Collaborative Research: Predicting Controls of Partitioning between Dissimilatory Ntirate Reduction to Ammonium (DNRA) and Dinitrogen Production in Marine Sediments

海洋沉积物中的微生物过程在全球氮循环中起着重要作用。由于溶解在海水中的氮化合物的存在在很大程度上控制着生物的生长，了解沉积氮收支如何随着环流、酸度和生物生产力的改变而变化，对于预测未来气候情景下海洋的功能至关重要。令人惊讶的是，我们不能确切地知道沉积物和水柱之间的氮交换是否平衡，如果不是，它是如何随时间和空间变化的。我们确实知道，两种细菌介导的化学反应通过将生物可用的溶解态氮转化为通常无法用于生物生产的氮气（N2），主要负责从海洋生态系统中去除氮。这些反应被称为反硝化和厌氧氨氧化（anammox）；后者只在氧为零的情况下工作。该项目将研究第三种反应，称为异化硝酸还原反应（DNRA），它与厌氧氨氧化反应直接竞争，以限制N2的产生和随之而来的氮的“损失”，从而保留氮用于海洋生态系统。DNRA的作用尚未被充分探索，量化这一反应有助于评估海洋系统的总体氮平衡。这里的研究人员将使用含有收集的海洋沉积物的新型实验反应器，并通过不同的环境条件（pH值、温度、氧气、有机碳），发现并量化控制沉积物中DNRA、反硝化和厌氧氨氧化速率的因素。这将为理论沉积氮模型提供直接测试和进一步发展，该模型可用于预测未来各种气候情景下全球氮循环的可能变化。两名研究生将参与缅因海岸观测联盟（MCOA）和缅因湾研究所（GOMI）的研究和合作，扩大参与研究所（IBP）将促进少数民族学生的参与和加强外联活动。本项目以热力学计算和经验证据为基础，评价了有效有机碳(C)与硝态氮（NO3-）的比值是氮氧化还原分配的关键控制因素；较高的比率被认为有利于异化硝酸盐还原（DNRA）而不是N2的产生。研究人员的理论模型预测了DNRA和N2产量之间的快速可逆转变，而C/NO3-的变化相对较小。这表明，分区可能对有机碳沉积的季节和年际差异以及控制硝酸盐流向沉积物的过程（如水柱分层）敏感。沉积C/NO3-与氮分配之间的定量关系仍然不明确，并且已知包括T、H2S和Fe（II）在内的许多其他因素会影响N分配。本研究将通过提出的理论沉积氮模型来预测氮氧化还原分配与C/NO3-之间的关系，进而探讨H2S/NO3-之间的关系。实验将在提供硫化氢（H2S）和13c标记的碎屑作为电子供体的情况下改变NO3通量，并测量15NO3-在薄板反应器中转化为15NH4+和29/30N2的速率，以确定DNRA和N2的生产速率和途径。建议将这些实验与基于理论的生物地球化学模型相结合，将对海洋氮循环产生可量化和可测试的理解。该研究为定量预测海洋生态系统氮保留与损失之间的沉积平衡提供了重要的应用前景。

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.