Methanotrophic activity and diversity of methanotrophs in volcanic-geothermal soils at Pantelleria island (Italy)

摘要

Volcanic and geothermal systems emit endogenous gases by widespread degassing from soils, including CH4, a greenhouse gas twenty-five times as potent as CO2. Recently, it has been demonstrated that volcanic/geothermal soils are not only a source of methane,udbut also sites of methanotrophic activity. Methanotrophs are able to consume 10-40 Tg ofudCH4 a-1 and to trap more than 50% of the methane degassing through the soils.udWe report on methane microbial oxidation in the geothermally most active site ofudPantelleria island (Italy), Favara Grande, whose total methane emission was previouslyudestimated in about 2.5 Mg a-1 (t a-1). Laboratory incubation experiments with three top-soiludsamples from Favara Grande indicated methane consumption values up to 59.2 nmol g-1udsoil d.w. h-1. One of the three sites, FAV2, where the highest oxidation rate was detected,udwas further analysed on a vertical soil profile and the maximum methane consumption wasudmeasured in the top-soil layer and values >6.23 nmol g-1 h-1 were still detected up to auddepth of 13 cm. The highest consumption rate was measured at 37°C, but a still detectableudconsumption at 80°C (>1.25 nmol g -1 h-1) was recorded.udThe soil total DNAs extracted from the three samples were probed by PCR using standardudproteobacterial primers and newly designed verrucomicrobial primers, targeting the uniqueudmethane monooxygenase gene pmoA; the presence of methanotrophs was detected inudsites FAV2 and FAV3, but not in FAV1, where harsher chemical-physical conditions andudnegligible methane oxidation were detected. The pmoA gene libraries from the most activeudsite FAV2 pointed out a high diversity of gammaproteobacterial methanotrophs, distantlyudrelated to Methylococcus/Methylothermus genera and the presence of the newlyuddiscovered acido-thermophilic methanotrophs Verrucomicrobia. Alphaproteobacteria of theudgenus Methylocystis were isolated from enrichment cultures, under a methane containingudatmosphere at 37°C. The isolates grow at a pH range from 3.5 to 8, temperatures of 18 –ud45 °C and consume 160 nmol of CH 4 h-1 ml-1 of culture. Soils from Favara Grande showedudthe largest diversity of methanotrophic bacteria until now detected in a geothermal soil.udWhile methanotrophic Verrucomicrobia are reported to dominate highly acidic geothermaludsites, our results suggest that slightly acidic soils, in high enthalpy geothermal systems,udhost a more diverse group of both culturable and uncultivated methanotrophs.