Artificial extracellular electron transfer engineering in nonclassical electroactive microorganisms for microbial electrochemical technologies.

Microbial electrochemical technologies (METs) couple microbial metabolism with extracellular electronic interfaces, but their broader application is limited by the small repertoire of naturally electroactive microorganisms. Nonclassical-electroactive microorganisms (nonclassical-EAMs) offer diverse metabolic functions, mature genetic tools, and environmental robustness, yet they usually lack efficient extracellular electron transfer (EET) pathways. This review critically examines artificial EET engineering as a strategy to convert function oriented nonclassical-EAMs for bioresource and environmental applications. We first summarize natural EET mechanisms as design templates, with an emphasis on cell-level direct and indirect electron transfer mechanisms as well as biofilm-level electron transfer mechanisms. We then compare three major engineering routes: genetically encoded or material-assisted direct electron transfer, endogenous or exogenous electron shuttles based indirect electron transfer, and electroactive biofilm engineering. Attention is given to host compatibility, redox-potential alignment, electron transfer directionality, metabolic coupling, and operational stability. Emerging applications in microbial biosensing, microbial electrosynthesis, carbon dioxide conversion, waste valorization, and pollutant remediation are discussed from the perspective of whether engineered electron flow is productively linked to target functions. Finally, we propose a demand-driven framework for selecting EET modules according to host physiology, application goals, electron-flow direction, and system constraints. Future progress will depend on quantitative electron-flow analysis, dynamic flux control, stable biofilm interfaces, and standardized performance metrics that distinguish functional electron transfer from apparent electroactivity. Artificial EET engineering of nonclassical-EAMs may expand METs into a more versatile platform for sustainable bioconversion and environmental biotechnology.