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发挥着重要作用。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，因此了解沉积氮收支如何随着环流、酸度和生物生产力的变化而变化，对于预测未来气候情景中的海洋功能至关重要。令人惊讶的是，我们不确定沉积物和水柱之间的氮交换是否平衡，如果不平衡，它如何随时间和空间变化。我们知道，两种细菌介导的化学反应主要负责从海洋生态系统中去除氮，方法是将生物可用形式的溶解氮转化为通常不能用于生物生产的氮气(氮气)。这些反应被称为反硝化和厌氧氨氧化(Anammox)；后者只在氧气为零的地方运行。该项目将研究第三种反应，称为异化硝酸盐还原(DNRA)，它与厌氧氨氧化直接竞争，限制氮气的产生和随之而来的氮素的“损失”，从而保留氮气用于海洋生态系统。DNRA的作用尚未得到充分研究，量化这一反应有助于评估海洋系统的整体氮平衡。这里的研究人员将使用包含收集的海洋沉积物的新型实验反应器，通过改变环境条件(pH、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的速率的因素。这将为理论沉积氮模型提供直接测试和进一步发展，这些模型可用于预测全球氮循环可能发生的变化，从而导致各种未来气候情景。两名研究生将参与这项研究，并与缅因州海岸观测联盟(MCOA)和缅因湾研究所(GOMI)以及扩大参与研究所(IBP)合作，将产生少数族裔学生的参与和增强的外展活动。该项目使用热力学计算和经验证据作为基础，评估有效有机碳(C)与硝酸盐(NO3-)的比率作为氮氧化还原分配的关键控制因素；与氮气生产相比，更高的比率被认为有利于异化硝酸盐还原(DNRA)。研究人员的理论模型预测，通过C/NO3-相对较小的变化，DNRA和氮气生产之间快速且可逆的转变。这表明，分配可能对有机碳沉积的季节性差异以及可能的年际差异以及控制硝酸盐通量到沉积物的过程(如水柱分层)很敏感。沉积C/NO3-和氮分配之间的定量关系仍然不明确，而且已知还有许多其他因素，包括T、H 2S和Fe(II)，影响N分配。本研究将探讨氮的氧化还原分配与C/NO3-的关系以及由此引申出来的H_2S/NO_3~-之间的关系是由所提出的沉积氮理论模型来预测的假设。实验将在提供硫化氢(H_2S)和标记13C的碎屑作为电子供体的情况下改变NO_3的通量，并测量薄盘反应器中15NO_3~-到15NH_4~+和29/30N_2的转化速率，以确定DNRA和N_2的生成速率和途径。建议将这些实验与以理论为基础的生物地球化学模型相结合，将发展对海洋氮循环的可量化和可检验的理解。这项研究应提供一项重大进展，可广泛应用于定量预测海洋生态系统中氮的保留和损失之间的沉积平衡。

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.