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The fate of ammonium in anaerobic oxidation of methane

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Piepgras,  Laura
Department of Biogeochemistry, Max Planck Institute for Marine Microbiology, Max Planck Society;

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Citation

Piepgras, L. (2013). The fate of ammonium in anaerobic oxidation of methane. Master Thesis, University of Bremen, Bremen / Germany.


Cite as: https://hdl.handle.net/21.11116/0000-0001-C723-7
Abstract
Ammonium (NH4+) plays a key role in dissimilatory and assimilatory processes of the marine nitrogen cycle. Current geochemical evidence indicates that ammonium is lost from anoxic sediments where no oxygen, nitrate or nitrite is present (Schrum et al., 2009). Therefore the dissimilatory processes such as aerobic nitrification, denitrification and anammox have been considered unlikely to be responsible for the observed NH4+ removal in these sediments. Interestingly, upwards-diffusing ammonium disappears in the same zone in which downwards-diffusing sulfate (SO4 2) is reduced. It has therefore been proposed that lithotrophic ammonium oxidation coupled to sulfate reduction (SR) might be responsible for ammonium removal in this zone. The available porewater profiles suggest that the oxidation of ammonium occurs in the same zone where methane (CH4) is oxidized via anaerobic oxidation of methane (AOM). In AOM, sulfate serves as the electron acceptor for methane oxidation. It is not yet clear whether the AOM - the main process for sulfate reduction in this sediment depth - is also responsible for the observed ammonium removal. The goal of this study was therefore to investigate whether AOM consortia are also capable of anaerobic ammonium oxidation under sulfate-reducing conditions. To test this hypothesis an AOM enrichment culture dominated by ANME-2 and Desulfosarcinarelated bacteria was used for 15N-labeling experiments. Isotope labelling was used to determine consumption of ammonium and production of nitrogen compounds as possible intermediates or products of ammonium oxidation. Furthermore, ammonium assimilation was measured using single cell analysis. The enrichment culture showed an ongoing AOM process in CH4-supplemented incubation but the 15N-NH4 + determinations did not show an ongoing oxidation of ammonium. Yet, an increase in nitrogen gas (N2) concentrations over time was measured and an oxidized nitrogen species was formed and consumed within the first 12 hours of incubation. The latter results lead us to suggest that NH4+ oxidation was an ongoing process in this enrichment culture under AOM conditions. Based on the current state of knowledge about the species and formation of this nitrogen oxide we were not able to link this compound to a known pathway in the nitrogen cycle.Further investigations are necessary to elucidate the identity and the role of this nitrogen compound in anaerobic oxidation of ammonium under SR conditions.