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

摘要在亚热带低营养地区，固氮在提高初级生产和出口方面发挥着举世公认的作用。热带海洋。然而，最近的证据表明，固氮可能在全球范围内具有更重要的意义，可能通常发生在靠近强烈次表层反硝化带的表层海洋区域。在这个项目中，来自俄勒冈州立大学、夏威夷大学和南卡罗来纳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提高这样一个地区-加利福尼亚州海湾和东热带北太平洋(ETNP)附近水域的初级和出口产量。由于其丰富的生物生产力和反硝化中间水的优势，该海洋区为研究这些过程提供了一个极好的野外实验室。太平洋这一区域夏季表层水域的特点是，硝酸盐与磷酸盐(N：P)值实际上为零，磷浓度充足(0.3-0.8m)，温度层化明显，为显著固氮和这种初级生产对出口生产的贡献提供了潜在的最佳栖息地。鉴于这一前景及其与增进对整个海洋碳和氮循环的认识的相关性，这项多学科生物地球化学研究将解决一个重要的假设：加利福尼亚州海湾和邻近的ETNP反硝化水域的上升流，如果随后发生层化和红场类型的营养盐下降，则为固氮奠定基础，从而导致潜在的出口生产反馈(正反馈和负反馈)，有利于反硝化的亚氧条件的维持，以及反硝化带产生的溶解N：P比率的大小。该项目有三个关键目标：(1)证明固氮是该区域夏季分层表层水中一个常见的、在数量上很重要的初级生产过程；(2)确定负责氮固定的特定生物，并确定每个已确定的固氮生物的碳生产率；(3)量化新固定氮的出口，并评估固氮群落的不同成员对出口的可能贡献。为了实现这些目标，该团队将使用显微镜、遗传鉴定、脂质生物标记物以及稳定同位素和放射性同位素测量的组合，对2008年夏季在加利福尼亚州海湾地区进行研究巡航期间收集的样本进行测量。将进行以下观察：(1)确定加利福尼亚州海湾和ETNP诊断区的总和净氮固定的空间格局和速率；(2)确定负责固氮的特定生物，并使用新建立的同位素示踪技术直接确定每个已确定生物的碳生产速率；(3)评估特定生物的碳和氮输出生产速率；以及(4)评估降雨的程度？这些新固定的氮在反硝化层的上部区域内分解，随后被硝化。在更广泛的影响方面，该项目将包括通过与一名墨西哥同事和他的本科生合作进行外联。高中、本科生(包括勤工俭学、S荣誉学院和俄勒冈州立大学的学生)、研究生和博士后研究人员参与该项目，将鼓励各级教育的国内和国际合作。

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.