Hydroxyl Radical-Driven Methanogenesis in Sunlit Surface Waters

Methane (CH4) production is commonly associated with anaerobic microbial respiration, yet substantial CH4 production has been observed in aerobic surface waters, presenting the “aerobic methane paradox”. Clarifying the mechanism behind aerobic CH4 production is essential for refining the global CH4 budget. Here, we demonstrate that sunlight-driven photochemical processes can effectively produce CH4 across various surface waters, including river, lake, and seawater. The CH4 formation rates ranged from 0.4 ± 0.1 to 0.7 ± 0.1 μmol/m2/h in the daytime, which were 2.3- to 3.9-fold higher as compared to those driven by the decomposition of organic matter by methanogenic archaea in the nighttime. Such aerobic CH4 production stems from the rapid oxidation of naturally abundant methyl donors (e.g., dimethyl sulfoxide) by a photochemically produced hydroxyl radical (•OH), which yields a methyl radical (•CH3) and subsequently leads to CH4 production. Simulations on CH4 productions across varying seasons, latitudes, altitudes, and regions at the global scale suggest that the photochemical processes contribute to 35.3–70.7% of marine CH4 emissions. Our study highlights a ubiquitous yet previously overlooked photochemical source of CH4 production in surface waters, shedding light on the “aerobic methane paradox”, which has implications on the global CH4 budget.