Bacteria living in surface-attached biofilm communities must maintain electrochemical gradients to support basic cellular functions, including chemo-osmotic transport and adenosine triphosphate synthesis. Central to this is the maintenance of electron flow to terminal electron acceptors. These acceptors can be soluble inorganic and organic molecules, such as oxygen, nitrate, sulfate, dimethyl sulfoxide, or fumarate, or solid metal oxides, such as Fe(III) and Mn(IV) oxides. When electrons are transferred to a solid substrate, they may be (1) carried directly to the acceptor via outer membrane cytochromes, (2) carried by electron shuttle molecules, (3) transferred along conductive protein nanowires, or (4) conducted through other extracellular matrices. No matter what the electron acceptor is, in the laboratory, bacterial biofilms are frequently studied while growing on inert surfaces, incapable of electron transfer. However, in natural environments, as well as many industrial and biotechnology settings, biofilms grow on electrically active surfaces. In this review, we propose that the study of bacterial biofilms on redox-active surfaces is important both for the development of industrial processes, such as microbial fuel cells and wastewater treatment systems, as well as for our understanding of how these communities of microbes affect global nutrient cycling, other geobiological processes, and even human disease.
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