Utilization of dissolved phosphorus compounds by bacteria and algae in lakes.

Abstract

Uptake of phosphate and hydrolysis and uptake of dissolved organic phosphorus (DOP) by bacteria and algae were examined in two lakes. Alkaline phosphatase was purified from three strains of bacteria isolated from pelagic, epiphytic and epipelic habitats in a lake and kinetic parameters were measured. The epipelic strain had the greatest capacity to hydrolyze organic phosphorus and the pelagic strain the least capacity. All three strains were competitively inhibited by phosphate. In situ measurements of alkaline phosphatase activity indicated that enzyme activity was greater in the littoral region of the lake than the pelagic region. Therefore, it was suggested that organic carbon availability to bacteria may be important in regulating their ability to hydrolyze DOP. Differences in 5$\sp\prime$-nucleotidase (5PN) activity were compared at two sites in an oligotrophic lake and a eutrophic lake. Activities of 5PN were comparable between sites within oligotrophic Lake Michigan and eutrophic Third Sister Lake. However, a greater percent of the hydrolyzed phosphate was taken up into particles at the inshore site in Lake Michigan and in Third Sister Lake. Differential size-fractionation of hydrolyzed phosphate uptake showed that uptake into particles in the bacterial size-fraction was greater than algal uptake at the nearshore Lake Michigan site and in Third Sister Lake, whereas uptake of hydrolyzed phosphate into the algal size-fraction was greater than bacterial uptake at the offshore Lake Michigan site. It is hypothesized that these differences are related to differences in loading and availability of organic carbon at these sites. Differences in phosphate and uptake and hydrolysis of DOP by algae and bacteria were compared in Third Sister Lake. Most phosphate uptake was by bacteria at low phosphate concentrations and by algae at high phosphate concentrations. Both phytoplankton and bacterioplankton took up phosphate more readily than phosphorus hydrolyzed from DOP. Phytoplanktonic and bacterioplanktonic uptake rates of phosphate from DOP were nearly comparable at low concentrations of DOP and most uptake was by phytoplankton at high concentrations, which suggests that phytoplankton used DOP more effectively as a phosphorus source than bacteria. Long-term measurements with size-fractionated microflora indicated that some of the phosphorus initially taken up into the bacterial size-fraction was eventually lost. It is concluded that phytoplankton can compete with bacteria by assimilating and storing excess phosphate and phosphorus from DOP when they are available at above-ambient concentrations.