Evidence against incorporation of exogenous thymidine by sulfate‐reducing bacteria

The uptake of exogenous [ 3 H]thymidine by SO 4 2− ‐reducing bacteria (SRB) was tested in both pure culture and in marine sediments. Ten cultures isolated from mesohaline sediments of Chesapeake Bay, plus ATCC strain Desulfovibrio desulfuricans aestuarii , were incubated in SO 4 2− ‐reducing media with [ 3 H]TdR concentrations ranging from 2 to 100 nM. In most cases [ 3 H]TdR uptake levels were no more than a few times higher than Formalin‐killed controls; SRB cells took up a maximum of 1.7 × 10 −22 moles TdR cell −1 h −1 . Across all incubations, an average of 3.3 × 10 23 SRB cells were produced (mol [ 3 H]TdR) −1 taken up, compared to an average value of ∼2 × 10 18 cells (mol [ 3 H]TdR) −1 in natural, oxic waters. In anoxic sediments the addition of molybdate, an inhibitor of SO 4 2− reduction, significantly reduced amino acid metabolism but did not decrease [ 3 H]TdR incorporation into TCA‐insoluble material. These data suggest that the thymidine uptake method may drastically underestimate bacterial production in samples where SO 4 2− reduction is quantitatively important, such as nonsurficial marine and estuarine sediments and anoxic saline waters.