Steering Bioelectrochemical CO2 Reduction Toward Methanogens Suppression and Acetogen Bioaugmentation

Biological strategies for converting CO2 into valuable compounds are attractive ap-proaches for a carbon-neutral future. Bioelectrochemical systems (BES) represent an innovative strategy for the control of microbial metabolism, in which electrochemical techniques are adopted to stimulate reductive and oxidative processes. Acetogenesis and methanogenesis are the two main chemoautotrophic pathways of CO2 reduction usually present in anaerobic environments. Due to the syntrophic and competitive re-lationship between acetogens and methanogens, methanogenesis inhibition strategies should be adopted to direct CO2 reduction towards acetate and fatty acids. In this work, an acetogen-enriched inoculum was produced by the bioaugmentation of Acetobacte-rium Woodii in the heat-shocked and acid-treated inoculum. Then, by using H-cell re-actors, without the use of any chemical inhibitor, this inoculum was tested in semi-continuous mode by imposing a dilution rate previously identified from growth kinetic assessment. Bioelectrochemical tests, conducted at -0.9 V and -0.7 V vs SHE, shown the overcoming of acetogenesis on methanogenesis. At −0.7 V vs SHE, acetate was produced at 0.0385 ± 0.009 mmol d⁻¹ and 0.0343 ± 0.010 mmol d⁻¹ in the absence and presence of bioaugmentation, respectively, with acetate CCEs of 68% and 64%. At −0.9 V vs SHE, bioaugmentation markedly reduced methanogenesis, decreasing the methane production rate from 0.105 ± 0.012 to 0.007 ± 0.004 mmol d⁻¹, while acetate production reached 0.061 ± 0.025 mmol d⁻¹ with a CCE of 43%. Finally, the effect of bioaugmentation was demonstrated by cyclic voltammetry of the biocathode, which showed the increase of biocatalytic activity due to the presence of Acetobacterium Woodii.