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

点击翻译按钮获取中文摘要

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.