Unraveling the Microbial Mechanisms and Electron Transfer Networks of Cr(VI) Driven Anaerobic Methane Oxidation

Anaerobic oxidation of methane (AOM) plays a critical role in mitigating methane emissions, while hexavalent chromium [Cr(VI)] contamination remains a significant challenge in environmental remediation. The coupling of AOM with Cr(VI) reduction offers a promising strategy to simultaneously address heavy metal pollution and methane emissions reduction. However, the exact contribution of Cr(VI)-dependent AOM (Cr-AOM) to methane mitigation, the key functional microorganisms involved, and the underlying methane metabolic pathways and electron transfer mechanisms remain poorly understood. In this study, we systematically investigated these aspects using a combination of ¹³C-labeled methane microcosm experiments, DNA-stable isotope probing (DNA-SIP), meta-omics, electrochemical analysis, and molecular docking simulations. Our findings indicated that Cr-AOM enables concurrent Cr(VI) removal and methane emission reduction, with the AOM potential showing a positive correlation with the isotopic fractionation factor. ¹³CH₄-DNA-SIP analysis identified Methyloceanibacter, Methylocystis, and Candidatus Bathyarchaeia as the key genera mediating Cr-AOM, which synergistically facilitate efficient methane metabolism through inter-microbial cooperation. Metatranscriptomic analysis revealed the presence of an active reverse methanogenesis pathway. Meanwhile, Actinomycetota harbored genes such as norB, narB/G/H, and nirK, indicating a potential nitrogen metabolic capacity that may be linked to AOM through bacterial cross-feeding, thereby coupling methane oxidation with the nitrogen cycle. Furthermore, electron transfer in Cr-AOM was primarily mediated by chromate reductases from Pseudomonadota, cytochrome c, PilA-encoded proteins, and riboflavin, supplemented by a chemical reduction pathway (involving H₂S). The presence of trace amounts of sulfate was found to enhance the electron transfer process in Cr-AOM, thereby coupling methane oxidation with the sulfur cycle. This study elucidates the metabolic pathways and electron transfer mechanisms of Cr-AOM, providing a theoretical foundation for controlling methane emissions and remediating heavy metal pollution, advancing the conceptual framework of Metal-AOM, and furthering our understanding of the coupling between metal oxide reduction and the biogeochemical cycles of C, N, and S.