Discussion on the sources and mechanism of supersaturated methane in euphotic seawater

Methane supersaturation occurs widely in the euphotic zone of oceans, especially in the areas with natural gas hydrate. It is closely related to atmospheric methane emission and global greenhouse effect due to the proximity of the sea-air interface. Up to date, it remains controversy concerning the source of supersaturated methane in euphotic seawater. This paper focuses on synthesizing the previous research results in order to sort out the sources of supersaturated methane, summarizing the influencing factors of supersaturated methane formation, and further exploring the mechanism of methane metabolism that in-situ microbes may participate in. The sources of supersaturated methane in euphotic zone may be transported from sediments, near-rivers or generated by in-situ microbes, and affected by various factors such as region, season, nutrient, and biological activities. Due to the influence of oxygen, the particularity of methanogenic mechanism is showed in euphotic seawater. Currently, it is speculated that conventional methanogenic pathways may be still performed by microorganisms, which exist in the micro-anaerobic environment of seawater, or generate the ability of resistance to oxygen; in addition, microorganisms may also choose new methanogenic pathways that are not sensitive to oxygen. Therefore, for the methane supersaturation phenomenon in euphotic seawater in natural gas hydrate area, the study of the sources and metabolic mechanisms of methane was carried out. It was hoped to provide theoretical support for the environmental assessment of gas hydrate test mining and development, and provide a theoretical basis for exploring the impact of seawater methane on the atmosphere and global climate.