Summary: A freshwater sp. was grown in continuous culture under steady-state conditions in L-lactate-, succinate-, glucose- or ammonium-limited media. Under carbon limitation, the NAD(H) (i.e. NAD + NADH) concentration of the organisms increased exponentially from approximately 2 to 7 μmol/g dry wt as the culture dilution rate was decreased from 0.5 to 0.02 h. Organisms grown at a given in any of the carbon-limited media possessed very similar levels of NAD(H). Therefore, under these conditions, cellular NAD(H) was only a function of the culture and was independent of the nature of the culture carbon source. had no influence on the NAD(H) content of cells grown under ammonium limitation. In contrast, cellular NADH concentration was not influenced by in carbon- or ammonium-limited media. In L-lactate-limited medium, bacteria possessed 0.14 μmol NADH/g dry wt: very similar levels were found in organisms grown in the other media. The results are consistent with those of Wimpenny & Firth (1972) that bacteria rigidly maintain a constant NADH level rather than a constant NADH:NAD ratio. NADP(H) (i.e. NADP + NADPH) and NADPH levels were also not influenced by changes in the culture carbon source or in in L-lactate-limited medium these concentrations were 0.97 and 0.53 μmol/g cell dry wt, respectively. The NADPH:NADP(H) ratio was much higher than the NADH:NAD(H) ratio, averaging 55% in carbon-limited cells.


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