Variations in microbial community compositions and processes imposed under contrast geochemical contexts in Sicilian mud volcanoes, Italy

Terrestrial mud volcanoes are important atmospheric methane source in natural environments. Methane regulation involving microbial processes has not been fully examined in terrestrial mud volcanoes with various geological origins. This study demonstrated the geochemical characteristics and microbial community in four mud volcanoes and seeps sites associated with different geological settings in Sicily. At the sites within accretionary wedge where methane and sulfate are more abundant, the community members related to methane and sulfate metabolisms and involved in the organic degradation. Both anaerobic and aerobic methanotrophy controlled the methane release from terrestrial mud volcanoes related to the subduction processes. Unlike being a dominated metabolic scheme for methane consumption in marine cold seeps, sulfate reduction coupling with anaerobic methane oxidation might only be partly involved in terrestrial seeps. In contrast, the sites near Mt. Etna were characterized by high salinity and CO2, and low methane and sulfate, where the community compositions were composed of halophilic organic degraders and sulfur metabolizers along with a minor fraction of aerobic methanotrophs. The depth variation in community composition suggests the local physiochemical context imposed a stronger control on the community compositions and functions. Apparently, the geological context combined with the fluid path and source characteristics drives the proliferation of specific community members at different depth ranges and sites. Owning to diverse geological frameworks in Sicily, a wide spectrum of community compositions and functions is involved into the cycling of carbon and sulfur in Sicilian terrestrial mud volcanoes.