COLLABORATIVE RESEARCH: Nitrogen Fixation and its Coupling with Denitrification in the Eastern Tropical North Pacific

摘要固氮作用在亚热带贫营养区提高初级生产力和输出方面发挥着重要作用。热带海洋然而，最近的证据表明，固氮作用可能更具全球意义，可能通常发生在靠近强烈地下反硝化区的表层海洋区域。在这个项目中，来自俄勒冈州州立大学，夏威夷大学和南卡罗来纳州大学的一组研究人员将研究反硝化和固氮之间的相互作用如何增强这样一个地区的初级和出口生产，加州湾和东热带北太平洋（ETNP）的邻近沃茨。这一海洋学区域提供了一个很好的野外实验室，在其中研究这些过程，由于其丰富的生物生产力和主要的中间沃茨。太平洋这一区域夏季表层沃茨的特点是硝酸盐与磷酸盐（N：P）的比值实际上为零，而P的浓度却很高（0.3- 0.8 M），而且温度分层明显，这为发生显著的固氮作用和这种初级生产对出口生产的贡献提供了潜在的主要生境。鉴于这一前景及其与推进海洋碳和氮循环整体知识的相关性，这项多学科生物地球化学研究将解决一个总体假设：在加州湾和邻近的ETNP，如果随后出现分层和Redfield型营养盐下降，则会使表层沃茨进行固氮，从而导致潜在的反馈（积极和消极）的出口生产、有利于反硝化的亚氧条件的维持以及反硝化区产生的溶解氮磷比的大小。该项目有三个主要目标：（1）证明固氮是该地区夏季分层表层沃茨中一种常见的、数量上重要的初级生产过程;（2）确定负责固氮的特定生物，并确定每种已确定的固氮生物的碳生产率;以及（3）量化新固定氮的输出，并评估固氮群落的不同成员对输出的可能贡献。为了实现这些目标，该小组将使用显微镜，遗传鉴定，脂质生物标志物和稳定同位素和放射性同位素测量在2008年夏季研究巡航期间收集的样本在加州地区的组合。将对以下方面进行观察：（1）确定加州湾和ETNP诊断区域的总氮和净氮固定的空间格局和速率;（2）确定哪些特定生物负责氮固定，并使用新建立的同位素示踪技术直接确定每种确定的生物的碳生产速率;（3）评估特定生物体碳和氮输出生产的速率;（4）评估在何种程度上？下雨了？在反硝化层的上部区域内分解并随后硝化。就更广泛的影响而言，该项目将包括通过与一位墨西哥同事及其本科生合作开展外联活动。涉及高中、本科（含勤工俭学、荣誉？该项目的研究生和博士后研究人员将鼓励各级教育的国家和国际合作。

Abstract Nitrogen fixation plays a now well-recognized role in enhancing primary production and export in oligotrophic regions of the subtropical ? tropical ocean. However, recent evidence suggests that nitrogen fixation may be even more globally significant, perhaps occurring commonly in regions of the surface ocean proximate to zones of intense subsurface denitrification. In this project, a team of researchers from Oregon State University, University of Hawaii, and University of South Carolina will examine how the interactions between denitrification and nitrogen fixation enhance primary and export production in one such area, the Gulf of California and adjacent waters of the eastern tropical North Pacific (ETNP). This oceanographic region provides an excellent field laboratory in which to study these processes due to its rich biological productivity and the predominance of denitrified intermediate waters. Summertime surface waters in this region of the Pacific, characterized by effectively zero nitrate-to-phosphate (N:P) values with replete P concentrations (0.3-0.8M) and pronounced thermal stratification, provide potentially prime habitat for the occurrence of significant nitrogen fixation and contribution of such primary production to export production. Given this prospect and its relevance to advancing knowledge of the overall marine carbon and nitrogen cycles, this multidisciplinary biogeochemical study will address an overarching hypothesis: Upwelling of denitrified waters in the Gulf of California and adjacent ETNP, if followed by stratification and Redfield-type nutrient drawdown, primes surface waters for nitrogen fixation and thus leads to potential feedbacks (positive and negative) for export production, the maintenance of the suboxic conditions that favor denitrification, and the magnitude of dissolved N:P ratios that are generated in the denitrification zone. The project has three key objectives: (1) to demonstrate that nitrogen fixation is a common, quantitatively important primary production process in summer stratified surface waters of this region; (2) to identify the specific organisms responsible for the N2 fixation and determine the carbon production rate for each identified N-fixing organism; and (3) to quantify the export of the newly fixed nitrogen and assess the possible contribution that different members of the N-fixing community directly contribute to export. To achieve these objectives the team will use a combination of microscopy, genetic identification, lipid biomarkers and both stable isotopic and radioisotopic measurements on samples collected during a 2008 summer research cruise in the Gulf of California region. Observations will be made to: (1) determine the spatial pattern and rates of gross and net N2 fixation in a diagnostic region of the Gulf of California and ETNP; (2) identify what specific organisms are responsible for the nitrogen fixation fixation and, use a novel, newly established isotopic tracer technique to directly determine the carbon production rate for each identified organism; (3) assess the rates of organism-specific carbon and nitrogen export production; and (4) evaluate the extent to which ?rain? of this newly fixed nitrogen is decomposed within the upper zone of the denitrification layer and subsequently nitrified. In terms of broader impacts, this project will include outreach through collaboration with a Mexican colleague and his undergraduates. Involvement of high school, undergraduate (including work-study, Honor?s College and REU students), graduate and postdoctoral researchers in the project will encourage national and international collaboration at all levels of education.