Fig. 3.
Examples of microbial electrosynthesis (cathodic electro-fermentation) based on direct EET (A and B) and shuttle-mediated EET (C, D, E, and F). (A) Electricity-driven metabolic shift of C. pasteurianum for butanol production from glucose and 1,3-propandiol production from glycerol [49]. (B) 2,3-butanediol production in a hydrogenase-deficient S. oneidensis using native Mtr proteins and exogenous light-driven proton pump (proteorhodopsin) for NADH accumulation [52]. (C) Different effects of neutral red (NR) and the barely studied redox mediator brilliant blue (BB) on the growth and product formation of C. pasteurianum grown on glycerol in a newly developed bioelectrochemical system [54]. (D) Chiral alcohol (R)-1-phenylethanol production from acetophenone in engineered E. coli by using methyl viologen (MV) as mediator to achieve EET [55]. (E) Hydrogen-driven microbial electrosynthesis from CO2 to acetate and methane in S. ovate and M. maripaludis via highly biocompatible transition-metal-based cathodes, respectively [62]. (F) Formate dehydrogenase (FDH)-assisted electrosynthesis of (3-hydroxybutyrate) (PHB) in R. eutropha, which overexpressed the ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) [65]. Formate reduced from CO2 and NR both served as the electron carriers to transfer electrons derived from cathodes into R. eutropha. PEM: proton exchange membrane; Rnf: a membrane-bound NADH:ferredoxin oxidoreductase; PEC: periplasmic electron carriers (FccA, CctA); OM: outer membrane; IM: inner membrane; LED: green light source; BBox: oxidized brilliant blue; BBred: reduced brilliant blue; NRox: oxidized neutral red; NRred: reduced neutral red; MVox: oxidized methyl viologen; MVred: reduced methyl viologen; LbADH: alcohol dehydrogenase from L. brevis; MtrA, STC and CymA: proteins of the electron transfer pathway in S. oneidensis MR-1.