EVIDENCE AGAINST INCORPORATION OF EXOGENOUS THYMIDINE BY SULFATE-REDUCING BACTERIA

The uptake of exogenous [H-3]thymidine by SO4(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 SO4(2-)-reducing media with [H-3]TdR concentrations ranging from 2 to 100 nM. In most cases [H-3]TdR uptake levels were no more than a few times higher than Formalin-killed controls; SRB cells took up a maximum of 1.7 x 10(-22) moles TdR cell-1 h-1. Across all incubations, an average of 3.3 x 10(23) SRB cells were produced (mol [H-3]TdR)-1 taken up, compared to an average value of approximately 2 x 10(18) cells (mol [H-3]TdR)-1 in natural, oxic waters. In anoxic sediments the addition of molybdate, an inhibitor of SO4(2-) reduction, significantly reduced amino acid metabolism but did not decrease [H-3]TdR incorporation into TCA-insoluble material. These data suggest that the thymidine uptake method may drastically underestimate bacterial production in samples where SO4(2-) reduction is quantitatively important, such as nonsurficial marine and estuarine sediments and anoxic saline waters.