CONVERSION FACTORS FOR ESTIMATION OF CELL PRODUCTION-RATES OF SOIL BACTERIA FROM [H-3] THYMIDINE AND [H-3] LEUCINE INCORPORATION

Thymidine and leucine incorporation into NaOH-extractable macromolecul es were examined as methods for estimating bacterial production rates in soil. Six out of ten bacterial strains isolated from soil incorpora ted thymidine and all incorporated leucine. Three of the strains which incorporated [H-3]thymidine were grown as a mixture in four continuou s cultures with generation times of 1.12-4.12 days. Empirical conversi on factors were determined by simultaneous measurement of thymidine an d leucine incorporation and bacterial cell and carbon production. Isot ope dilution was determined at each generation time to enhance the acc uracy of incorporation measurements. Empirical conversion factors for thymidine ranged from 0.2 to 1.1 x 10(18) cells mol-1 with a mean of 0 .54 x 10(18) cells mol-1. Empirical conversion factors for leucine (0. 38-1.8 x 10(16) cells mol-1) were low when compared to previously dete rmined conversion factors. Significant differences among empirical con version 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. Leu cine incorporation, however, was significantly increased by an enhance d extraction of leucine-labelled macromolecules. We conclude that conv ersion factors for calculation of in situ cell production of soil bact eria from [H-3]thymidine incorporation are in the same range as conver sion factors published for aquatic bacteria. Thymidine incorporation o ffers a direct quantitative estimate of bacterial cell production in s oil. A drawback of the thymidine method is that not all soil bacteria are able to incorporate thymidine, which may lead to conservative prod uction estimates. The leucine method needs further investigation befor e it is applicable to natural soil samples.