Fig. 1.

Mechanisms of bidirectional extracellular electron transfer (EET) during the electro-fermentation process, including cytochromes, nanowires, and other redox proteins (such as ferredoxins) based direct electron transfer and redox mediators or primary metabolites mediated electron transfer. (A) Electrons generated by intracellular metabolism in the cytoplasm of microbes are transferred to the anode for oxidized chemicals production. Both c-type cytochromes and conductive pili (nanowires) have the ability to transfer electrons directly to extracellular electron acceptors. Electron mediators and artificial mediators can shuttle between electrode and microorganisms and exchange electrons between them through their own redox transformation. (B) Microbes uptake electrons from the cathode for reduced chemicals production. Both c-type cytochromes and ferredoxins have the ability to acquire electrons directly from extracellular electron donors. Primary metabolites and artificial mediators can shuttle between electrode and microorganisms and exchange electrons between them through their own redox transformation. RF_ox: oxidized riboflavin; RF_red: reduced riboflavin; MV_ox: oxidized methyl viologen; MV_red: reduced methyl viologen; NR_ox: oxidized neutral red; NR_red: reduced neutral red.