Evidence for Nitrous Oxide Emissions by Nitrite-Dependent Anaerobic Methane Oxidizing Bacteria.

Nitrite-dependent anaerobic methane oxidizing (n-DAMO) bacteria generally convert nitrite to dinitrogen and bypass the nitrous oxide (NO) formation step. However, NO is often detected in n-DAMO bacteria dominated cultures and it remains an open question as to the microbial origin of NO in these enrichments. Using a stable nitrite consuming microbial community enriched for n-DAMO bacteria, we demonstrated that NO production was coupled to methane oxidation and the higher initial nitrite concentrations led to increased quantities of NO being formed. Moreover, continuous exposure of the enrichment culture to about 5 mg of N L nitrite resulted in constant NO being produced (12.5% of nitrite was reduced to NO). Metatranscriptomic analyses revealed that nitrite reductase () and nitric oxide reductase () transcripts from n-DAMO bacteria increased in response to nitrite exposure. No other bacteria significantly expressed genes under these conditions, suggesting n-DAMO bacteria are responsible for NO being produced. In a 35-day bioreactor experiment, NO produced by the n-DAMO bacteria accumulated when nitrite was in excess; this was found to be up to 3.2% of the nitrogen that resulted from nitrite removal. Together, these results suggested that excess nitrite is an important driver of NO production by n-DAMO bacteria. To this end, proper monitoring and control of nitrite levels in wastewater treatment plants would be effective strategies for mitigating NO emissions to the atmosphere.