P-limitation drives changes in DOM production by aquatic bacteria

Heterotrophic bacteria are key biogeochemical regulators in freshwater systems. Through both decomposition and production of organic matter, bacteria link multiple biogeochemical cycles together. While there has been a significant amount of work done on understanding the role of microbes in the aquatic carbon cycle, important linkages with other biogeochemical cycles will require more information about how organic matter transformations impact other nutrients, such as phosphorus. In this study, we conducted a culture-based laboratory experiment to examine the production of dissolved organic matter (DOM) by heterotrophic bacteria under varied nutrient conditions. In addition to quantifying the production of dissolved organic carbon (DOC), we also measured the production of dissolved organic phosphorus (DOP) and characterized the microbially produced organic matter using optical properties. Results demonstrated that measurable amounts of DOC and DOP were produced by heterotrophic bacteria under nutrient regimes ranging from carbon-limitation to strong phosphorus-limitation. Additionally, optical characterization of DOM revealed that the organic matter produced by bacteria grown under high phosphorus conditions was highly aromatic with similar optical properties to terrestrially derived organic matter. 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.