SUMMARY: A strain of the gas-vacuolate cyanobacterium was found to float in cultures grown at low light intensities and to sink in those grown at high intensities. The loss of buoyancy that occurred within 1 to 5 h on increasing the photon flux density from 10 to 100 μmol m-2 s-1 was investigated by centrifuging the cell suspensions in a horizontally placed capillary with a rectangular cross-section and then separately counting the floating cells under the upper tube surface and sinking cells on the lower surface. Buoyancy loss was not accompanied by loss of gas vesicles, as occurs in some other planktonic cyanobacteria, but was caused by a relative increase in dry matter, principally carbohydrate, without a corresponding increase in gas vesicles. The increase in light intensity gave an increase in cell turgor pressure but this was insufficient to collapse the strong gas vesicles present in this strain, which had a median critical pressure of 0·75 MPa (7·5 bar).


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