THYMIDINE INCORPORATION IN LAKE CISO - PROBLEMS IN ESTIMATING BACTERIAL SECONDARY PRODUCTION ACROSS OXIC-ANOXIC INTERFACES

Heterotrophic bacterial activity was measured by means of the H-3-thymidine (H-3-TdR) incorporation technique in Lake Cis6, a small holomictic lake with an anoxic hypolimnion. We tested several methodological questions across the vertical profile: TdR concentration at which maximal incorporation is reached, linearity of incorporation and isotope dilution, during holomixis and stratification periods. The TdR concentration at which maximal incorporation is reached changed seasonally and vertically. During holomixis, maximal incorporation was not always reached at concentrations up to 40 nM. Uptake was always linear in short incubation times and decreased from epi- to hypolimnion. The isotope dilution technique indicated a degree of participation in DNA synthesis higher than 50%, although a linear relationship between the inverse of H-3-TdR incorporation and increasing 'cold' thymidine concentration was not always observed. Autoradiographic experiments showed a low percentage of bacteria taking up H-3-TdR in both aerobic and anaerobic samples. The percentage of total labeled bacteria seemed to be generally higher in the metalimnion (11% maximal value) than in the hypolimnion. Labeled Amoebobacter and Chromatium cells were detected in field samples. Amoebobacter cells photoassimilated TdR in culture. Therefore, our results show that H-3-TdR incorporation is not an appropriate technique to estimate bacterial secondary production in anaerobic systems and in oxic-anoxic interfaces.