Higher butanol titer and selectivity in electro-fermentation experiments with Clostridium acetobutylicum ATCC 824 are due mainly to methyl viologen rather than electrode polarization

Butanol selectivity is crucial for the development of an industrial process targeting butanol as the main product of acetone-butanol-ethanol (ABE) fermentation by solventogenic Clostridia. This study evaluated electro-fermentation (EF), with an electron carrier, methyl viologen (MV), as a strategy to modify solvent production in Clostridium acetobutylicum ATCC 824 under optimized batch fermentation conditions. Cathodic EF was performed with or without 0.5 mM MV in single-compartment reactors with two-chamber and a negative potential was applied to the cathode. The greatest differences in final products concentrations were observed with the addition of MV compared to the control and EF fermentations. The fermentation with MV only was slowed down and resulted in 11% more butanol (control: 14.2 g.L^-1, MV: 15.8 g.L^-1) and half as much acetone, increasing the butanol/acetone (B/A) ratio by 137% from 1.93 to 4.57 g C.g C^-1. The pH profile was also modified with a final pH 0.5 unit higher. Conversely, EF did not appear to alter product formation significantly, even if slight differences were observed under our conditions. In addition, the reductase activity (RA) was measured by flow cytometry and showed that EF alone affected the intracellular redox state compared to the control fermentation, as did the addition of MV. However, electrode polarization had no effect on the extracellular oxidoreduction potential (ORP) profiles.