Collaborative Research: Direct Oxidation of Organic Nitrogen by Marine Ammonia Oxidizing Organisms

氮是浮游植物的必需营养素，通常限制海洋中的初级生产，因此其可用性在全球海洋生产力中起着关键作用。氮的数量和存在形式是由微生物控制的。一种微生物介导的过程被称为硝化作用，其将氨或铵氧化为亚硝酸盐，亚硝酸盐氧化为硝酸盐，硝酸盐是氮的生物可利用形式。虽然这是公认的硝化过程，但初步结果强烈表明，一种称为多胺氮的含氮化合物可能会被某些微生物直接转化为硝酸盐。然而，这一过程对全球非地球化学氮循环的重要性尚不清楚。本研究的目的是评估生物地球化学意义的直接氧化的多胺氮，作为一种模式的有机氮化合物，硝化相比，典型的硝化氨。该项目将培养一名博士后研究员，并为本科生提供机会，获得微生物地球化学和沿海生态系统过程研究方面的实践经验。项目人员还将与格鲁吉亚沿海生态系统长期生态研究计划合作，聘请一名K-12科学教师参与该项目。氨氧化是将固定氮转化为二氮气体过程中的关键步骤，因此对全球氮循环和从具有高浓度营养物的沿海沃茨中去除多余的固定氮至关重要。最近的研究表明，Thaumarchaeota在海洋中的氨氧化中发挥着重要作用。富集培养物和沿海水样中的氨氧化古菌是主要的氨氧化剂的实验表明，某些形式的有机氮可以直接氧化为氮氧化物，而无需首先再生为铵。在测试的底物中，多胺和特别是腐胺氮似乎直接氧化成氮氧化物，而氨基酸和尿素氮首先再生为铵，然后氧化。研究人员将在三年内使用富集培养物和沿海浮游细菌进行的实验详细研究这一过程。具体而言，他们将旨在更好地了解1）这种新过程对海洋地球化学的后果，2）多胺中存在的碳的命运，3）什么生物体负责直接氧化，以及4）直接氧化的有机氮化合物的化学特征。

Nitrogen is an essential nutrient for phytoplankton that often limits primary production in the ocean, and its availability therefore plays a key role in global ocean productivity. The amounts and form in which nitrogen exist are controlled by microorganisms. One microorganism-mediated process is known as nitrification, which oxidizes ammonia or ammonium to nitrite and nitrite to nitrate, nitrate being the bioavailable form of nitrogen. While this is the well-accepted process of nitrification, preliminary results strongly suggest that a nitrogen-containing compound know as polyamine nitrogen may be directly converted by some microorganisms to nitrate. However, the importance of this process for global biogeochemical nitrogen cycling is unknown. The goal of this study is to evaluate the biogeochemical significance of direct oxidation of polyamine nitrogen, as a model organic nitrogen compound, to nitrification compared to canonical nitrification of ammonia. The project will result in training a postdoctoral researcher and provide opportunities for undergraduates to gain hands-on experience with research on microbial geochemistry and coastal ecosystem processes. Project personnel will also work with the Georgia Coastal Ecosystems Long-Term Ecological Research program to engage a K-12 science teacher in the project.Ammonia oxidation is a key step in the process of converting fixed nitrogen to dinitrogen gas and thus is central to the global nitrogen cycle and to removing excess fixed nitrogen from coastal waters with high concentrations of nutrients. Recent research has shown that Thaumarchaeota play a major role in ammonia oxidation in the ocean. Experiments with enrichment cultures and coastal water samples where ammonia oxidizing archaea are the dominant ammonia oxidizers, show that some forms of organic nitrogen may be oxidized directly to nitrogen oxides without first being regenerated as ammonium. Of the substrates tested, polyamine and particularly putrescine nitrogen appear to be oxidized directly to nitrogen oxides, while amino acid and urea nitrogen is first regenerated as ammonium and then oxidized. The investigators will examine this process in detail over three years using enrichment cultures and experiments conducted with coastal bacterioplankton. Specifically, they will aim to better understand 1) the consequences of this novel process to ocean geochemistry, 2) the fate of the carbon present in polyamines, 3) what organisms are responsible for the direct oxidation, and 4) the chemical characteristics of the organic nitrogen compounds accessible to direct oxidation.