Growth competition between Halobacterium salinarium strain PHH1 and mutants affected in gas vesicle synthesis

To investigate the role of the buoyancy provided by gas vesicles in the facultative anaerobe Halobacterium salinarium PHH1, the growth of a gas-vacuolate (Gv⁺) strain in competition with two gas-vesicle-defective (Gv^def) 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^def1 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^def2 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^def strains grew equally well in shaken cultures, but in static cultures, where steep vertical gradients of oxygen concentration were established, Gv^def1 was outgrown by the Gv⁺ strain. Gv^def2 outcompeted the Gv⁺ strain in shallow static cultures, perhaps because Gv^def2 carried a smaller protein burden, which offset the benefits of buoyancy. This selection for Gv^def2 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.