Electron transfer mechanisms between microorganisms and electrodes in bioelectrochemical systems

Microbes have been shown to naturally form veritable electric grids in which different species acting as electron donors and others acting as electron acceptors cooperate. The uptake of electrons from cells adjacent to them is a mechanism used by microorganisms to gain energy for cell growth and maintenance. The external discharge of electrons in lieu of a terminal electron acceptor, and the reduction of external substrates to uphold certain metabolic processes, also plays a significant role in a variety of microbial environments. These vital microbial respiration events, viz. extracellular electron transfer to and from microorganisms, have attracted widespread attention in recent decades and have led to the development of fascinating research concerning microbial electrochemical sensors and bioelectrochemical systems for environmental and bioproduction applications involving different fuels and chemicals. In such systems, microorganisms use mainly either (1) indirect routes involving use of small redox-active organic molecules referred to as redox mediators, secreted by cells or added exogenously, (2) primary metabolites or other intermediates, or (3) direct modes involving physical contact in which naturally occurring outer-membrane c-type cytochromes shuttle electrons for the reduction or oxidation of electrodes. Electron transfer mechanisms play a role in maximizing the performance of microbe–electrode interaction-based systems and help very much in providing an understanding of how such systems operate. This review summarizes the mechanisms of electron transfer between bacteria and electrodes, at both the anode and the cathode, in bioelectrochemical systems. The use over the years of various electrochemical approaches and techniques, cyclic voltammetry in particular, for obtaining a better understanding of the microbial electrocatalysis and the electron transfer mechanisms involved is also described and exemplified.