Summary: The lengths and cylinder diameters of gas vesicles isolated from eight cyanobacteria were determined by electron microscopy. From these measurements the volumes and buoyant densities of the gas vesicles were calculated. A quantitative analysis was made of the effects of length and diameter on efficiency in providing buoyancy. In seven of the cyanobacteria the mean length exceeded that which would give 90% of the maximum efficiency. It is postulated that the mean diameter () in each organism is constrained by the requirement for a particular critical pressure () to withstand the combination of turgor pressure and hydrostatic pressure experienced in the natural habitat, such that = 59 (/MPa) nm. The mean diameter of gas vesicles from was estimated to be 84.0 nm with a standard deviation of 4.1 nm. The weakest 10% of the vesicles had a diameter 6.2 nm wider than the strongest 10%, indicating that the variation was not an artifact of measurement. The intrinsic variation in width accounts for about a third of the observed critical pressure variation. The width of new gas vesicles formed in cells of grown in 100 m-sucrose, which reduced turgor pressure by half (to 0.17 MPa), was only marginally greater than in normal conditions, and it is concluded that the mean width is genetically determined and not directly altered by pressure.


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