Measurements of bacterial production in coastal marine environments using leucine: application of a kinetic approach to correct for isotope dilution

Abstract

Bacterial carbon production was measured daily by means of leucine and thymidine incorporation in samples from 2 locations in the eutrophic Roskilde Fjord, Denmark. During periods of 2 wk in March, June, and September 1992, rates of leucine incorporation using 50 nM leucine were significantly correlated (r = 0.88, n = 77) to thymidine-based production rates. To correct for external isotope dilution, the maximal rate of leucine incorporation (V,,,,,) was calculated from additions of subsaturatIng concentrations of I4C leucine (1 to 100 nM). During September, the average isotope dilution was 2.7 (SD = 1.4, n = 25) and 2.0 [SD = 0.8, n = 25) times higher than rates obtained from additions of 50 or 100 nM leucine. respectively. The correlation coefficient r between leucine and thymidine increased from 0.79 to 0.88 when leucine incorporation rates were corrected for isotope dilution. In these eutrophic samples, it is necessary to add more than 200 nM to maximize leucine incorporation and to reduce isotope dilution.