Heterotrophic bacteria are key biogeochemical regulators in freshwater systems. Bacteria are key drivers of both the decomposition and production of organic matter. In this way, bacteria link multiple biogeochemical cycles including carbon and phosphorus. To date, much of the work on dissolved organic matter transformations has been carbon centric, but improving our understanding of the important links between carbon and phosphorus cycling will require more information about how organic matter transformations impact multiple nutrient. In this study, a culture-based laboratory experiment was used to examine the production of dissolved organic matter by heterotrophic bacteria under varied nutrient conditions, ranging from likely carbon limitation to strong phosphorus limitation. In addition to quantifying the production of dissolved organic carbon, we also measured the production of dissolved organic phosphorus and characterized the microbially-produced organic matter using optical properties.
Results/Conclusions
We found measurable amounts of dissolved organic carbon and dissolved organic phosphorus were produced by heterotrophic bacteria under nutrient conditions ranging from carbon-limitation to strong phosphorus-limitation. Dissolved organic phosphorus production was highest when phosphorus was abundant, but even under strongly phosphorus limited conditions as much as 5% of the provided phosphate was converted to dissolved organic phosphorus. Additionally, the varied nutrient conditions resulted in strong differences in the optical properties of the produced organic matter. SUVA254 values revealed that organic matter produced under carbon limited conditions was highly aromatic with similar optical properties to terrestrially derived organic matter. Lastly, estimated growth efficiencies showed that under strong phosphorus limitation production of dissolved organic carbon was similar in magnitude to respired carbon, indicating that dissolved organic production may be important for dealing with excess carbon in phosphorus limited systems. Overall, these findings suggest that heterotrophic bacteria can be important producers of organic matter in freshwaters and that continued trends of increased nutrient concentrations (eutrophication) may fundamentally change the composition of microbially produced organic matter in freshwater systems.