Methane oxidation and emissions in cold seeps: from South China Sea to global scale

Cold seeps are chemosynthetic ecosystems on the seafloor that harbor diverse benthic communities by the supply of methane-rich fluids from subsurface reservoirs. Despite the global significance in biogeochemical cycling of cold seeps, the relative importance of methane-related microbial processes and the impact of methane leakage on the upper ocean remains not fully understood. We integrated a suite of biogeochemical approaches to elucidate microbial activity of methane oxidation in cold seeps sediment and overlying waters of South China Sea, and further estimate the role of methane oxidation in the regulation of methane emissions. Stable carbon isotope of methane suggested a biological origin and δ¹³C values of DIC indicated the dominance of methane oxidation. Radiotracer labelling showed that methanogenesis, anaerobic oxidation of methane and sulfate reduction concurrently occurred in seep sediments. In the overlying waters, methane concentrations in the vicinity of the seeps (up to ~71 µM) were well above background levels (~1−2 nM) and methane oxidation rates reached up to 8658 nmol L⁻¹ day⁻¹, among the highest rates documented in pelagic ocean. Using a machine learning model, we complied a database of methane emission and oxidation from global seeps. We estimated a global methane emission rate of 57.8 Tg yr⁻¹ from seeps to the overlying water columns and 31%−63% of this methane could be oxidized aerobically around seeping waters, suggesting that aerobic methanotrophy significantly reduces the emissions of methane released from submarine seeps. Our results also indicated that methane leakage from seeps could impact metabolic activity and carbon cycling in the deep ocean.