Sodium Energetic Cycle in the Natronophilic Bacterium Thioalkalivibrio versutus

As inhabitants of soda lakes, Thioalkalivibrio versutus are halo- and alkaliphilic bacteria that have previously been shown to respire with the first demonstrated Na(+)-translocating cytochrome-c oxidase (CO). The enzyme generates a sodium-motive force (Δs) as high as −270 mV across the bacterial plasma membrane. However, in these bacteria, operation of the possible Δs consumers has not been proven. We obtained motile cells and used them to study the supposed Na(+) energetic cycle in these bacteria. The resulting motility was activated in the presence of the protonophore 2-heptyl-4-hydroxyquinoline N-oxide (HQNO), in line with the same effect on cell respiration, and was fully blocked by amiloride—an inhibitor of Na(+)-motive flagella. In immotile starving bacteria, ascorbate triggered CO-mediated respiration and motility, both showing the same dependence on sodium concentration. We concluded that, in T. versutus, Na(+)-translocating CO and Na(+)-motive flagella operate in the Na(+) energetic cycle mode. Our research may shed light on the energetic reason for how these bacteria are confined to a narrow chemocline zone and thrive in the extreme conditions of soda lakes.