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

海洋表层氮的可用性在调节海洋初级生产力方面起着关键作用，因此，通过改变碳循环，氮具有影响气候的能力。海洋可利用的生物氮的主要来源是通过一种称为双氮（N2）固定的过程，该过程涉及被称为重氮营养体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重氮营养体多样性方面取得了重大进展，但量化全球氮固定率以及确定哪个海洋盆地支持最高通量仍然是一个令人困惑的问题。本研究将量化N2固定速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对N2固定（温度、铁浓度）的敏感性，以及海洋中氮源和氮汇的时空耦合程度。这项工作将由一名早期职业科学家进行，并涉及对年轻女性、中学女生和少数民族的指导，对本科生和研究生研究人员的培训，以及国际合作。确定海洋中最大的二氮（N2）固定通量的空间分布仍然是化学海洋学的一个关键目标。空间分布可以帮助我们了解海洋氮固定的环境敏感性，以及主要的海洋氮源和氮汇过程相互响应的能力，从而影响全球碳循环和气候。除了温度之外，目前关于影响海洋中N2固定的空间分布的两个因素是争论的核心：1)存在足够的铁来满足N2固定微生物的需要，以及2)相对于“Redfield”比率而言，海洋表面硝酸盐和磷酸盐浓度的绝对浓度和比例较低，这被认为有利于N2固定微生物。本项目将测试在地表水具有相对稳定但有利的硝酸盐对磷酸盐浓度时，大气粉尘沉降梯度对N2固定率的影响。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受到新的建模工作的强调，预计将有利于显著的N2固定通量。将测量在法国邮轮上收集的水柱样品中的硝酸盐+亚硝酸盐d15N以及总溶解氮（TDN）浓度和d15N，并部署沉积物捕集器来捕获下沉的颗粒N通量。这些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.