phosphorus availability

We investigated bacterial responses to variations in dissolved organic carbon (DOC) and nutrient availability by a comparative analysis of bacterial metabolism in lakes ranging from oligotrophic to eutrophic. Bacterial growth, respiration, and growth efficiency were quantified in lake water dilution cultures performed in 20 lakes located in eastern Quebec, Canada, which varied with respect to both DOC and nutrient concentrations. Intrinsic growth rates of the bacteria ranged from 0.1 to 1.4 d21, bacterial cell-specific respiration rates ranged from 0.4 to 7.2 fg C cell21 h21, and growth efficiencies ranged from 6.7 % to 51.6%. These variations were unrelated to bulk DOC concentra-tions. Instead, growth rate and efficiency were positively related to total phosphorus concentrations. Specific res-piration rate, on the other hand, decreased with increasing phosphorus concentrations, and the magnitude of respi-ration, on a per-cell basis, strongly influenced observed growth efficiencies. In a series of substrate enrichment experiments, additions of glucose alone failed to stimulate a response in growth rate, mean cell biovolume, or the potential biomass yield in dilution cultures, but all responded positively to phosphorus additions. Our results show that bacterial metabolism and the fate of DOC input to lake microbial communities are strongly dependent on phosphorus availability, rather than total carbon availability. Extreme oligotrophy appears to place high respiratory demands on the bacterioplankton, resulting in very low bacterial growth efficiencies and consequently greater DO