This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). Nitrogen is an essential element for life on Earth, yet fixed nitrogen is relatively scarce in the oceans. Simultaneously, global nitrogen budgets are difficult to balance, suggesting unknown nitrification and denitrification reactions may account for a large flux of nitrogen between reservoirs. Over the last decade, new microbial transformations of nitrogen have been discovered using geochemical, cultivation, and molecular approaches, and genes responsible for the oxidation of ammonium have been found in a wide variety of prokaryotes. Among these processes, anammox, which anaerobically oxidizes ammonium to N2 in the presence of nitrite or nitrate, may contribute to 30-50% of denitrification in marine environments. Other thermodynamically favorable pathways hypothesized to play a significant role in the transformation of nitrogen in sediments include iron- and manganese-coupled nitrification and denitrification. The overall objective of this project is to demonstrate that manganese-reducing prokaryotes play a significant role in the nitrogen cycle by providing, through anaerobic oxidation of ammonium, an alternative source of nitrite and nitrate to denitrifying and nitrate- reducing microorganisms. Several studies, including ours, have concluded that manganese- coupled anaerobic nitrification occurs in marine sediments, though direct evidence for such processes is still lacking and pure strains have yet to be isolated. A series of incubations with sediments will be complemented with cultivation-independent molecular phylogenetic studies to identify the dominant microbial species in incubations displaying MnO2-dependent nitrifying activities. These incubations will be supplemented with a cultivation approach to isolate pure or mixed cultures catalyzing this reaction. This study will demonstrate that dissimilatory manganese reduction plays a significant role in anaerobic nitrification and, ultimately, denitrification through the anammox process. Denitrification and the dissimilatory reduction of manganese play a significant role in the cycling of carbon in sediments, thus the new processes identified in this study will have broader implications on our understanding of the global carbon cycle. This study will also refine diagenetic models and expand the number of microbial processes involved in the nitrogen cycle to help balance nitrogen budgets more accurately.
