The effect of bacterial specific growth rates of abundance (mu) and protein synthesis (b) on conversion factor (CF) variability was explored in order to provide an alternative approach to the controversial application of just one universal CF to field data. Nine regrowth cultures (RCs) were set up from very diverse aquatic ecosystems, controlling temperature and adding N and P to avoid mineral limitation and force organic carbon limitation. The values of p varied one order of magnitude from 0.26 to 3.34 d(-1), whereas b values varied two orders of magnitude from 0.28 to 34.87 d(-1). We found no relationships between V or b values and the dissolved organic carbon (DOC) concentration or the dissolved organic matter (DOM) quality indexes assayed. Abundance and protein synthesis increased exponentially and synchronously in four RCs, leading to balanced growth (mu = b). In contrast, abundance and protein synthesis increased logistically in the other five RCs and b values were significantly higher than p values, leading to unbalanced growth (mu not equal b). Us ranged from 0.0062 to 0.0576 x 10(18) cells mol leucine(-1) with an average of 0.0305 x 10(18) cells mol leucine(-1). Us obtained in RCs with balanced growth were generally higher than Us obtained in RCs with unbalanced growth and were not alike, impeding the establishment of an upper limit for Us. A positive and significant relationship (n = 8, p < 0.01**, r(2) = 0.71) was found between Us and DOC concentration (CF (x10(18) cells, mol leucine(-1)) = 0.0104 + 0.0094 DOC (mM)) when the value for the most productive system was excluded. This function permits the estimation of site-specific Us based on DOC concentration instead of the controversial use of a single CIF for different systems.
