Extracellular electron transfer (EET) between electrochemically active microorganisms and electrodes plays a key role in microbial electrochemical systems (MESs) functioning of energy generation, bioremediation, etc. At present, researchers have a very limited understanding of the mechanism of EET, which is one of the major bottlenecks in application of MESs. Compared with direct electron transfer which requires a direct contact between microbial functional proteins and electrode, mediated electron transfer use electron transfer mediators (ETMs) which have reversible redox activities accompanies by high-efficiency EET for transporting electrons. ETMs serve as the middle electron acceptor, once reduced, can transfer electrons to terminal electron acceptor where upon it becomes re-oxidized. In principle, ETMs molecules could cycle thousands of times,thus, have a significant effect on the turnover of the terminal oxidant (e.g.iron) in certain circumstances.This review summarizes the recent advances of EET mechanisms with focus on mediated EET in MESs. Furthermore, we have highlighted the research trends of ETMs in MES,which will promote the practical applications of MESs in bioremediation, energy generation and so on.

Contents
1 Introduction
2 Roles of electron transfer mediators in extracellular electron transfer
3 Properties of electron transfer mediators
4 Classification of electron transfer mediators
5 Electron transfer mediators and their electron transfer mechanism
5.1 Exogenous electron transfer mediators
5.2 Endogenous electron transfer mediators
6 Outlook