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 th e facultative anaerobe Halobacterium salinarium PHH1, the growth of a gas-vacuolate (Gv(+)) strain in competition with two gas-vesicle-defec tive (Gv(def)) mutants was examined. The Gv(+) strain synthesized gas vesicles throughout ifs growth cycle, and floated up to form a thick s urface scum during the exponential growth phase in static culture. Mut ant 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 much lower gas vesicle content in shaken culture and produced negligible amounts of gas vesicles in static cul ture. The Gv(+) and the two Gv(def) strains grew equally well in shake n 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 cu ltures, perhaps because Gv(def2) carried a smaller protein burden, whi ch offset the benefits of buoyancy. This selection for Gv(def2) was lo st in deeper static cultures, although it could be restored by aeratin g 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.