Incorporation of thymidine and leucine in the subarctic Pacific: Application to estimating bacterial production

Abstract

This study examines the use of incorporation rates of thymidine (TdR) and leucine (Leu) in order to estimate bacterial production in the oceanic subarctic Pacific. Incorporation rates at different concentrations suggest that isotope dilution is low when 5 nM TdR is used, but other data indicate at least 3-fold and 2-fold dilution for TdR and Leu, respectively. The ratio of incorporation into the hot acid insoluble fraction, compared to the cold acid insoluble fraction, is 0.66 for TdR and 0.86 for Leu. Empirical conversion factors were measured in 10 experiments during four 1 mo-long cruises. Conversion factors did not vary systematically and were not affected by additions of organic compounds or ammonium. Average conversion factors were 1.74 × 1018 cells mol-1 of incorporated TdR (SD = 0.89 × 1018 cells mol-1) and 0.108 × 1018 cells mol-1 of incorporated Leu (SD = 0.085 × 1018 cells mol-1). It was possible to estimate 'bottle-less' conversion factors during May 1988 when bacterial numbers increased greatly; in situ bottle-less conversion factors were 1.55 and 0.185 × 1018 cells mol-1 for TdR and Leu, respectively. Estimates of bacterial production based on the TdR and Leu methods agreed within 10 and 25 % for 0 to 40 m (ca the mixed layer) and 40 to 80 m (bottom half of the euphotic zone), respectively.