UPTAKE OF DISSOLVED INORGANIC AND ORGANIC PHOSPHORUS-COMPOUNDS BY PHYTOPLANKTON AND BACTERIOPLANKTON

Potential sources of dissolved P to phytoplankton and bacterioplankton were examined in a small meso-eutrophic lake. Kinetic analyses of whole lake water on three dates demonstrated that the maximal rate (V(max)) for phosphate uptake was highest (5.2 nM min-1) in spring. On all dates, size fractionation of plankton and kinetic analyses of uptake indicated that most (>50%) uptake of phosphate was by phytoplankton at V(max) and by bacteria at ambient concentrations. Isotope dilution assays, with either unlabeled phosphate or various dissolved organic P (DOP) compounds, demonstrated that phosphate was the preferred substrate for uptake into both algae and bacteria. Phytoplankton had greater capacity for uptake of P from both phosphate and DOP than bacterioplankton. We concluded that phytoplankton use both phosphate and DOP, particularly at high substrate concentrations, and that bacterial utilization of P may be limited by the availability of organic C or other nutrients.