This study describes a novel biological pathway for anaerobic methane oxidation driven by nitrite reduction in oxygenic bacteria. The researchers discovered a bacterium named 'Candidatus Methylomirabilis oxyfera' that can oxidize methane without oxygen, using nitrite as an electron acceptor. This bacterium encodes the well-established aerobic pathway for methane oxidation but lacks genes for complete denitrification. The absence of genes for nitrous oxide reductase suggests that 'M. oxyfera' bypasses this intermediate by converting two nitric oxide molecules to dinitrogen and oxygen, which then oxidizes methane. This mechanism explains the biochemical mechanism of a poorly understood freshwater methane sink and provides insights into the evolution of metabolic pathways on early Earth. The discovery of this 'intra-aerobic' pathway challenges the traditional view of denitrification and suggests that oxygen production may have preceded the evolution of photosynthetic organisms.This study describes a novel biological pathway for anaerobic methane oxidation driven by nitrite reduction in oxygenic bacteria. The researchers discovered a bacterium named 'Candidatus Methylomirabilis oxyfera' that can oxidize methane without oxygen, using nitrite as an electron acceptor. This bacterium encodes the well-established aerobic pathway for methane oxidation but lacks genes for complete denitrification. The absence of genes for nitrous oxide reductase suggests that 'M. oxyfera' bypasses this intermediate by converting two nitric oxide molecules to dinitrogen and oxygen, which then oxidizes methane. This mechanism explains the biochemical mechanism of a poorly understood freshwater methane sink and provides insights into the evolution of metabolic pathways on early Earth. The discovery of this 'intra-aerobic' pathway challenges the traditional view of denitrification and suggests that oxygen production may have preceded the evolution of photosynthetic organisms.