Collaborative Research: Predicting controls of partitioning between dissimilatory nitrate reduction to ammonium (DNRA) and dinitrogen production in marine sediments

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Microbial processes in marine sediments play a major role in the global nitrogen cycle. Because the presence of nitrogen compounds dissolved in seawater largely controls biological growth, understanding how the sedimentary nitrogen budget changes with altered circulation, acidity, and biological productivity is of critical importance to predict oceanic function in future climate scenarios. Surprisingly, we do not know definitively if nitrogen exchange between sediments and the water column is in balance, and if not, how it varies over time and space. We do know that two bacterially mediated chemical reactions are primarily responsible for removing nitrogen from marine ecosystems by converting biologically usable forms of dissolved nitrogen back to nitrogen gas (N2) that is not generally available for biological production. These reactions are called denitrification and anaerobic ammonium oxidation (anammox); the latter operating only where oxygen is zero. This project will investigate a third reaction, called dissimilatory nitrate reduction (DNRA), which competes directly with anammox to limit N2 production and the consequent "loss" of nitrogen, thus retaining nitrogen for use in marine ecosystems. The role of DNRA has not been fully explored and quantifying this reaction could help evaluate the overall nitrogen balance in ocean systems. The researchers here will use novel experimental reactors that contain collected marine sediments and, by varying environmental conditions (pH, temperature, oxygen, organic carbon), will discover and quantify what controls rates of DNRA, denitrification, and anammox in sediments. This will provide a direct test and further development of theoretical sedimentary nitrogen models that can be used to predict possible changes in the global nitrogen cycle resulting with various future climate scenarios. Two graduate students will participate in the research and collaborations with the Maine Coastal Observing Alliance (MCOA) and the Gulf of Maine Institute (GOMI), as well as the Institute for Broadening Participation (IBP) will generate minority student involvement and enhanced outreach activity.This project uses thermodynamic calculations and empirical evidence as a basis to evaluate the ratio of available organic carbon (C) to nitrate (NO3-) as a key controlling factor of nitrogen redox partitioning; with higher ratios believed to favor dissimilatory nitrate reduction (DNRA) over N2 production. The investigator's theoretical model predicts rapid and reversible transitions between DNRA and N2 production over relatively small changes in C/NO3-. This suggests that partitioning could be sensitive to seasonal and possibly inter-annual differences in organic C deposition as well as processes that control nitrate flux to the sediments such as water column stratification. Quantitative relationships between sedimentary C/NO3- and nitrogen partitioning remain poorly defined, and a number of other factors including T, H2S, and Fe(II), are known to influence N partitioning. This study will investigate the hypothesis that relationships between nitrogen redox partitioning and C/NO3-, and by extension H2S/NO3-, are predicted by the proposed theoretical sedimentary nitrogen model. Experiments will varying NO3 fluxes while providing hydrogen sulfide (H2S) and 13C-labelled detritus as electron donors, and measure transformation rates of 15NO3- to 15NH4+ and 29/30N2 in thin disc reactors to determine rates and pathways of DNRA and N2 production. The proposed integration of these experiments with a theoretically-based biogeochemical model will develop a quantifiable and testable understanding of the marine nitrogen cycle. This study should provide a major advance that could be broadly applied to quantitatively predict the sedimentary balance between nitrogen retention and loss across marine ecosystems.