Phosphorus dynamics in freshwater lakes: Examining the factors that control phosphate uptake by bacterioplankton communities

Although bacterioplankton are responsible for the majority of the phosphate taken up by planktonic communities in most freshwater lakes, factors that controls phosphate uptake by bacterioplankton are not well understood. In this study, the hypothesis that phosphate and dissolved organic carbon concentrations are the major factors that control bacterial phosphate uptake was tested using laboratory culture studies, nutrient amendment experiments on natural bacterioplankton assemblages, and field investigations in lakes of different trophic status. The role of community structure of bacterioplankton assemblages on phosphate uptake was also studied using cross-dialysis experiments on bacterioplankton communities of different species compositions.; Both phosphate and labile dissolved organic carbon (LDOC) concentrations significantly influenced phosphate uptake by bacterioplankton. Bacterial phosphate uptake increased with increasing phosphate concentrations but decreased with increasing LDOC concentration. The decline of bacterial phosphate uptake velocity at high LDOC concentration was apparently not caused by growth rate suppression. Bacterial cellular phosphorus content was lower at higher LDOC concentration, suggesting that bacterioplankton may use phosphate more efficiently when LDOC is readily available.; Chemical environment, especially LDOC concentration, is more important than community structure in controlling phosphate uptake by bacterioplankton. Phosphate uptake by communities of different species compositions differed significantly when dialyzed against different chemical environments, but it became similar when dialyzed against the same chemical environment, In contrast, phosphate uptake by the same community differed when dialyzed against different chemical environments. These findings indicated that phosphate uptake by bacterioplankton is mainly determined by chemical environments. Species composition of a community or the contribution from each component species to overall phosphate uptake by the community may change accordingly when exposed to different chemical environments,; These findings allow development of a novel bioassay technique, bacterial phosphate uptake and growth (BPUG) test, for estimation of available phosphate concentration in ambient water by comparing bacterial phosphate uptake and phosphate requirement for bacterial growth. Ambient phosphate concentration estimated using this technique ranged from subnanomolar to nanomolar, suggesting that the ambient phosphate concentrations estimated using conventional techniques may be overestimations.