Bacterial carbon production in the northern Baltic: A comparison of thymidine incorporation and FDC based methods

Bacterial production in the open Gulf of Finland was estimated by H-3-thymidine incorporation rate (TTI) and by the frequency of dividing cells (FDC), counted by transmission electron microscopy (TEM). To relate TTI and FDC to bacterial growth rate, batch cultures with the natural bacterial community from the study area were used. Conversion factors between 0.2 and 7.2 x 10(18) cells mol(-1) TdR, or 0.3 and 24.3 x 10(17) mu m(3) mol(-1) TdR, were obtained for TTI, depending on the season and calculation model used (integrative, cumulative, or modified derivative). From FDC and growth rate (mu) based on bacterial cell numbers, the following equation was derived: mu = 0.002FDC - 0.001. Bacterial carbon production in the study area, estimated using the FDC method, was 20 to 80% higher than TTI-based estimates. Virus abundances were followed in the batch cultures and in the field study. Viral lysis may have been a significant cause of bacterial mortality in the field, but it remains to be shown whether viral-induced mortality was already included in the empirical conversion factor.