Metabolic versatility of aerobic methane-oxidizing bacteria under anoxia in aquatic ecosystems

The potential positive feedback between global aquatic deoxygenation and methane (CH₄) emission emphasizes the importance of understanding CH₄ cycling under O₂-limited conditions. Increasing observations for aerobic CH₄-oxidizing bacteria (MOB) under anoxia have updated the prevailing paradigm that MOB are O₂-dependent; thus, clarification on the metabolic mechanisms of MOB under anoxia is critical and timely. Here, we mapped the global distribution of MOB under anoxic aquatic zones and summarized four underlying metabolic strategies for MOB under anoxia: (a) forming a consortium with oxygenic microorganisms; (b) self-generation/storage of O₂ by MOB; (c) forming a consortium with non-oxygenic heterotrophic bacteria that use other electron acceptors; and (d) utilizing alternative electron acceptors other than O₂. Finally, we proposed directions for future research. This study calls for improved understanding of MOB under anoxia, and underscores the importance of this overlooked CH₄ sink amidst global aquatic deoxygenation.