An explanation for the variability in the conversion factor deriving bacterial cell production from incorporation of [3 H] thymidine

The empirical conversion factor deriving bacterial cell production from incorporation of [3H]thymidine (cells nmol−1) varies by orders of magnituide in the literature. An empirical conversion factor is not realistic unless the concentration of [3H]thymidine used maximizes the degree of participation (DP) of [3H]thymidine in DNA synthesis equally in both unaltered lakewater samples and in lake-water cultures. I find that a higher [3H]thymidine concentration (>20 nM) than normally used for routine samples (5–10 nM) may be required for maximal incorporation into DNA in lake-water cultures, at least in spring. Thus a conversion factor with the latter [3H]thymidine concentrations will be an overestimate. This hypothesis is supported by an analysis of the literature indicating that with concentrations of [3H]thymidine > 10 nM, the empirical conversion factors most often approach the expected theoretical factor (≈1 × 1018 cells mol−1).