The impact of methanotrophic activity on methane emissions through the soils of geothermal areas

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Methane plays an important role in the Earth’s atmospheric chemistry and radiative balance being themost important greenhouse gas after carbon dioxide. It has recently been established that geogenicgases contribute significantly to the natural CH4 flux to the atmosphere (Etiope et al., 2008).Volcanic/geothermal areas contribute to this flux, being the site of widespread diffuse degassing ofendogenous gases (Chiodini et al., 2005). In such an environment soils are a source rather than a sinkfor atmospheric CH4 (Cardellini et al., 2003; Castaldi and Tedesco, 2005; D’Alessandro et al., 2009;2011; 2013). Due to the fact that methane soil flux measurements are laboratory intensive, very fewdata have been collected until now in these areas. Preliminary studies (Etiope et al., 2007) estimateda total CH4 emission from European geothermal and volcanic systems in the range 4-16 kt a-1. Thisestimate was obtained indirectly from CO2 or H2O output data and from CO2/CH4 or H2O/CH4 valuesmeasured in the main gaseous manifestations. Such methods, although acceptable to obtainorder-of-magnitude estimates, completely disregard possible methanotrophic activity within the soil.At the global scale, microbial oxidation in soils contributes for about 3-9% to the total removal ofmethane from the atmosphere. But the importance of methanotrophic organisms is even largerbecause they oxidise the greatest part of the methane produced in the soil and in the subsoil before itsemission to the atmosphere. Environmental conditions in the soils of volcanic/geothermal areas (i.e.low oxygen content, high temperature and proton activity, etc.) have been considered inadequate formethanotrophic microrganisms. But recently, it has been demonstrated that methanotrophicconsumption in soils occurs also under such harsh conditions due to the presence of acidophilic andthermophilic Verrucomicrobia. These organisms were found in Italy at the Solfatara di Pozzuoli (Pol etal., 2007), in New Zealand at Hell’s Gate (Dunfield et al., 2007) and in Kamchatka, Russia (Islam et al.,2008).Both the Italian and the Hellenic territories are geodynamically very active with many activevolcanic and geothermal areas. Here we report on methane flux measurements made at Pantelleria(Italy) and at Sousaki and Nisyros (Greece). The total methane output of these three systems is about10, 19 and 1 t a-1, respectively (D’Alessandro et al., 2009; 2011; 2013). The total emissions obtainedfrom methane flux measurements are up to one order of magnitude lower than those obtained throughindirect estimations. Clues of methanotrophic activity within the soils of these areas can be found inthe CH4/CO2 ratio of the flux measurements which is always lower than that of the respective fumarolicmanifestations, indicating a loss of CH4 during the travel of the gases towards earth’s surface.Furthermore laboratory methane consumption experiments made on soils collected at Pantelleriaand Sousaki revealed, for most samples, CH4 consumption rates up to 9.50 μg h-1 and 0.52 μg h-1respectively for each gram of soil (dry weight). Only few soil samples displayed no methane2consumption activity.Finally, microbiological and molecular investigations allowed us to identify the presence ofmethanotrophic bacteria belonging to the Verrucomicrobia and to the Alpha- andGamma-Proteobacteria in the soils of the geothermal area of Favara Grande at Pantelleria. While thepresence of the former was not unexpected due to the fact that they include acidophilic andthermophilic organisms that were previously found in other geothermal environments,
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2013


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