Under anaerobic conditions, many microorganisms are capable of extracellular respiration involving electron transfer to or from extracellular substrates such as iron (hydr)oxides and humic substances. Electron shuttling is one of the significant strategies for extracellular electron transfer, however, the involved mechanism has not been thoroughly understood. Electron shuttles can be divided into endogenous electron shuttles that are self-produced by microbes themselves and exogenous electron shuttles that are natural substances or artificially synthesized materials. Electron shuttle-mediated extracellular electron transfer generally involves the following reactions: the oxidized form of electron shuttles (ES_ox) accept electrons from the oxidization of organic matter and become as the reduced form of electron shuttles (ES_red), then ES_red transfer electrons to extracellular electron acceptors and return to ES_ox. Through these steps, electron shuttles can be reversibly oxidized and reduced. This review mainly focuses on the electron transfer mechanisms of different electron shuttles, and the factors affecting extracellular electron transfer such as the molecule diffusion, redox potential and electron transfer capacity of electron shuttles. Electron shuttle-mediated extracellular electron transfer has significant influence on contaminants degradation and microbial electrogenesis, thus the better understanding of their mechanisms is very important to their implications in bioremediation and bioenergy.

Contents
1 Introduction
2 Electron transfer mechanisms of different electron shuttles
2.1 Endogenous electron shuttles
2.2 Exogenous electron shuttles
3 Factors affecting extracellular electron transfer
3.1 Molecule diffusion
3.2 Redox potential
3.3 Electron transfer capacity
4 Environmental implications
4.1 The applications of electron shuttles in pollutant biodegradation
4.2 The applications of electron shuttles in bioelectrochemical systems
5 Conclusion and outlook