Chaeho Im ^1* Minsoo Kim ² Jung Rae Kim ² Kaspar Valgepea ³ Oskar Modin ⁴ Yvonne Nygård ⁴ Carl J. Franzén ^1*

• ¹ Department of Life Sciences, Division of Industrial Biotechnology, Chalmers University of Technology, Göteborg, Vastra Gotaland County, Sweden
• ² School of Chemical Engineering, Pusan National University, Busan, Busan, Republic of Korea
• ³ Institute of Bioengineering, University of Tartu, Tartu, Tartu County, Estonia
• ⁴ Department of Architecture and Civil Engineering, Division of Water Environment Technology, Chalmers University of Technology, Göteborg, Sweden

Fossil resources must be replaced by renewable resources in production systems to mitigate greenhouse gas emissions and combat climate change. Electro-fermentation utilizes a bioelectrochemical system (BES) to valorize industrial and municipal waste. Current electro-fermentation research is mainly focused on microbial electrosynthesis using CO2 for producing commodity chemicals and replacing petroleum-based infrastructures. However, slow production rates and low titers of metabolites during CO2-based microbial electrosynthesis impede its implementation to the real application in the near future. On the other hand, CO is a highly reactive gas and an abundant feedstock discharged from fossil fuel-based industry. Here, we investigated CO and CO2 electrofermentation, using a CO-enriched culture. Fresh cow fecal waste was enriched under an atmosphere of 50% CO and 20% CO2 in N2 using serial cultivation. The CO-enriched culture was dominated by Clostridium autoethanogenum ( 89%) and showed electro-activity in a BES reactor with CO2 sparging. When 50% CO was included in the 20% CO2 gas with 10 mA applied current, acetate and ethanol were produced up to 12.9  2.7 mM and 2.7  1.1 mM, respectively. The coulombic efficiency was estimated to 148% ± 8% without an electron mediator. At 25 mA, the culture showed faster initial growth and acetate production but no ethanol production, and only at 86% ± 4% coulombic efficiency. The maximum optical density (OD) of 10 mA and 25 mA reactors were 0.29 ± 0.07 and 0.41 ± 0.03, respectively, whereas it was 0.77 ± 0.19 without electric current. These results show that CO electro-fermentation at low current can be an alternative way of valorizing industrial waste gas using a bioelectrochemical system.