Consequences of Thymidine Catabolism for Estimates of Bacterial Production: An Example from a Coastal Marine Sediment

Radioactively labeled thymidine (TdR) has been used extensively to measure bacterial production in aquatic environments, but critical assumptions of the TdR technique often have gone untested. In this study of a coastal marine sediment, the metabolic fate of methyl [3H]TdR and methyl [14C]TdR was at variance with the assumptions necessary for determining bacterial production. Only 2% of incorporated radioactivity was recovered in the DNA fraction of TCA-insoluble material following time-course incubations of l-300 min. At least the methyl group of TdR was extensively catabolized, as shown by copious production of 14C0,. The temporal patterns of 3H : 14C ratios in macromolecuAcknowledgments We thank P. LaRock for helpful discussions and use of his laboratory, M. Foy for assistance with the experiment, and D. Karl for advice on technical aspects of DNA extraction. K. Hofer and M. Schneiderman provided comments on our results. The manuscript was improved by comments from R. Iverson, P. LaRock, D. Thistle, and three reviewers. This research is Florida State University Marine Laboratory Contribution No. 1046 and was funded in part by Sigma Xi Grants-in-Aid-of Research (F.C.D. and K.R.C.) and NSF Grant OCE 83-09967 (P. A. LaRock). lar fractions indicated that products of catabolism were recycled into the DNA fraction. The accuracy of the TdR technique depends in large part on the degree to which such catabolism occurs. Estimation of bacterial production in aquatic environments is of interest to microbial ecologists, organic geochemists, limnologists, and oceanographers, among others. There has been a recent proliferation of such estimates, accompanied by controversy regarding the technique of choice (e.g. Hagstriim et al. 1979; Karl 1986; Moriarty 1986; Fuhrman et al. 1986; Karl and Winn 1986). Many investigators use radiolabeled precursors of nucleic acids and, for example, incorporation of radioactivity into deoxyribonucleic acid (DNA) as a measure of bacterial growth. Undoubtedly the most popular precursor has been the nucleoside thymidine (thymidine deoxyribose, hereafter TdR); methyl [3H]TdR is the most frequently used form. The expectation has been