1. Department of Microbiology, IWWR, Radboud University Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands
2. Dipartimento di Scienze Ambientali, Seconda Università di Napoli, Via Vivaldi 43, 81100 Caserta, Italy

Correspondence to: Mike S. M. Jetten¹Huub J. M. Op den Camp¹ Correspondence and requests for materials should be addressed to M.S.M.J. (Email: m.jetten@science.ru.nl) or H.J.M.O.d.C. (Email: h.opdencamp@science.ru.nl).

Mud volcanoes, mudpots and fumaroles are remarkable geological features characterized by the emission of gas, water and/or semi-liquid mud matrices¹ with significant methane fluxes to the atmosphere (10^-1 to 10³ t y^-1)^{2, 3, 4}. Environmental conditions in these areas vary from ambient temperature and neutral pH to high temperatures and low pH. Although there are strong indications for biological methane consumption in mud volcanoes^{4, 5}, no methanotrophic bacteria are known that would thrive in the hostile conditions of fumaroles (temperatures up to 70 °C and pH down to 1.8)². The first step in aerobic methane oxidation is performed by a soluble or membrane-bound methane mono-oxygenase. Here we report that pmoA (encoding the -subunit of membrane-bound methane mono-oxygenase) clone libraries, made by using DNA extracted from the Solfatara volcano mudpot and surrounding bare soil near the fumaroles, showed clusters of novel and distant pmoA genes. After methanotrophic enrichment at 50 °C and pH 2.0 the most distant cluster, sharing less than 50% identity with any other described pmoA gene, was represented in the culture. Finally we isolated an acidiphilic methanotrophic bacterium Acidimethylosilex fumarolicum SolV belonging to the Planctomycetes/Verrucomicrobia/Chlamydiae superphylum⁶, 'outside' the subphyla of the Alpha- and Gammaproteobacteria containing the established methanotrophs. This bacterium grows under oxygen limitation on methane as the sole source of energy, down to pH 0.8—far below the pH optimum of any previously described methanotroph. A. fumarolicum SolV has three different pmoA genes, with two that are very similar to sequences retrieved from the mudpot. Highly homologous environmental 16S rRNA gene sequences from Yellowstone Park show that this new type of methanotrophic bacteria may be a common inhabitant of extreme environments. This is the first time that a representative of the widely distributed Verrucomicrobia phylum, of which most members remain uncultivated⁶, is coupled to a geochemically relevant reaction.

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