Steering Bioelectrochemical CO₂ Reduction Toward Methanogens Suppression and Acetogen Bioaugmentation

Biological strategies for converting carbon dioxide (CO₂) into valuable compounds are attractive approaches for a carbon-neutral future. Bioelectrochemical systems (BESs) 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 relationship 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 Acetobacterium woodii in the heat-shocked and acid-treated inoculum. Then, by using H-cell reactors, 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, showed 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 cathodic coulombic efficiency (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 in biocatalytic activity due to the presence of Acetobacterium woodii.