Nitrogen Fixation in the Open Ocean: Assessing the Role of Recently Discovered Diazotrophs

Zehr 9977460和Montoya 9977528营养物质的可用性，主要是磷，氮和铁，限制了海洋的生产力。氮通常被认为是贫营养海洋中的一种重要营养物质，如果不是限制性的话。氮，以二氮气体的形式存在，是地球大气的主要成分，并以溶解气体的形式存在于海水中。然而，大多数生物可以以硝酸盐，铵或有机氮的形式使用氮，但不能直接使用二氮。这种形式的氮只能被固氮生物利用，固氮生物可以通过固氮酶的作用将氮“固定”成铵。各种原核生物都具有固氮能力，包括异养细菌、蓝藻和蓝藻。尽管可以固氮的微生物种类繁多，但以前很少有关于公海固氮类群的报道。 最近在大西洋和太平洋的研究表明，固氮作用在氮动力学中比以前认为的更重要，可能有大量的、无法解释的氮流入混合层，这可能是由于固氮作用。已知固氮生物，蓝细菌Trichodesmium和内共生体Richelia固氮的估计，不能占这个估计的N flux.Using分子的方法，它最近被证明，有不同的微生物在开放的海洋环境中，具有固氮的遗传能力。 在这个项目中，这些微生物的生态和生物学意义将使用分子方法，培养努力，固氮率测量和稳定同位素测量进行检查。RNA技术（nifH逆转录酶聚合酶链反应，RT-PCR）的最新发展使得有可能在季节性研究和实验操作中检查独特的固氮酶β型的表达。这些研究将集中在HOT网站，以确定和表征新的固氮蓝藻，以确定控制其生长和活动的因素，并量化其对当地氮预算使用稳定同位素技术的贡献。与此同时，将通过N-15示踪测量和RT-PCR研究与无脊椎动物相关的固氮活性实验，以确定负责观察到的固氮活性的固氮微生物的类型。最后，新的固氮生物的特点是通过检查固氮装置（固氮酶基因以外的nifH），以确定这些生物的固氮装置是否是不寻常的，这可能有生物技术的应用以及进化的影响。 这些新的固氮微生物在开放的海洋中的识别和表征是及时的，并可能对评估海洋的氮收支的重要。

Zehr 9977460 and Montoya 9977528The availability of nutrients, primarily phosphorus, nitrogen and iron, limits the productivity of the oceans. Nitrogen is often believed to be an important, if not limiting, nutrient in oligotrophic oceans. Nitrogen, in the form of dinitrogen gas, is a major component of the Earth's atmosphere and is abundant in seawater as dissolved gas. However, most organisms can use nitrogen in the form of nitrate, ammonium, or organic nitrogen, but cannot directly use dinitrogen. This form of nitrogen can be used only by diazotrophs, which can "fix" nitrogen into ammonium through the action of the enzyme nitrogenase. Diverse prokaryotic taxa have the ability to fix nitrogen, including heterotrophic bacteria, cyanobacteria and Archaea. Despite the diversity of types of microorganisms that can fix nitrogen, very few nitrogen--fixing taxa have previously been reported from the open ocean. Recent studies in the Atlantic and Pacific Oceans indicate that nitrogen fixation is more important in nitrogen dynamics than previously believed, and that there may be a large, unaccounted for flux of nitrogen into the mixed layer, which may be due to nitrogen fixation. Estimates of the nitrogen fixation by known diazotrophs, the cyanobacterium Trichodesmium and endosymbiont Richelia, cannot account for this estimated N flux.Using a molecular approach, it has very recently been shown that there are diverse microorganisms in the open ocean environment, which have the genetic capacity for nitrogen fixation. In this project, the ecological and biological significance of these microorganisms will be examined using molecular approaches, cultivation efforts, nitrogen fixation rate measurements, and stable isotope measurements. Recent developments in RNA technology (nifH reverse-transcriptase polymerase chain reaction, RT-PCR) make it possible to examine the expression of the unique nitrogenase phylotypes in seasonal studies, and in experimental manipulations. These studies will focus at the HOT site in order to identify and characterize the novel nitrogen-fixing cyanobacterium, to determine the factors controlling its growth and activity, and to quantify its contribution to the local nitrogen budget using stable isotope techniques. In parallel, experiments on nitrogen-fixation activity associated with invertebrates will be investigated by N-15 tracer measurements and RT-PCR to identify the types of nitrogen-fixing microorganisms responsible for observed nitrogen fixation activity. Finally, the novel diazotroph will be characterized by examining the nitrogen fixation apparatus (nitrogenase genes other than nifH) to determine whether the nitrogen fixation apparatus of these organisms is unusual, which may have biotechnological applications as well as evolutionary implications. The identification and characterization of these novel nitrogen-fixing microorganisms in the open ocean is timely and potentially important for evaluating the nitrogen budget of the sea.