Nano-magnetite effects on CO2 reduction of homoacetogens with different extracellular electron transfer pathways

This study aimed to investigate the effect of nano-magnetite on the CO₂ reduction of homoacetogens with different extracellular electron transfer (EET) pathways in the microbial electrosynthesis system (MES). Homoacetogens were cathode-cultured in the MES, which was operated under voltages of 0.5 and 0.8 V. Compared with the MES without magnetite, acetate (i.e., a C2–C4 carboxylic acid) yield was doubled in the MES with nano-magnetite under 0.5 V. However, the nano-magnetite effect was not significant in the MES under 0.8 V. The result suggested that homoacetogens in the MES had different EET pathways under different voltages. Based on SEM and CV results, magnetite promoted microorganism adherence to the cathode, and enhanced catalytic activity of the biocathode at 0.5 V, which increased acetate yield. Carbonyl cyanide m-chlorophenylhydrazone inhibited the ATP synthesis of homoacetogens in the H₂-cultured condition but increased the current output in the biocathode, suggesting that the pathway for homoacetogens to harvest electrons from the cathode was different from the H₂-mediated pathway. Rotenone inhibited the current output of biocathode at 0.5 V by 3.6% and 15% with and without nano-magnetite, respectively, indicating that NADH dehydrogenase played an important role on the EET pathways of homoacetogens.