The existence of ferric hydroxide links the carbon and nitrogen cycles by promoting nitrite-coupled methane anaerobic oxidation.

Microorganism-mediated anaerobic oxidation of methane can efficiently mitigate methane atmospheric emissions and is a key process linking the biogeochemical cycles of carbon, nitrogen, and iron. The results showed that methane oxidation and nitrite removal rates in the CF were 1.12 and 1.28 times higher than those in CK, respectively, suggesting that ferric hydroxide can enhance nitrite-driven AOM. The biochemical process was mediated by the enrichment of methanogens, methanotrophs, and denitrifiers. Methanobacterium and Methanosarcina were positively correlated with Fe3+ and Fe2+, whereas Methylocystis and Methylocaldum were positively correlated with methane, and denitrifiers were positively correlated with nitrite. Metagenomic analysis revealed that the genes related to methane oxidation, nitrogen reduction, and heme c-type cytochrome were upregulated in CF, indicating that a synergistic action of bacteria and methanogens drove AOM via diverse metabolic pathways, within which ferric hydroxide played a crucial role. This study provides novel insights into the synergistic mechanism of ferric iron and nitrite-driven AOM.