show Abstracthide AbstractDNA was extracted from a culture enriched from Twentekanaal sediment, designated Twente, and a culture from Ooijpolder ditch sediment, designated Ooij. Only three biological pathways are known to produce molecular oxygen (O2): photosynthesis, microbial chlorite respiration and the detoxification of reactive oxygen species. Here we present evidence for a fourth pathway, possibly of considerable geochemical and evolutionary importance. The pathway was discovered after metagenomic sequencing of an enrichment culture that couples the anaerobic oxidation of methane (CH4) with the reduction of nitrite (NO2-) to dinitrogen gas (N2). The complete genome of the dominant bacterium, named herein Methylomirabilis oxyfera, could be assembled.