The Permeability of Heterocysts to the Gases Nitrogen and Oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retain gas vacuoles for several days after differentiation. It is demonstrated that the rate of gas diffusion into a heterocyst that is near an overlying gas phase can be determined approximately from observations on the rate of gas pressure rise required to collapse 50% of its gas vacuoles. The mean permeability coefficient (α ) of heterocysts to O₂ and N₂ was found to be 0.3 s^-1. From this it was calculated that the average permeability (kappa ) of the heterocyst surface layer is about 0.4 μ m s^-1 (within a factor of 2). This is probably within the range that could be provided by a few layers of the 26-C glycolipids in the heterocyst envelope. It is likely, but not proven, that the main route for gas diffusion is through the envelope rather than through the terminal pores of the heterocyst. From measurements of cell nitrogen content (2.7 pg), doubling time (3 days) and heterocyst: vegetative cell ratio (1:24) it was calculated that the average heterocyst fixed 5.9 × 10^-18 mol N₂ s^-1; this must equal the diffusion rate of N₂ into the heterocyst. This rate would be sustained by a concentration of N₂ inside the average heterocyst that was 22% below the outside air-saturated concentration. The maximum N₂ fixation rate allowed by the estimated permeability coefficient would be 2.7 × 10^-17 mol s^-1 per heterocyst, slightly greater than the maximum calculated N₂ fixation rate. The observed permeability coefficient is low enough for the oxygen concentration in the heterocyst to be maintained close to zero by the probable rate of respiration, providing an anaerobic environment for nitrogenase. The rate of O₂ diffusion will limit the N₂-fixation rate in the dark by limiting the rate at which ATP is supplied by oxidative phosphorylation.