Humin-promoted microbial electrosynthesis of acetate from CO₂ by Moorella thermoacetica

Humin, an insoluble fraction of humic substances at any pH, has been reported to be an extracellular electron mediator (EEM) that functions in carbon dioxide (CO₂ )-fixing acetogenesis. Here, we show that humin promotes the microbial electrosynthesis (MES) of acetate from CO₂ using Moorella thermoacetica. Yeast extract, essential for the reaction of M. thermoacetica, resulted in the heterotrophic production of organic acids including acetate, hydrogen, and methane. Excluding the effect of yeast extract, MES with 13 g/L of suspended humin poised at -510 mV (vs. Ag/AgCl) achieved a CO₂ -fixing acetate production of 24.2 mg-acetate/L/day (1.9 mg-acetate/day/g-humin); this is 10-folds higher than the humin-free MES, with 90.3% of the coulombic efficiency. Although M. thermoacetica is an electroactive bacterium, it obtains electrons for acetogenesis mostly via humin. The suspended humin-assisted MES poised at -810 mV (vs. Ag/AgCl) increased the acetate production rate to 39.3 mg-acetate/L/day using electrons mainly from electrolyzed hydrogen and humin. Immobilization increased the humin's EEM efficiency, as indicated by the acetate production rate of 20.8 mg-acetate/L/day (6.9 mg-acetate/day/g-humin) with a 98.7% coulombic efficiency in MES with 3 g/L of immobilized humin poised at -510 mV (vs. Ag/AgCl). These results suggest that humin-assisted MES has high potential for microbial CO₂ fixation.