Cold seeps and hydrothermal vents 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 and hydrothermal vent, 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, methane production, and sulfate reduction from cold seeps and hydrothermal systems, 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 (AOM) and sulfate reduction (SR) concurrently occurred in seep sediments. In the overlying waters of South China Sea seeps, methane concentrations in the vicinity of the seeps (up to ~71 µM) were well above background levels (~1−2 nM) and MOx rates in overlying water reached up to 8658 nmol L⁻¹ day⁻¹, among the highest rates documented in pelagic ocean. Using a machine learning model, we estimated a global methane emission rate of 57.8 Tg yr⁻¹ from seeps to the overlying water columns and 36%−66% of this methane could be oxidized aerobically around seeping waters, suggesting that aerobic methanotrophy significantly reduces the emissions of methane released from submarine seeps. In the hydrothermal plume-impacted waters, both methanotrophic and heterotrophic activity was elevated, indicating that microbial communities quickly responded to hydrothermal inputs. Collectively, methane leakage from seeps and heterotrophic production near hydrothermal vents may profoundly impact metabolic activity and carbon cycling in these deep-sea habitats.
 

methane,Cold seep,Hydrothermal vent