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Summary

Submarine mud volcanoes are a significant source of methane to the atmosphere. The Napoli mud volcano, situated in the brine-impacted Olimpi Area of the Eastern Mediterranean Sea, emits mainly biogenic methane particularly at the centre of the mud volcano. Temperature gradients support the suggestion that Napoli is a cold mud volcano with moderate fluid flow rates. Biogeochemical and molecular genetic analyses were carried out to assess the methanogenic activity rates, pathways and diversity in the hypersaline sediments of the centre of the Napoli mud volcano. Methylotrophic methanogenesis was the only significant methanogenic pathway in the shallow sediments (0–40 cm) but was also measured throughout the sediment core, confirming that methylotrophic methanogens could be well adapted to hypersaline environments. Hydrogenotrophic methanogenesis was the dominant pathway below 50 cm; however, low rates of acetoclastic methanogenesis were also present, even in sediment layers with the highest salinity, showing that these methanogens can thrive in this extreme environment. PCR-DGGE and methyl coenzyme M reductase gene libraries detected sequences affiliated with anaerobic methanotrophs (mainly ANME-1) as well as Methanococcoides methanogens. Results show that the hypersaline conditions in the centre of the Napoli mud volcano influence active biogenic methane fluxes and methanogenic/methylotrophic diversity.