Variability and predictability of the empirical conversion factor for converting ³H-thymidine uptake into bacterial carbon production for a eutrophic lake

To assess the variability and predictability of the empirical conversion factor (CF) for converting the uptake of ³H-thymidine (TDR) to bacterial carbon production rates in eutrophic environments, we performed 11 dilution culture experiments and results were related to a set of important concomitantly recorded environmental variables. TDR incorporation data from a field study were then converted to carbon production with the established empirical CF and a theoretical CF and compared to production estimates derived from leucine incorporation. Mean empirical CF varied between 0.5 and 7.0 × 10⁶ cells (pmol TDR)^-1 over the year and showed highly significant negative correlation with TDR incorporation rate and a highly significant positive correlation to temperature. Values of carbon production derived with the variable empirical CF were lower than the values obtained by the use of the theoretical CF of 1 × 10⁶ cells (pmol TDR) ^-1. They showed less seasonal variation than values obtained by leucine incorporation, and periods of uncoupling were observed. However, when the empirical CF was calculated from the multiple regression equation including TDR incorporation and temperature, the resulting carbon production rates showed a high correspondence with the leucine-derived production rates. Results of the analyses were interpreted as an indication that under favourable conditions (resulting in high TDR incorporation) bacteria may be able to optimize DNA duplication over protein synthesis as a possible strategy to persist and to maintain their potential to divide under limiting conditions (e.g. decrease in temperature and substrate availability). In unfavourable conditions (resulting in low incorporation rates), bacteria may then use already produced DNA copies for rapid growth, when the environmental conditions turn favourable again. Thus, an experimental set-up causing nutrient enrichment of the sampled water by autoclaving and filtering, as generally used for dilution culture experiments, does not always reflect in situ situations, especially during periods of low nutrient concentrations.