MICROBIAL CONTROL OF DISSOLVED ORGANIC-CARBON IN LAKES - RESEARCH FORTHE FUTURE

The dissolved organic carbon (DOC) of lakes dominates any budget of or ganic carbon in these systems. Limnologists are still limited by techn iques and particularly by the lack of measurements of rates of microbi al transformation and use of this DOC. There are now four different ap proaches to the study of the microbial control of DOC in lakes. The fi rst is through measurements of the total DOC. Recent advances in measu rement with high temperature combustion will likely lead to higher and more consistent measurements in freshwaters than previously. It is po ssible that a biologically active fraction may be identified. The seco nd approach is through measurements of microbial incorporation and res piration of C-14-labeled organic matter. The kinetics of this process are well known but advances in measurement of the size of the substrat e pool are still being made. The third approach is to use bacterial gr owth in batch or continuous flow experiments in order to understand ho w much of the total DOC can be decomposed by microbes. The assay in th is approach may be microbial growth (thymidine incorporation, biomass changes) or change in the DOC (total concentrations, specific compound s, or fractions of the DOC by molecular weight). These methods are pro mising but are not developed enough for routine use. For example, grow th measurements in the laboratory are all subject to experimental arti facts caused by changes in the DOC and in the bacterial populations. F inally, the fourth approach is through the use of isotopes of the natu ral DOC. In the sea this approach has given the age of the bulk DOC (C -14 data). In freshwaters it has great potential for differentiating b etween bacterial use of terrestrial DOC vs. use of algal-derived DOC ( C-13 data). Stable isotopes are also useful for experimentally labelin g DOC produced by algae and following the use of this material by bact eria.