Cyanobacterial heterocysts: terminal pores proposed as sites of gas exchange

In many filamentous cyanobacteria, oxygenic photosynthesis is restricted to vegetative cells, whereas N2 fixation is confined to microoxic heterocysts. The heterocyst has an envelope that provides a barrier to gas exchange: N2 and O2 diffuse into heterocysts at similar rates, which ensures that concentrations of N2 are high enough to saturate N2 fixation while respiration maintains O2 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 N2 fixation is most active, and decreasing it at night, minimizing O2 entry. Comparisons are made with stomata, which regulate gas exchange in plants.