Conversion factors for estimation of cell production rates of soil bacteria from [ 3H]thymidine and [ 3H]leucine incorporation

Abstract

Thymidine and leucine incorporation into NaOH-extractable macromolecules were examined as methods for estimating bacterial production rates in soil. Six out of ten bacterial strains isolated from soil incorporated thymidine and all incorporated leucine. Three of the strains which incorporated [ 3H]thymidine were grown as a mixture in four continuous cultures with generation times of 1.12–4.12 days. Empirical conversion factors were determined by simultaneous measurement of thymidine and leucine incorporation and bacterial cell and carbon production. Isotope dilution was determined at each generation time to enhance the accuracy of incorporation measurements. Empirical conversion factors for thymidine ranged from 0.2 to 1.1 × 10 18 cells mol −1 with a mean of 0.54 × 10 18 cells mol −1. Empirical conversion factors for leucine (0.38–1.8 × 10 16 cells mol −1) were low when compared to previously determined conversion factors. Significant differences among empirical conversion factors measured for leucine at the four generation times were found, suggesting a dependence of the empirical conversion factor for leucine on generation time. Addition of gamma-sterilized soil before incorporation measurements did not effect thymidine incorporation. Leucine incorporation, however, was significantly increased by an enhanced extraction of leucine-labelled macromolecules. We conclude that conversion factors for calculation of in situ cell production of soil bacteria from [ 3H]thymidine incorporation are in the same range as conversion factors published for aquatic bacteria. Thymidine incorporation offers a direct quantitative estimate of bacterial cell production in soil. A drawback of the thymidine method is that not all soil bacteria are able to incorporate thymidine, which may lead to conservative production estimates. The leucine method needs further investigation before it is applicable to natural soil samples.