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

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更为重要，可能通常发生在靠近强烈的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加利福尼亚湾和热带北太平洋东部邻近水域（ETNP）的初级和出口产量。这个海洋学区域由于其丰富的生物生产力和反硝化中间水的优势，为研究这些过程提供了一个极好的野外实验室。该太平洋地区夏季地表水的特点是硝酸盐对磷酸盐（N:P）值为零，P浓度丰富（0.3-0.8M），热分层明显，为显著固氮的发生和这种初级生产对出口生产的贡献提供了潜在的主要栖息地。鉴于这一前景及其与推进整体海洋碳氮循环知识的相关性，这项多学科生物地球化学研究将解决一个总体假设：加利福尼亚湾和邻近ETNP的反硝化水上涌，如果随之而来的是分层和redfield型的养分下降，将为地表水的固氮提供条件，从而导致出口生产的潜在反馈（正反馈和负反馈），有利于反硝化的缺氧条件的维持，以及反硝化区产生的溶解N:P比率的大小。该项目有三个主要目标：(1)证明固氮是该地区夏季分层地表水常见的、数量上重要的初级生产过程；(2)确定负责固定N2的特定生物，并确定每种确定的固定n生物的产碳速率；(3)量化新固定氮的出口，评估固定氮群落不同成员对出口的可能直接贡献。为了实现这些目标，研究小组将结合显微镜、基因鉴定、脂质生物标志物以及稳定同位素和放射性同位素测量，对2008年夏季在加利福尼亚湾地区进行的研究巡航中收集的样本进行测量。将进行以下观察：(1)确定加利福尼亚湾和ETNP诊断区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固氮固定，并使用一种新的、新建立的同位素示踪技术直接确定每种已确定生物的碳产率；(3)评估生物特异性碳氮输出速率；(4)评估？雨？这些新固定的氮在反硝化层的上部区域分解并随后硝化。就更广泛的影响而言，该项目将包括通过与一名墨西哥同事及其本科生合作开展外展活动。高中，本科(包括勤工俭学，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内和国际合作。

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.