Accumulation of ambient phosphate into the periplasm of marine bacteria is proton motive force dependent

BSTRACT Bacteria acquire phosphate (P i ) by maintaining a periplasmic concentration below environmental levels. We recently described an extracellular P i buffer which appears to counteract the gradient required for P i diffusion. Here, we demonstrate that various treatments to outer membrane (OM) constituents do not affect the buffered P i because bacteria accumulate P i in the periplasm, from which it can be removed hypo-osmotically. The periplasmic P i can be gradually imported into the cytoplasm by ATP-powered transport, however, the proton motive force (PMF) is not required to keep P i in the periplasm. In contrast, the accumulation of P i into the periplasm across the OM is PMF-dependent and can be enhanced by light energy. Because the conventional mechanism of P i -specific transport cannot explain P i accumulation in the periplasm we propose that periplasmic P i anions pair with chemiosmotic cations of the PMF and millions of accumulated P i pairs could influence the periplasmic osmolarity of marine bacteria.