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.