Carbon and phosphorus regulating bacterial metabolism in oligotrophic boreal lakes

This study focused on how phosphorus and carbon control pelagic bacteria in lakes over a gradient of dissolved organic carbon (DOC from 6.7 to 29.5 mg C L−1) and phosphorus (P-tot from 5 to 19 µg L−1). Five oligotrophic lakes in southern Sweden were sampled in late autumn. Phosphate-P and glucose-C alone or in combination (0.01 and 0.3 mg L−1, respectively) was added to 1.0 µm filtered lake water and incubated in darkness at 20°C. Additions of glucose (C) and phosphorus (P) alone did not lead to changes in the rates of bacterial metabolic processes, whereas bacterial respiration and bacterial production responded positively to C + P enrichment for most of the lakes sampled. Bacterial growth efficiency showed a wide range (2.5–28.7%) and low mean value (12%). These variations were not correlated with the DOC concentration. Our results show that heterotrophic bacterial carbon mineralization in this kind of system during autumn is conditioned by the combined availability of labile carbon and phosphorus, with the assimilated carbon mainly transformed to inorganic carbon in respiration, contributing to CO2 supersaturation in these systems.