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要旨:
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.
