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

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