The Anaerobic Oxidation of Methane (AOM) is a central microbial activity in marine sediment experiencing upward migration of this potent greenhouse gas. In the Gulf of Cadiz (East Atlantic margin), over 30 mud volcanoes act as important methane escape pathways from deep-rooted hydrocarbon reservoirs toward sediment surface, and fuel active AOM microbial communities. These mud volcanoes span a wide range of water depths (300 to 2300 m deep), temperatures (4 to 13 °C), salinities (brackish to saturated brines), methane fluxes, geofluid composition and sediment depths. They hence constitute an ideal natural laboratory to study shifts in microbial community structure, functional gene repertoires and expression along these natural gradients. In a recent study (Maignien et al., in press), we have examined how AOM-mediating microorganisms can thrive up to saturated salinity in the center and rim of the Mercator mud volcano. This study raised numerous hypothesis regarding functioning of anaerobic methanotroph Archaea, and electron transfer during methane oxidation. Thanks to the support of C-DEBI, the results of this study could be presented in the 14th meeting of the International Society for Microbial Ecology (ISME14, August 19-14 2012), in Copenhagen. The talk, which was part of the “Microbial Life in Extreme Environments” session, sparked some interesting discussions that were a great incentive to continue the work!