Quantification of Trichodesmium spp. vertical and horizontal abundance patterns and nitrogen fixation in the western North Atlantic

该奖项是根据2009年美国复苏和再投资法案（公法111-5）资助的。重氮营养菌Trichodesmium spp.是海洋浮游生态系统氮流的主要途径，但其对全球氮收支的影响估计差异很大。由于菌体的脆弱性，取样变得困难，结果是在野外和实验室实验中都难以操作。因此，对生物体进行无损取样的光学方法很有吸引力。最近，一项使用视频浮游生物记录仪（VPR）进行的跨大西洋调查显示，在深海中，trichodesium spp.的丰度出乎意料地高，这表明该生物在光带内的垂直分布可能比以前认为的更均匀。将一个简单的生物光学生产力模型应用于观测到的丰度剖面表明，深度整合的固氮速率可能比基于丰度随深度指数下降的典型剖面的固氮速率高3到5倍。然而，Davis和McGillicuddy（2006）中所描述的观察结果来自一个纬度范围，在这个纬度范围内，毛菌属的数量并不是特别丰富。这就提出了一个关键的问题：在更南的水域是否也有类似的垂直分布，在那里，毛菌属的数量总体上要多一个数量级？如果是这样，深海生物是否在积极地修复氮？如果是这样，对全球氮收支的影响将是巨大的。为了回答这些问题，pi将在两次巡航中调查南马尾藻海和热带大西洋的水域。沿着轨道的VPR测量将用于记录生物在米到数千公里范围内的丰度和分布。标准的水文测量站工作将提供基于vpr的估计与显微镜计数的比较，以及一些额外的原位光学方法。将结合nifH基因表达测定和直接测定氮固定率来评估深层种群是否积极固定氮。这些观测结果将在涡流解析数值模式的背景下进行综合。这将允许在多个尺度上研究控制毛霉菌的垂直和水平分布和丰度的机制，包括相对最大丰度与反气旋涡流的神秘联系。此外，将这些观测结果整合到数值模型中，将有助于修订北大西洋Trichodesmium spp.的固氮估计。该项目的智力优势在于其跨学科方法（物理学和生物学），先进的观测技术（光学成像，分子方法）以及在最先进的物理-生物地球化学耦合模型背景下的综合分析。该项目将为在海洋盆地尺度上调控固氮及其对生物地球化学循环的影响的复杂海洋现象提供新的见解。该项目的更广泛影响包括对研究生和本科生的培训，以及加强现有的一项旨在提高8-14岁儿童科学素养的网络推广计划www.whyville.net。

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).The diazotroph Trichodesmium spp. constitutes a major pathway of nitrogen flow into marine planktonic ecosystems, but estimates of its impact on global nitrogen budgets vary widely. Sampling is made difficult by the fragility of the organism with the consequence that Trichodesmium spp. are difficult to manipulate in both field and laboratory experiments. Optical methods that sample the organism nondestructively are thus appealing. A recent transatlantic survey using the Video Plankton Recorder (VPR) revealed unexpectedly high abundance of Trichodesmium spp. at depth, suggesting the vertical distribution of the organism within theeuphotic zone may be more uniform than previously thought. Application of a simple bio-optical model of productivity to the observed profile of abundance suggests the depth-integrated nitrogen fixation rate could be three to five times higher than that based on the canonical profile of exponential decrease in abundance with depth. However, the observations described in Davis and McGillicuddy (2006) come from a latitude range where Trichodesmium spp. are not especially abundant. This raises a key question: is there a similar vertical distribution in waters further to the south, where Trichodesmium spp. are an order of magnitude more abundant overall? If so, are the deep populations actively fixing nitrogen? If so, the implications for the global nitrogen budget would be substantial.To answer these questions, the PIs will survey the waters of the southern Sargasso Sea and tropical Atlantic on two cruises. Along-track VPR measurements will be used to document the abundance and distribution of the organism on the scale of meters to thousands of kilometers. Standard hydrographic station work will provide for comparison of VPR-based estimates with microscope counts, as well as some additional in situ optical methods. A combination of nifH gene expression assays and direct determinations of N2-fixation rates will be made to assess whether or not the deep populations are actively fixing nitrogen.These observations will be synthesized in the context of an eddy-resolving numerical model. This will permit investigation of the mechanisms controlling the vertical and horizontal distribution and abundance of Trichodesmium spp. at multiple scales, including the enigmatic association of relative maxima in abundance with anticyclonic eddies. Moreover, integration of these observations into the numerical model will facilitate revised estimates of nitrogen fixation by Trichodesmium spp. in the North Atlantic.The intellectual merit of this project is its interdisciplinary approach (physics and biology), advanced observational techniques (optical imaging, molecular methods) and integrated analysis in the context of state-of-the-art coupled physical-biogeochemical models. This project will provide new insights into the complex oceanographic phenomena regulating nitrogen fixation and its impact on biogeochemical cycling at the scale of an ocean basin.Broader impacts of this project include training of graduate and undergraduate students, and enhancement of an existing web-based outreach program www.whyville.net geared toward promoting science literacy in children ages 8-14.