pores that connect heterocysts with vegetative cells. Transmembrane proteins would make the narrow pores permeable enough and they might provide a means of regulating the rate of gas exchange, increasing it by day, when N-2 fixation is most active, and decreasing it at night, minimizing O-2 entry. Comparisons are made with stomata, which regulate gas exchange in plants.