Fig. 2.

Mechanisms of Electron Transfer in MES.

(a) Direct electron transfer (DET) occurs via (i) electron conduit transmembrane protein complex or (ii) pili/nanowires. Microbes with electron conduit transmembrane proteins can transfer electrons directly to and from electrodes through conductive porin-cytochrome protein complexes (such as MtrCAB), enabling efficient electron flow to drive redox reactions. Certain microbes utilize conductive pili or nanowires to transfer electrons directly to the electrode. This involves mechanisms such as electron transport through stacked aromatic residues or coherent/incoherent hopping through redox-active amino acid groups. (b) Indirect electron transfer (IET) uses electron carriers such as neutral red or hydrogen to shuttle electrons between microbial cells and the electrode, facilitating the reduction of NAD(P)⁺ to NAD(P)H and driving metabolic processes.