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

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.