To investigate the role of the buoyancy provided by gas vesicles in the facultative anaerobe PHH1, the growth of a gas-vacuolate (Gv) strain in competition with two gas-vesicle-defective (Gv) mutants was examined. The Gv strain synthesized gas vesicles throughout its growth cycle, and floated up to form a thick surface scum during the exponential growth phase in static culture. Mutant Gv produced significantly fewer gas vesicles than the Gv strain in corresponding stages of growth, although in late stationary phase a small proportion of cells floated up to the surface of static cultures. Mutant Gv had a much lower gas vesicle content in shaken culture and produced negligible amounts of gas vesicles in static culture. The Gv and the two Gv strains grew equally well in shaken cultures, but in static cultures, where steep vertical gradients of oxygen concentration were established, Gv was outgrown by the Gv strain. Gv outcompeted the Gv strain in shallow static cultures, perhaps because Gv carried a smaller protein burden, which offset the benefits of buoyancy. This selection for Gv was lost in deeper static cultures, although it could be restored by aerating static cultures from below. The results support the hypothesis that the role of buoyancy in halobacteria is to maintain cells at the more aerated surface of brine pools.


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