Geothermal sites host microbial communities adapted to extreme environments, steep geochemical gradients, and high CO₂ fluxes. In this study, we exploit the unique potential of soil microbial communities from the Monte Amiata geothermal area, Italy (Fig.1a) to drive CO₂ reduction into value-added compounds using bioelectrochemical systems (BESs). Under a fixed applied voltage of 6 V (E cell) over 120 days, gas chromatography revealed CH4 production rates of 7.2 L·m⁻²·day⁻¹, while acetate accumulated to concentration of 5000 ppm. Microbial community analysis based on 16S rRNA gene sequencing shows the acetogen Sporomusa as the dominant member of the community followed by Methanobrevibacter, explaining both acetate accumulation and methane production (Fig.1c). qPCR analyses of mcrA gene further confirms methanogens colonization on the cathode surface, reaching 106 copies per cm-2 compared to 103 copies per cm-2 of unpolarized controls. Colonization by active microbial community has been further revealed by scanning electron microscopy (SEM) as shown in Fig.1d. Cyclic voltammetry measures a progressive increase in the electrochemical active surface area (ECSA) of the bioreactors, reaching 90 cm2 at the end of the experiment compared to 5 cm2 in controls. CO2 microsensor measurements revealed complete substrate consumption within 48 hours (Fig.1b); therefore, the set-ups were operated by CO2 sparging every 2 days. As the investigated geothermal area harboured sites with varying levels of sulfate, we further tested the effect of initial sulfate concentration on the bioelectrochemical CO2 reduction process. Elevated sulfate negatively affected the process, promoting enrichment of sulfate reducers. Overall, our results demonstrate that geothermal soils represent a promising source of microorganisms for bioelectrochemical CO₂ reduction, provided that sulfate concentrations are low. This approach offers a sustainable strategy to valorize geothermal CO₂ fluxes while recovering high-energy and valuable products.
Harnessing Geothermal Soils for CO2 Bioelectrochemical Reduction / G. Caucia, E. Cazzulani, A. Franzetti, F. Pittino, F. Pini, P. Fedeli, P. Cristiani. 29. International Symposium on Bioelectrochemistry and Bioenergetics of the Bioelectrochemical Society Bari 2026.
Harnessing Geothermal Soils for CO2 Bioelectrochemical Reduction
E. Cazzulani;
2025
Abstract
Geothermal sites host microbial communities adapted to extreme environments, steep geochemical gradients, and high CO₂ fluxes. In this study, we exploit the unique potential of soil microbial communities from the Monte Amiata geothermal area, Italy (Fig.1a) to drive CO₂ reduction into value-added compounds using bioelectrochemical systems (BESs). Under a fixed applied voltage of 6 V (E cell) over 120 days, gas chromatography revealed CH4 production rates of 7.2 L·m⁻²·day⁻¹, while acetate accumulated to concentration of 5000 ppm. Microbial community analysis based on 16S rRNA gene sequencing shows the acetogen Sporomusa as the dominant member of the community followed by Methanobrevibacter, explaining both acetate accumulation and methane production (Fig.1c). qPCR analyses of mcrA gene further confirms methanogens colonization on the cathode surface, reaching 106 copies per cm-2 compared to 103 copies per cm-2 of unpolarized controls. Colonization by active microbial community has been further revealed by scanning electron microscopy (SEM) as shown in Fig.1d. Cyclic voltammetry measures a progressive increase in the electrochemical active surface area (ECSA) of the bioreactors, reaching 90 cm2 at the end of the experiment compared to 5 cm2 in controls. CO2 microsensor measurements revealed complete substrate consumption within 48 hours (Fig.1b); therefore, the set-ups were operated by CO2 sparging every 2 days. As the investigated geothermal area harboured sites with varying levels of sulfate, we further tested the effect of initial sulfate concentration on the bioelectrochemical CO2 reduction process. Elevated sulfate negatively affected the process, promoting enrichment of sulfate reducers. Overall, our results demonstrate that geothermal soils represent a promising source of microorganisms for bioelectrochemical CO₂ reduction, provided that sulfate concentrations are low. This approach offers a sustainable strategy to valorize geothermal CO₂ fluxes while recovering high-energy and valuable products.
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