Microbial control of dissolved organic carbon in lakes: research for the future

The dissolved organic carbon (DOC) of lakes dominates any budget of organic carbon in these systems. Limnologists are still limited by techniques and particularly by the lack of measurements of rates of microbial transformation and use of this DOC. There are now four different approaches to the study of the microbial control of DOC in lakes. The first is through measurements of the total DOC. Recent advances in measurement with high temperature combustion will likely lead to higher and more consistent measurements in freshwaters than previously. It is possible that a biologically active fraction may be identified. The second approach is through measurements of microbial incorporation and respiration of ¹⁴C-labeled organic matter. The kinetics of this process are well known but advances in measurement of the size of the substrate pool are still being made. The third approach is to use bacterial growth in batch or continuous flow experiments in order to understand how much of the total DOC can be decomposed by microbes. The assay in this approach may be microbial growth (thymidine incorporation, biomass changes) or change in the DOC (total concentrations, specific compounds, or fractions of the DOC by molecular weight). These methods are promising but are not developed enough for routine use. For example, growth measurements in the laboratory are all subject to experimental artifacts caused by changes in the DOC and in the bacterial populations. Finally, the fourth approach is through the use of isotopes of the natural DOC. In the sea this approach has given the age of the bulk DOC (¹⁴C data). In freshwaters it has great potential for differentiating between bacterial use of terrestrial DOC vs. use of algal-derived DOC (13C data). Stable isotopes are also useful for experimentally labeling DOC produced by algae and following the use of this material by bacteria.