Improved robustness of microbial electrosynthesis by adaptation of a strict anaerobic microbial catalyst to molecular oxygen

Abstract Microbial electrosynthesis (MES) and other bioprocesses such as syngas fermentation developed for energy storage and the conversion of carbon dioxide into valuable chemicals often employs acetogens as microbial catalysts. Acetogens are sensitive to molecular oxygen, which means that bioproduction reactors must be maintained under strict anaerobic conditions. This requirement increases cost and does not eliminate the possibility of O2 leakage. For MES, the risk is even greater since the system generates O2 when water splitting is the anodic reaction. Here, we show that O2 from the anode of a MES reactor diffuses into the cathode chamber where strict anaerobes reduce CO2. To overcome this drawback, a stepwise adaptive laboratory evolution (ALE) strategy is used to develop the O2 tolerance of the acetogen Sporomusa ovata. Two heavily-mutated S. ovata strains growing well autotrophically in the presence of 0.5 to 5% O2 were obtained. The adapted strains were more performant in the MES system than the wild type converting electrical energy and CO2 into acetate 1.5 fold faster. This study shows that the O2 tolerance of acetogens can be increased, which leads to improvement of the performance and robustness of energy-storage bioprocesses such as MES where O2 is an inhibitor.