| Summary: | Bioelectrochemical systems (BESs) is a term that encompasses a group of novel technologies able to interconvert electrical energy and chemical energy by means of a bioelectroactive biofilm. Microbial electrosynthesis (MES) systems, which branch off from BESs, are able to convert CO<sub>2</sub> into valuable organic chemicals and fuels. This study demonstrates that CO<sub>2</sub> reduction in MES systems can be enhanced by enriching the inoculum and improving CO<sub>2</sub> availability to the biofilm. The proposed system is proven to be a repetitive, efficient, and selective way of consuming CO<sub>2</sub> for the production of acetic acid, showing cathodic efficiencies of over 55% and CO<sub>2</sub> conversions of over 80%. Continuous recirculation of the gas headspace through the catholyte allowed for a 44% improvement in performance, achieving CO<sub>2</sub> fixation rates of 171 mL CO<sub>2</sub> L<sup>−1</sup>·d<sup>−1</sup>, a maximum daily acetate production rate of 261 mg HAc·L<sup>−1</sup>·d<sup>−1</sup>, and a maximum acetate titer of 1957 mg·L<sup>−1</sup>. High-throughput sequencing revealed that CO<sub>2</sub> reduction was mainly driven by a mixed-culture biocathode, in which <i>Sporomusa</i> and <i>Clostridium</i>, both bioelectrochemical acetogenic bacteria, were identified together with other species such as <i>Desulfovibrio</i>, <i>Pseudomonas</i>, <i>Arcobacter</i>, <i>Acinetobacter</i> or <i>Sulfurospirillum</i>, which are usually found in cathodic biofilms. Moreover, results suggest that these communities are responsible of maintaining a stable reactor performance.
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