Methane plays an important role in the Earth’s atmospheric chemistry and radiative balance being the
most important greenhouse gas after carbon dioxide. It has recently been established that geogenic
gases 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 of
endogenous gases (Chiodini et al., 2005). In such an environment soils are a source rather than a sink
for 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 few
data have been collected until now in these areas. Preliminary studies (Etiope et al., 2007) estimated
a total CH4 emission from European geothermal and volcanic systems in the range 4-16 kt a-1. This
estimate was obtained indirectly from CO2 or H2O output data and from CO2/CH4 or H2O/CH4 values
measured in the main gaseous manifestations. Such methods, although acceptable to obtain
order-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 of
methane from the atmosphere. But the importance of methanotrophic organisms is even larger
because they oxidise the greatest part of the methane produced in the soil and in the subsoil before its
emission 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 for
methanotrophic microrganisms. But recently, it has been demonstrated that methanotrophic
consumption in soils occurs also under such harsh conditions due to the presence of acidophilic and
thermophilic Verrucomicrobia. These organisms were found in Italy at the Solfatara di Pozzuoli (Pol et
al., 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 active
volcanic 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 about
10, 19 and 1 t a-1, respectively (D’Alessandro et al., 2009; 2011; 2013). The total emissions obtained
from methane flux measurements are up to one order of magnitude lower than those obtained through
indirect estimations. Clues of methanotrophic activity within the soils of these areas can be found in
the CH4/CO2 ratio of the flux measurements which is always lower than that of the respective fumarolic
manifestations, 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 Pantelleria
and Sousaki revealed, for most samples, CH4 consumption rates up to 9.50 μg h-1 and 0.52 μg h-1
respectively for each gram of soil (dry weight). Only few soil samples displayed no methane
2
consumption activity.
Finally, microbiological and molecular investigations allowed us to identify the presence of
methanotrophic bacteria belonging to the Verrucomicrobia and to the Alpha- and
Gamma-Proteobacteria in the soils of the geothermal area of Favara Grande at Pantelleria. While the
presence of the former was not unexpected due to the fact that they include acidophilic and
thermophilic organisms that were previously found in other geothermal environments, the latter are
generally considered not adapted to live in harsh geothermal environments. Their presence in the soils
of Pantelleria could be explained by the fact that these soils do not have extremely low pH values (>5).
Indeed thermotollerant methanotrophic Gamma-proteobacteria, have been previously found in the
sediments of thermal springs in Kamchatka (Kizilova et al., 2012). Such species could find their niches
in the shallowest part of the soils of Favara Grande were the temperatures are not so high and they
thrive on the abundant upraising hydrothermal methane.
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