Quantifying nitrogen fixation along unique geochemical gradients in the southwest Pacific Ocean

氮在海洋表层的可用性在调节海洋初级生产率方面起着关键作用，因此，通过改变碳循环，氮有能力影响气候。海洋生物可利用氮的主要来源是通过一个称为二氮（N2）固定的过程，该过程涉及通过称为固氮菌的微生物将溶解在海水中的N2气体还原为铵。虽然近年来利用分子工具在确定海洋固氮生物多样性方面取得了重大进展，但量化全球N2固定率以及确定哪个海洋盆地支持最高通量仍然是一个棘手的问题。这项研究将量化N2固定率及其对支持西南太平洋生产的重要性。这项研究的结果将揭示N2固定（温度，铁浓度）的敏感性以及海洋中氮源和汇的时空耦合程度。这项工作将由一名早期职业科学家进行，并涉及指导年轻女性，中学女生和少数民族，本科生和研究生研究人员的培训，以及国际合作。确定最大的二氮（N2）固定通量的空间分布仍然是化学海洋学的一个重要目标。空间分布可以为我们了解N2固定的环境敏感性以及主要海洋氮（N）源和汇过程相互响应的能力提供信息，从而影响全球碳循环和气候。除了温度之外，还有两个因素是目前关于影响海洋中N2固定空间分布的争论的核心：1）存在足够的铁以满足N2固定微生物的需要，和2）相对于“Redfield”比率低的表面海洋硝酸盐和磷酸盐浓度的绝对浓度以及比率，这被认为有利于固氮微生物。本项目将测试大气粉尘沉积梯度对N2固定率的影响，当地表沃茨具有相对恒定但有利的硝酸盐-磷酸盐浓度时。这项工作将在西南太平洋进行，该地区因其独特的地球化学特征而突出了新的建模工作，预计将有利于显着的N2固定通量。硝酸盐+亚硝酸盐d15 N以及总溶解氮（TDN）浓度和d15 N将在法国邮轮和沉积物捕集器上收集的水柱样品进行测量，以捕获下沉颗粒氮通量。结果将与已发表的工作进行比较，以评估哪些海洋区域支持最大的N2固定通量。

The availability of nitrogen in the surface ocean plays a critical role regulating rates of primary productivity in the ocean, and thus through modification of the carbon cycle, nitrogen has the capacity to influence climate. The dominant source of biologically available nitrogen to the ocean is through a process known as di-nitrogen (N2) fixation, which involves the reduction of N2 gas dissolved in seawater to ammonium by microbes referred to as diazotrophs. While significant progress has been made identifying a diversity of marine diazotrophs in recent years using molecular tools, quantifying global rates of N2 fixation, and identifying which ocean basin supports the highest fluxes, has remained a vexing question. This research will quantify rates of N2 fixation as well as its importance for supporting production in the southwest Pacific Ocean. Results from this research will shed light on the sensitivities of N2 fixation (temperature, iron concentrations) as well as the extent of spatial and temporal coupling of nitrogen sources and sinks in the ocean. The work will be carried out by an early career scientist, and involve mentoring of young women, middle school girls and minorities, training of undergraduate and graduate researchers, and international collaborations.Identifying the spatial distribution of the largest di-nitrogen (N2) fixation fluxes to the ocean remains a critical goal of chemical oceanography. The spatial distribution can inform our understanding of the environmental sensitivities of N2 fixation and the capacity for the dominant marine nitrogen (N) source and sink processes to respond to each other and thus influence the global carbon cycle and climate. In addition to temperature, two factors are at the heart of the current debate over what influences the spatial distribution of N2 fixation in the ocean: 1) the presence of adequate iron to meet the needs of N2 fixing microbes, and, 2) the absolute concentrations as well as ratios of surface ocean nitrate and phosphate concentrations that are low relative to the "Redfield" ratio, which are thought to favor N2 fixing microbes. This project will test the effects of gradients in atmospheric dust deposition on N2 fixation rates when surface waters have relatively constant but favorable nitrate to phosphate concentrations. The work will be carried out in the southwest Pacific, a region highlighted by new modeling work for its unique geochemical characteristics that are expected to favor significant N2 fixation fluxes. Nitrate+nitrite d15N as well as total dissolved nitrogen (TDN) concentration and d15N will be measured in water column samples collected on a French cruise and sediment traps were deployed to capture the sinking particulate N flux. The results will be compared with published work to evaluate which ocean regions support the largest N2 fixation fluxes.