High diversity of methanotrophic bacteria in geothermal soils affected byudhigh methane fluxes

摘要

Volcanic and geothermal systems emit endogenous gases by widespread degassing from soils, including CH4, audgreenhouse gas 25 times as potent as CO2. Recently, it has been demonstrated that volcanic/geothermal soils actudas source, but also as biological filter for methane release to the atmosphere. For long time, volcanic/geothermaludsoils has been considered inhospitable for methanotrophic microorganisms, but new extremophile methanotrophsudbelonging to Verrucomicrobia were identified in three different areas (Pozzuoli, Italy; Hell’s Gate, New Zealand;udKamchatka, Russia), explaining anomalous behaviours in methane leakages of several geothermal/volcanic sites.udOur aim was to increase the knowledge of the relationship between methane emissions from volcanic/geothermaludareas and biological methane oxidation, by investigating a geothermal site of Pantelleria island (Italy). PantelleriaudIsland hosts a high enthalpy geothermal system characterized by high temperature, high CH4 and very low H2Sudfluxes. Such characteristics are reflected in potentially great supply of methane for methanotrophs and scarce presenceudof inhibitors of their activity (H2S and NH3) in the Pantelleria soils. Potential methanotrophic activity withinudthese soils was already evidenced by the CH4/CO2 ratio of the flux measurements which was lower than that ofudthe respective fumarolic manifestations indicating a loss of CH4 during the gas travel towards the earth’s surface.udIn this study laboratory incubation experiments using soils sampled at Favara Grande, the main hydrothermaludarea of Pantelleria, showed very high methane consumption rates (up to 9500 ng CH4 h1 g1). Furthermore,udmicrobiological and culture-independent molecular analyses allowed to detect the presence of methanotrophsudaffiliated to Gamma- and Alpha-Proteobacteria and to the newly discovered acidothermophilic methanotrophsudVerrucomicrobia. Culturable methanotrophic Alpha-proteobacteria of the genus Methylocystis were isolated byudenrichment cultures. The isolates showed a wide range of tolerance to pH (3.5 – 8) and temperatures (18 – 45 C),udand an average methane oxidation rate of 450 ppm/h. A larger diversity of proteobacterial and verrucomicrobialudmethanotrophs was detected by the amplification of the methane mono-oxygenase gene pmoA.udThis study demonstrates the coexistence of both the methanotrophic phyla Verrucomicrobia and Proteobacteria inudthe same geothermal site. The presence of proteobacterial methanotrophs was quite unexpected because they areudgenerally considered not adapted to live in such harsh environments. Their presence at Favara Grande could beudexplained by not so low soil pH values (> 5) of this specific geothermal site and by the high methane availability.udSuch species could have found their niches in the shallowest part of the soils, were the temperatures are not soudhigh, thriving on the abundant upraising methane. Understanding the ecology of methanotrophy in geothermaludsites will increase our knowledge of their role in methane emissions to the atmosphere.