Soil from geothermal environments host microbial communities adapted to elevated CO₂ levels and reducing conditions, yet their potential for bioelectrochemical CO₂ conversion remains unexplored. In this study, we investigated methane and acetate production in microbial electrolysis cells inoculated with soil samples collected from a high-CO₂–impacted geothermal area with different sulfate concentrations. We evaluated the effects of sulfate level and reactor configuration (with and without a cation exchange membrane) on electrochemical performance, product formation, and microbial community composition under high polarization conditions (Ecell: 6 V). Double-chamber reactors with high-sulfate concentration soil showed no detectable methane and low acetate concentration after 90 days (2.15 ± 1.65 mmol L⁻¹) with microbial communities dominated by sulfate-reducing bacteria (Desulfosporosinus) and nitrogen fixing bacteria (Azospirillum). In contrast, low-sulfate soil reactors showed substantial acetate and methane production, with acetate concentrations reaching up to 120 mmol L⁻¹, and methane production rates up to 350 mmol m⁻¹ day⁻¹ (Fig.1a). CO2 microsensor measurements showed complete substrate consumption within 48 hours, suggesting a highly active microbial community driving CO2 reduction processes. 16S rRNA gene sequencing revealed that acetogenic Sporomusa and hydrogenotrophic methanogenic Methanobrevibacter populations dominate the microbial community (Fig.1b). qPCR of mcrA gene further confirms methanogens colonization on the cathode surface, reaching 106 copies per cm-2. Overall, this study provides the first evidence that high-CO₂ geothermal soils can be directly used as inoculum source for CO₂-driven electrosynthesis, enabling sustained biologically mediated acetate and methane production. Sulfate concentration is identified as a key environmental driver of microbial community structure and product formation.
Microbial Enrichment from CO₂-Rich Geothermal Soil Drives Acetate and Methane Production in Bioelectrochemical Systems / G. Caucia, E. Cazzulani, R. Veerubhotla, U. Marzocchi, S. Ravasi, F. Pittino, A. Franzetti, P. Cristiani. EU ISMET Tolouse 2026.
Microbial Enrichment from CO₂-Rich Geothermal Soil Drives Acetate and Methane Production in Bioelectrochemical Systems
E. Cazzulani;
2026
Abstract
Soil from geothermal environments host microbial communities adapted to elevated CO₂ levels and reducing conditions, yet their potential for bioelectrochemical CO₂ conversion remains unexplored. In this study, we investigated methane and acetate production in microbial electrolysis cells inoculated with soil samples collected from a high-CO₂–impacted geothermal area with different sulfate concentrations. We evaluated the effects of sulfate level and reactor configuration (with and without a cation exchange membrane) on electrochemical performance, product formation, and microbial community composition under high polarization conditions (Ecell: 6 V). Double-chamber reactors with high-sulfate concentration soil showed no detectable methane and low acetate concentration after 90 days (2.15 ± 1.65 mmol L⁻¹) with microbial communities dominated by sulfate-reducing bacteria (Desulfosporosinus) and nitrogen fixing bacteria (Azospirillum). In contrast, low-sulfate soil reactors showed substantial acetate and methane production, with acetate concentrations reaching up to 120 mmol L⁻¹, and methane production rates up to 350 mmol m⁻¹ day⁻¹ (Fig.1a). CO2 microsensor measurements showed complete substrate consumption within 48 hours, suggesting a highly active microbial community driving CO2 reduction processes. 16S rRNA gene sequencing revealed that acetogenic Sporomusa and hydrogenotrophic methanogenic Methanobrevibacter populations dominate the microbial community (Fig.1b). qPCR of mcrA gene further confirms methanogens colonization on the cathode surface, reaching 106 copies per cm-2. Overall, this study provides the first evidence that high-CO₂ geothermal soils can be directly used as inoculum source for CO₂-driven electrosynthesis, enabling sustained biologically mediated acetate and methane production. Sulfate concentration is identified as a key environmental driver of microbial community structure and product formation.
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