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The permeability of heterocysts to the gases nitrogen and oxygen

Heterocysts of the cyanobacterium Anabaena flos-aquae retina 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 O2 and N2 was found to be 0.3s-1. From this it was calculated that the average permeability (k) of the heterocyst surface layer is about 0.4 μm 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 x 10-18 mol N2 s-1; this must equal the diffusion rate 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 coefficeint would be 2.7 x 10-17 mol s-1 per heterocyst, slightly greater than the maximum calcualted 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.