The marine bacterium , containing 470 mM-K and 70 mM-Na inside its cells, was able to regulate the cytoplasmic pH (pH) in the narrow range 7·6–7·8 over the external pH (pH) range 6·0–9·0 in the presence of 400 mM-Na and 10 mM-K. In the absence of external K, however, pH was regulated only at alkaline pH values above 7·6. When the cells were incubated in the presence of unusually high K (400 mM) and 4 mM Na, the pH was regulated only at acidic pH values below 7·6. These results could be explained by postulating a K/H antiporter as the regulator of pH over the pH range 6·0–9·0. When Na-loaded/K-depleted cells were incubated in 400 mM-Na in the absence of K, an inside acidic ΔpH was generated at pH values above 7·0. After addition of diethanolamine the inside acidic ΔpH collapsed transiently and then returned to the original value concomitant with the extrusion of Na, suggesting the participation of a Na/H antiporter for the generation of an inside acidic ΔpH. In the presence of 400 mM-K, at least 5 mM-Na was required to support cell growth at pH below 7·5. An increase in Na concentration allowed the cells to grow at a more alkaline pH. Furthermore, cells containing more Na inside could more easily adapt to grow at alkaline pH. These results indicated the importance of Na in acidification of the cell interior via a Na/H antiporter in order to support cell growth at alkaline pH under conditions where the activity of a K/H antiporter is marginal.


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