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.

Activation of extracellular electron network in non-electroactive bacteria by Bombyx mori silk.

Extracellular electron transfer material (EETM) has increasingly attracted attentions for the enhancing effect on multiple microbial reactions. Especially, EETM is known to be essential to activate the energy network in non-electroactive bacteria. It is motivated to find out an EETM which is natural-based, environmentally friendly, and easily produced at large-scale. In this study, Bombyx mori silk is found, for the first time, to function as an EETM by using an EETM-dependent pentachlorophenol (PCP) dechlorinating anaerobic microbial culture. Subsequently, by dividing fibroin fiber into different soluble/insoluble fractions and correlating their EET functions with their structural properties based on various spectroscopic analyses, the ß-sheet configuration is suggested as an essential structure supporting the EET function of silk materials. The analyses also suggested the involvement of sulfur-containing amino acids in this function. The EET function is not degraded by boiling or acid/alkaline treatments and the material can be utilized multiple times, although it is susceptible to UV irradiation. Bombyx mori silk also enhance other microbial reactions, including Fe(III)OOH reduction, CO2 reduction to acetate, and nitrogen fixation. This discovery provides a basis for developing biotechnology for environmental remediation, global warming reduction, and biofertilizer production using Bombyx mori silk and its wastes.