Incorporation of thymidine, adenine and leucine into natural bacterial assemblages

Incorporation of 3 ~-t h y m i d i n e and 3H-adenine into DNA and 'H-leucine into protein were measured in diluted batch cultures of natural assemblages of coastal marine and freshwater bacteria. Incorporation rates of DNA and protein-synthesis were compared with changes in bacterial DNA and carbon pools. Incorporation rates of all 3 substances peaked before maximum cell densities were reached. 3 ~-l e u c i n e incorporation was somewhat higher than both "-thymidine and 3H-adenine incorporation during early exponential and stationary growth periods. The molar ratio of incorporated %-adenine and 3H-thymidine was 1:l in freshwater and 3:1 in seawater cultures, and the cellular DNA content ranged from 2 to 4 fg cell-', corresponding to a n average of 5.5 "i, of cell carbon. DNA contents predicted from incorporation rates of adenine were 66 to 86 % and of thymidine 28 to 77 % of the measured DNA content. Conversion factors to derive bacterial cell numbers were 0.85 to 1.75 X 1018 cells mol-' thymidlne ~ncorporated and 0.47 to 0.92 X 10" cells mol-' adenine incorporated. Carbon production predicted from incorporation rates of 'H-thymidine, 'H-adenine or 'H-leucine differed by a t most 31 % from measured carbon production, suggesting that all 3 methods were applicable to determine growth rates and carbon production in the batch cultures.