An Explanation for the Variability in the Conversion Factor Deriving Bacterial Cell Production from Incorporation of $[^3H]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 lake-water 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 $(\thickapprox 1 \times 10^18 cells mol^-1)$.