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

氮是浮游植物必不可少的营养物质，往往限制海洋的初级生产，因此氮的供应在全球海洋生产力中起着关键作用。氮存在的数量和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，它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一些微生物直接转化为硝酸盐。然而，这一过程对全球生物地球化学氮循环的重要性尚不清楚。本研究的目的是评价多胺氮作为一种模式有机氮化合物，与氨的典型硝化反应相比，直接氧化对硝化作用的生物地球化学意义。该项目将培养一名博士后研究员，并为本科生提供获得微生物地球化学和沿海生态系统过程研究实践经验的机会。项目人员还将与乔治亚州海岸生态系统长期生态研究项目合作，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营养物质浓度高的沿海水域中去除过量固定氮至关重要。最近的研究表明，古细菌在海洋氨氧化中起着重要作用。对富营养化培养和以氨氧化古菌为主要氨氧化剂的沿海水样进行的实验表明，某些形式的有机氮可以直接氧化为氮氧化物，而不需要首先再生为铵。在测试的底物中，多胺，特别是腐胺氮似乎直接氧化为氮氧化物，而氨基酸和尿素氮首先再生为铵，然后被氧化。研究人员将在三年内通过富集培养和对沿海浮游细菌进行实验来详细研究这一过程。具体来说，他们将致力于更好地了解1)这一新过程对海洋地球化学的影响，2)多胺中存在的碳的命运，3)什么生物负责直接氧化，以及4)可直接氧化的有机氮化合物的化学特性。

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.