A conceptual model of dissolved P pools, their bioavailability, and P transformations across the prokaryotic cell membrane. The phosphate pool and pathway is indicated in black, phosphoesters in orange, and phosphonates in green. Note the relative size of the different P pools; their likely bioavailability is indicated on the right of the diagram. In this conceptual model, we indicate the potential for microbial metabolism of phosphate (through a high affinity system), general phosphoesters (via hydrolysis by nucleases or phospholipases, for example), phosphomonoesters (via hydrolysis by alkaline phosphatase – AP), and phosphonates (via a C-P lyase). We underscore that these are conceptual routes, and the presence and the localization of the transporters and enzymes shown here may differ substantially between microbes. For example, the route of phosphonate transport, hydrolysis, and accumulation as either Pi or LMW DOP is not well characterized. We also highlight here some important areas of P biogeochemistry that are poorly understood and deserve further attention: (1) the presence and functional role of viral P-related genes; (2) the reactivity of HMW phosphoesters, their modes of hydrolysis, and their transport into the cell; (3) the sources and cycling of dissolved phosphonates; (4) the composition and bioavailability of LMWDOP; and (5) the frequency and specificity of microbial phosphonate metabolism.