The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Methane (CH₄) is the second most important greenhouse gas after carbon dioxide (CO₂). In aquatic environments, methanogenesis is considered a strictly anaerobic process. Contradictorily, the oxygen-rich waters of the surface open ocean are almost always supersaturated with methane, a phenomenon termed the oceanic methane paradox (OMP). Here, we collected various waters along the land-ocean continuum and demonstrated that the abiotic degradation of chromophoric dissolved organic matter (CDOM) in the surface ocean represents an important mechanism of OMP. Such process maintains the supersaturation of methane in surface waters and sustains methane emissions to the atmosphere on a global scale. Generally, methane photoproduction rates decreased seaward, whereas its relative production efficiency and the methane‐to-carbon‐monoxide (CO) photoproduction ratio (ΔCH₄ /ΔCO) both followed a reversed trend. Remote‐sensing modeling incorporating a ΔCH₄ /ΔCO–CDOM absorption relationship yielded an annual methane photoproduction of 118 Gg for the global open ocean, accounting for 20–60% of the open‐ocean methane efflux and being of comparable magnitude to the upper‐ocean methane microbial‐oxidation sink. A potential issue is therefore arising in that, while higher phytoplankton biomass may enhance carbon sequestration, the degradation of increased phytoplankton-derived DOM could lead to greater CH₄ production, which has a stronger greenhouse gas effect than CO₂. The eventual impact of these processes on the carbon sequestration of the biological carbon pump is yet to be further explored.
 

methane photoproduction,oceanic methane paradox,chromophoric dissolved organic matter