[Methyl‐3H]thymidine macromolecular incorporation and lipid labeling: Their significance to DNA labeling during measurements of aquatic bacterial growth rate

It is essential during measurements of aquatic bacterial production with [methyl#x2010;3H]thymidine (Tdr) that only labeled DNA is measured. We found in 12 freshwater and marine systems that DNA labeling represented a variable proportion of total macromolecular labeling. Up to 87% of label appearing in precipitated labeled macromolecules from acid#x2010;base hydrolysis treatments was soluble in ethanol. Reverse#x2010;phase, high#x2010;pressure liquid chromatography showed that the composition of labeled molecules in the ethanol was 78–88% [3H]Tdr. The rate of labeling of the ethanol#x2010;soluble fraction was significantly correlated with the rate of total macromolecular labeling (r = 0.88, n = 40, P < 0.001) and less strongly with the DNA labeling rate (r = 0.49, n = 28, P = 0.005). Experiments in which bacterial cells were labeled with [3H]Tdr or32PO43− showed that above a total macromolecular labeling rate of ∼1 pmol Tdr liter−1 h−1, bacterial cells bind Tdr but do not incorporate it into phospholipids in the cell envelope.