Physiological and Stoichiometric Studies of Marine Diazotrophs

在过去的十年中，海洋固氮已经成为贫营养海洋生态系统氮（N）循环的重要组成部分，并且公认的固氮微生物的数量急剧增加。浮游蓝细菌束毛藻曾经被认为是主要的固氮生物在开放的海洋，它仍然被认为是占可能的一半的N2固定推断的几个地球化学指标。 然而，我们现在知道硅藻的蓝藻共生体和浮游细菌的其他成分也对这一过程有很大的贡献。目前的努力正在进行中，明确表示N2固定在海洋生物地球化学模型，但这些模型所需的关键参数化是相对原始的。光合参数为CO2和N2固定束毛藻只有粗略的特点，是不存在的其他光合固氮。 地球化学分析已经评估了过量N形成的程度（即相对于P的过量），并假设了固氮生物的非Redfield N：P比率，这是过量N的假定来源。假设的值极大地影响了N2固定的导出率，特别是当外推到全球海洋时。然而，固氮生物的N：P比率受到很差的限制。 本项目的主要目标是系统地评估限制固氮生长和原位固氮的因素。研究人员将研究两种光合海洋固氮生物，Trichodesmium IMS 101和球形蓝藻，作为模式生物。 研究人员将定义光和P对模型生物的生长和固氮活性的精细尺度控制，它们与光和营养物质的相互作用，以及它们对光合参数和细胞N：P化学计量的响应。这项研究中产生的数据和想法将增加对光和营养物质之间复杂相互作用的理解，这与全球重要的氮气固定过程有关。 更好地了解的生理控制在束毛藻和固氮的球状蓝藻固氮反过来又会导致更好地了解控制全球固氮作为一个整体。 这项研究的结果正在寻求，并将直接用于海洋生态系统和海洋地球化学模拟社区，这是试图预测海洋碳循环动力学。该项目将支持培训两名高年级女研究生和本科生。调查人员还将参加COSEE-West计划，该计划向高中教师传播海洋学的最新发现。

Over the last decade, marine N2 fixation has emerged as an important component of the nitrogen (N) cycle of oligotrophic marine ecosystems, and the number of recognized diazotrophic microorganisms has increased dramatically. The planktonic cyanobacterium Trichodesmium was once thought to be the predominant diazotroph in the open ocean, and it is still thought to account for possibly half of the N2 fixation inferred by several geochemical indices. However, we now know that cyanobacterial symbionts of diatoms, and other components of the bacterioplankton also contribute substantially to this process. Current efforts are underway to explicitly represent N2 fixation in marine biogeochemical models, but key parameterizations required for these models are relatively primitive. Photosynthetic parameters for both CO2 and N2 fixation by Trichodesmium have only been coarsely characterized and are non-existent for other photosynthetic diazotrophs. Geochemical analyses that have assessed the extent of excess N formation (i.e. excess relative to P) have assumed a non-Redfield N:P ratio for diazotrophs, the presumed source of the excess N. The value assumed greatly affects the derived rate of N2 fixation, especially when extrapolated to the global ocean. However, the N:P ratio of diazotrophs is poorly constrained. The principle aim of this project will be to systematically evaluate factors that limit diazotrophic growth and N2 fixation in situ. The investigators will examine two photosynthetic marine diazotrophs, Trichodesmium IMS 101 and coccoid cyanobacteria, as model organisms. The investigators will define the fine scale controls of light and P on growth and diazotrophic activity of the model organisms, their interaction with light and nutrients, and their responses with respect to photosynthetic parameters and cellular N:P stoichiometry. The data and ideas produced in this study will increase understanding of the complex interactions between light and nutrients with respect to the globally significant process of nitrogen gas fixation. A better understanding of the physiological controls on N2 fixation in Trichodesmium and diazotrophic coccoid cyanobacteria will in turn lead to a better understanding of controls on global N2 fixation as a whole. Results derived from this study are being sought by and will be of direct use to the marine ecosystem and biogeochemical modeling community, which is attempting to forecast ocean C cycle dynamics. This project will support training of two senior female graduate students, as well as undergraduates. The investigators will also participate in the COSEE-West program, which disseminates recent findings in oceanography to high school teachers.