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.
