Temperature control of bacterioplankton growth in a temperate large lake

In order to examine the significance of temperature in controlling growth of bacterioplankton in a large lake with a pronounced pelagic zone we performed 12 temperature shift-up experiments year-round in mesotrophic Lake Constance, Germany, with samples from 1, 6, 10 or 20 and 50 m depth. We measured incorporation of C-14-leucine (Leu) and H-3-thymidine (TdR) at 5, 10, 15, 20 and 25 degrees C including the in situ temperature +/-2 degrees C. The results show that during most of the year bacteria at 1 and 6 m were well adapted to the ambient temperature, ranging between 4 and 23 degrees C, because in situ and optimum growth temperatures agreed within 1 to 5 degrees C. At 28 and 50 m, in situ and optimum growth temperatures usually diverged by more than 5 degrees C, and often by more than 10 degrees C, indicating that bacterial growth at these depths was clearly Limited by temperature. Hence, in the upper water column, temperature controlled bacterial growth by selecting for a well-temperature-adapted community whereas in the lower part of the water column temperature directly controlled bacterial growth. In several experiments we observed 2 temperature optima. We interpret this observation as an indication of the existence of 2 different subpopulations with different temperature preferences. Q(10) values of Leu incorporation and TdR incorporation, ranging from 1.4 to 42.5, exhibited systematic differences. At 1, 6 and 10 m, Q(10) of Leu was higher than that of TdR except in 3 cases, whereas at 20 and 50 m Q(10) of TdR was higher than that of Leu except in 3 cases. This observation suggests that protein synthesis (Leu) and DNA replication (TdR) are controlled differently by temperature in differently adapted bacterial communities. In order to test the hypothesis that utilization of the most important substrates is optimized at the optimum growth temperature we carried out 5 additional experiments in which we measured uptake and respiration of dissolved free amino acids and monosaccharides. In 3 experiments we found that the fraction of amino acids respired had a distinct minimum at the optimum growth temperature whereas respiration of monosaccharides had such a minimum only in 1 experiment. The experiments gave no indication of enhanced respiration percentages at minimum growth temperatures.