Theoretical and empirical conversion factors for determining bacterial production in freshwater sediments via leucine incorporation

Bacteria play an extraordinarily important role in carbon transformations. It is therefore crucial to accurately measure bacterial production. One of the most widely used methods is the leucine method. From rates of leucine incorporation bacterial carbon production can be derived by empirical or theoretical conversion factors (CFs). However, only few empirical CFs have been established, and no estimation of the theoretical conversion factor for freshwater systems exists until today. Therefore, we determined both, the empirical and the theoretical conversion factor, of sediment bacteria from a headwater stream. The empirical conversion factor determined from exponentially growing bacteria was 1.445 kg C mol(-1). The theoretical conversion factor derived from the determination of the molar fraction of leucine in bacterial protein (0.081 +/- 0.001) was 1.442 kg C mol(-1). Both conversion factors are close to each other and similar to conversion factors established for marine bacterioplankton. Therefore, results of the present study indicate that high values of bacterial production determined in freshwater sediments by the leucine method in several studies were not overestimates caused by inappropriate use of CFs from marine systems but represent true high bacterial production in these environments. For studies that apply the leucine technique in freshwaters, we recommend using the theoretical conversion factor for calculation of bacterial carbon production: BCP (kg) = 1.44 x Leu(inc) (Leu(inc) = leucine incorporation in mol).