Diffuse hydrothermal methane output and evidence of methanotrophic activity within the soils at Sousaki (Greece)

Methane soil flux measurements have been made in 38 sites at the geothermal system of Sousaki (Greece) with the closed chamber method. Fluxes range from ?47.6 to 29 150 mg m^?2 day^?1, and the diffuse CH₄ output of the system has been estimated at 19 t a^?1. Contemporaneous CO₂ flux measurements showed a moderate positive correlation between CO₂ and CH₄ fluxes. Comparison of the CO₂/CH₄ soil flux ratios with the CO₂/CH₄ ratio of the gases of the main gas manifestations provided evidence for methanotrophic activity within the soil. Laboratory CH₄ consumption experiments confirmed the presence of methanotrophic microorganisms in soil samples collected at Sousaki. Consumption was generally in the range from ?4.9 to ?38.9 pmolCH₄ h^?1 g^?1 but could sometimes reach extremely high values (?33 000 pmolCH₄ h^?1 g^?1). These results are consistent with recent studies on other geothermal systems that revealed the existence of thermoacidophilic bacteria exerting methanotrophic activity in hot, acid soils, thereby reducing methane emissions to the atmosphere.