Collaborative Research: Dinitrogen fixation rates and diazotrophic communities in contrasting oxygen regimes of the Eastern Pacific Ocean

世界上存在三个主要的公海氧气最低区（OMZ）：东部热带北太平洋（ETNP）、东部热带南太平洋（ETSP）和阿拉伯海。 OMZ 是反硝化作用（包括厌氧氨氧化）的重要区域，代表着海洋中固定氮 (N) 的显着损失。然而，最近的多项证据表明，通过二氮（N2）固定和反硝化作用进行的氮输入在空间中的耦合可能比之前提出的更为紧密。尽管关于可能与 OMZ 相关的 N2 固定位置和强度的地球化学推论以及这些区域对于从海洋中去除固定 N 的重要性，人们认为 OMZ 不含有固氮生物。在一项初步研究中，在阿拉伯人体内检测到了 nifH 基因及其表达。海洋和 ETNP OMZ 以及在 ETNP OMZ 中测量的活性 N2 固定，证实 N2 固定发生在缺氧水域。 PI 建议测量 N2 固定和“固氮营养群落在氧、光和溶解氮 (N) 浓度垂直梯度方面的多样性”。他们将比较这些详细的垂直剖面——具有在 OMZ 附近的全含氧水域中制作的类似轮廓。此外，他们还将比较和对比两个截然不同的 OMZ 区域；一个包括地球上一些生产力最高的海洋水域（ETSP），另一个是生产力低得多的水域（ETNP）。作为“的一部分”在这个项目中，他们将更好地了解氮气固定发生的位置，即主动反硝化区域和参与这些过程的微生物。有了“对 OMZ 水域中 N2 固定和活性固氮群落相对于化学和生物梯度的垂直分布的更全面的了解”，他们将对这些区域内的氮循环有更现实的认识，并更好地了解包括含氧和缺氧无光水域在内的氮固定的深度综合速率。智力优点：尽管有关于氮固定的位置和程度及其与 OMZ 内反硝化作用并置的地球化学推论，但支持这些的速率估计和生物数据却很少。推测。我们掌握的生物学证据表明，固氮生物在阿拉伯海和 ETNP OMZ 内很活跃，并且在 ETNP OMZ 内存在可测量的 N2 固定。拟议的研究将使 PI 能够确定固氮营养对两个广阔的 OMZ 区域内及其附近氮库存的贡献。目前，由于地理上缺乏来自不同海洋环境的速率测量，协调海洋氮预算和构建准确的生物地球化学模型的能力受到限制。此外，虽然 PI 测量了无光水域中的活跃 N2 固定情况，但他们缺乏深度分辨率，无法将广阔的亚透光海洋区域纳入对任何地方的海洋 N2 固定情况的深度综合估计中。 Ward 和 Devol 测量了该 OMZ 中与反硝化相关的氮损失，通过与 Ward 和 Devol 的持续合作，PI 将能够构建这个全球重要系统中氮循环的“端到端”视图。该项目生成的分子数据不仅将揭示通过固氮和反硝化作用导致海洋氮输入的活跃分支，而且还可以深入了解固氮菌和反硝化菌沿氧、光和溶解氮的垂直梯度的相对分布和活性。更广泛的影响：结果将有助于我们的理解对海洋 N2 固定的控制；固氮营养沿着光、氧和养分浓度垂直梯度的变化；和
 N 投入与 OMZ 损失之间的平衡。该项目将“在塑造我们对海洋氮预算（输入端和输出端）的看法方面具有潜在的变革性。该项目还将为多个层面的教育做出贡献。奥多明尼恩大学和普林斯顿大学的本科生和研究生将有机会利用最先进的陆地和海上研究设施获得宝贵的研究经验。此外，我们将创建一个视频教室，在 ETNP 和 ETSP 巡游期间与中小学生进行交流，并通过博客向科学界和公众通报最新情况。

There are three major open ocean oxygen minimum zones (OMZs) in the world: the Eastern Tropical North Pacific (ETNP), the Eastern Tropical South Pacific (ETSP), and the Arabian Sea. OMZs 
are important areas of denitrification (including anammox) and represent a significant loss 
of fixed nitrogen (N) from the ocean. However, multiple lines of evidence have recently indicated that N inputs via dinitrogen (N2) fixation and denitrification may be more closely coupled in space than previously suggested. Despite the geochemical inferences regarding the location and magnitude of N2 fixation that might be associated with OMZs and the importance of these regions for removing fixed N from the ocean, it is thought that OMZs do not harbour diazotrophs. In a preliminary study, nifH genes and their expression were detected from within the Arabian
 Sea and ETNP OMZs and active N2 fixation was measured in the ETNP OMZ, confirming that N2 fixation occurs in oxygen deficient waters. The PIs propose to measure N2 fixation and the 
diversity of diazotrophic communities with respect to vertical gradients of oxygen, light, 
and dissolved nitrogen (N) concentrations. They will compare these detailed vertical profiles
 with similar profiles made in fully oxic waters adjacent to the OMZs. In addition, they will 
compare and contrast two very different OMZ regions; one that includes some of the most productive oceanic waters on Earth (ETSP), and another that is far less productive (ETNP). As part of
 this project, they will acquire a better understanding of where N2 fixation occurs with respect 
to areas of active denitrification and the microbes involved in these processes. Armed with 
this more comprehensive understanding of the vertical distribution of N2 fixation and active diazotrophic communities with respect to chemical and biological gradients in OMZ waters,
 they will garner a more realistic view of the N cycle within these regions and a better understanding 
of depth-integrated rates of N2 fixation that include oxic and anoxic aphotic waters.Intellectual Merit: Despite the geochemical inferences regarding the location and magnitude of N2 fixation and
its juxtaposition with denitrification within OMZs, there are few rate estimates and biological 
data supporting these conjectures. The biological evidence we do have suggests that diazotrophs are active within the Arabian Sea and ETNP OMZs and that there is measureable N2 fixation 
within the ETNP OMZ. The proposed research will allow the PIs to establish the contribution of diazotrophy to N inventories within and adjacent to two expansive OMZ regions. The ability 
to reconcile oceanic N budgets and construct accurate biogeochemical models is currently limited by the geographical paucity of rate measurements from diverse oceanic environments. In addition, while the PIs have measured active N2 fixation in aphotic waters, they lack the depth resolution 
to include the expansive sub-euphotic oceanic realm in depth-integrated estimates of oceanic 
N2 fixation anywhere. Through their ongoing collaboration with Ward and Devol, who have measured N losses associated with denitrification in this OMZ, the PIs will be able to construct an "end-to-end" view of the N cycle in this globally important system. The molecular data generated from 
this project will not only shed light on the active clades that contribute to oceanic 
N inputs through N2 fixation and N loss through denitrification, but also give insights 
into the relative distributions and activity of diazotophs and dentrifiers along vertical 
gradients of oxygen, light, and dissolved N.Broader Impacts: Results will contribute to our understanding of controls on marine N2 fixation; variations 
in diazotrophy along vertical gradients of light, oxygen, and nutrient concentrations; and
 the balance between N inputs and losses from OMZs. This project will
 be potentially transformative in shaping our view of the oceanic N budget, both the input and output ends. The project will also contribute to education on a number of levels. Undergraduate and graduate students at Old Dominion University and Princeton will have opportunities to gain valuable research experience using state of the art research facilities both on land and at 
sea. In addition, we will create a video classroom to communicate with elementary and middle school students during the cruises to the ETNP and ETSP and keep the scientific community and general public updated through blogs.