Since their first discovery 20 years ago, numerous cold seeps have been found, mainly along active and passive continental margins. Methane gas plays a key role in the global carbon cycle and, as a highly potent greenhouse gas, in the control of the Earth's climate. In diffusion-controlled sediments, methane is completely oxidized within the sediment column and does not reach the overlying water. In contrast, at cold seeps, methane is transported to the sediment surface by advective forces. For geologic reasons, fluid advection and the respective methane flux are spatially and temporarily variable. Microbial communities of methane-oxidizing archaea in syntrophy with sulfate-reducing bacteria have been found to play a key role in consuming methane and thus controlling methane efflux from the sediments. Anaerobic oxidation of methane leads to the production of dissolved inorganic carbon and subsequent precipitation of carbonate, representing a permanent sink for methane-derived carbon. The metabolic pathway and the role of the different groups of methanotrophic archaea and sulfate-reducing bacteria that are involved in the anaerobic oxidation of methane remain poorly understood. There seem to be site-specific differences in the composition and function of the microbial communities. The upwelling of methane- and sulfide-rich fluids supports rich benthic communities of sulfur-oxidizing bacterial mats and abundant macrofauna species harboring thiotrophic or methanotrophic symbionts. Variation in fluid flow, and thus methane supply and hydrogen sulfide concentrations, are key factors controlling the occurrence and community structure of benthic communities.
