Lower limit of the gas permeability coefficient of gas vesicles

Abstract

The gas vesicles found in various planktonic prokaryotes are hollow, rigid structures permeable to gases. They collapse when the difference between the external hydrostatic pressure and internal gas pressure exceeds their critical pressure (usually about 0.6 MPa). It was found that dried gas vesicles would survive exposure to gas pressures considerably in excess of this value (4 MPa or more), because gas diffused into them as the pressure was raised and the pressure difference required to cause collapse was not established. They survived the most rapid rates of pressure rise, 0–4.6 MPa in less than 2.5 ms, to which they were exposed. From this it can be calculated that the gas permeability coefficient of the average gas vesicle ( α ) exceeds 22 x 10 3 s -1 and the permeability of the gas vesicle wall ( k ) is greater than 332 μm s -1 . Gas molecules may diffuse through fixed pores in the gas vesicle wall. Since a gas molecule of collision diameter 0.63 nm is known to penetrate the gas vesicle, this would be the minimum diameter of such a fixed pore. It is shown by kinetic theory that the permeability coefficient of an average gas vesicle with one pore of this size would be 2.1 x 10 3 s -1 : there would, therefore, have to be at least 11 such pores to account for the observed minimum permeability coefficient. Gas vesicles in aqueous suspension will also survive a rapid rise in the overlying gas pressure in excess of their critical pressure if they are near enough to the gas-water interface for sufficient gas to reach them by diffusion during the pressure rise. The distance from the interface at which the gas vesicles survive can be used to calculate the diffusivity of the gas through the suspension. A modification of this method can be used to measure the gas-permeability of certain types of cells containing gas vesicles.