The mechanical properties of the Microcystis gas vesicle

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Abstract
Gas vesicles isolated from the cyanobacterium Microcystis sp. retained their high stability and critical pressure on storage and could be held under a sustained pressure of 0.5 MPa without collapse. Application and release of pressures up to 0.5 MPa gave reversible changes in volume that indicated an elastic compressibility of 0.0087 MPa-1, which was linear with pressure. The elastic bulk modulus of the Microcystis gas vesicle was 115 MPa. The average yield pressure of the gas vesicles, 4.3 MPa, was determined by infiltrating them with gas at high pressure and then releasing the pressure so that they exploded. From these two measurements the proteinaceous wall of the gas vesicle was calculated to have a Young's modulus of 3.8 GPa and a yield stress of 78 MPa. These values correspond closely to those of nylon, which has a similar secondary structure to that of the gas vesicle protein. The critical collapse pressure of the Microcystis gas vesicle, 0.8 MPa, is substantially higher than the theoretical buckling pressure, 0.2 MPa, for an unstiffened homogeneous cylinder of similar relative dimensions. The stiffening of the gas vesicle may be provided by its outer layer of GVPc, since the removal of this protein decreases the critical pressure to 0.23 MPa. The volumetric compressibility of the gas vesicle generates a small reversible change in turbidity (0.008 MPa-1). Spectrophotometric measurements showed that a gas vesicle suspension containing 1 µl ml-1 of gas gives a pressure-sensitive optical density of 2.03 cm-1 at a wavelength of 500 nm.

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