Delving through electrogenic biofilms: from anodes to cathodes to microbes

The study of electromicrobiology has grown into its own field over the last decades and involves microbially driven redox reactions at electrodes as part of a microbial electrochemical system (MES). The microorganisms known to use electrodes as either electron acceptors; electricigens, or electron donors; electrotrophs, drive the redox reactions within these systems through extracellular electron transfer (EET) processes. These exoelectrogenic microorganisms form biofilms, referred to as electroactive biofilms (EAB), in order to maximize adherence and contact with electrode surfaces and with one another. In this review, we will discuss the key differences between biofilms that utilize the electrode as an electron acceptor or donor, including their mechanisms for electron transfer, structural and functional compositions as well as which species are enriched for in each microenvironment. Lastly, we will discuss the intricacies of interspecies and intraspecies biofilm formation in electrode biofilms and considerations required for future bioengineering efforts.