Microscale Gradients in Electrochemically Active Biofilms and Their Role in Extracellular Electron Transfer Processes
Authors: Beyenal Haluk , Babauta Jerome T , Renslow Ryan Scott 
Published: February 2020

Electrochemically active biofilms (EABs) can exchange electrons with an inert, conductive surface - an electrode. Specifically, biofilms that deliver electrons to the electrode are described as anodic biofilms, whereas biofilms that accept electrons or facilitate reduction reactions at the electrode surface are cathodic biofilms. Because EABs consist of many individual cells contributing to the overall electron transfer processes of the biofilm, the chemical and electrochemical gradients that form within EABs play a critical role in understanding macroscopic properties of EABs. Simply, the optimal conditions expected for individual cells in the bulk phase may not exist for individual cells within the EAB because of the biofilm structure and metabolic activity.
EAB processes are an interfacial phenomenon: EABs interact with the electrode below the biofilm diffusive and reactive layers at the electrode surface. The microscale layers close to the electrode are directly related to extracellular electron transfer, whereas diffusion processes above these layers are linked indirectly. We expect that the surface concentrations of the redox-active compounds and local solution properties inside EABs are more relevant and critical than the corresponding values in the bulk. Correlating and fitting lines to bulk data may have little significance to the fundamental processes occurring inside EABs. Direct measurements inside EABs are preferred, such as measuring pH inside EABs or measuring the spectroelectrochemical properties of EABs (Babauta et al. 2011, Franks et al. 2009, Liu et al. 2011). This is especially important since the cell density inside some EABs is not uniformly distributed and predictions based on simple diffusion may not apply (Renslow et al. 2010).
Although a significant amount of research has been undertaken to elucidate EAB electron transfer processes, there is only a limited set of direct measurements of the chemical and electrochemical gradients found within EABs. We believe the chemical and electrochemical gradients are critical for explaining electron transfer mechanisms. The presence and absence of microscale gradients within EABs correlates well with the observed electron transfer rates and can explain how reactor configuration and operating conditions can limit EAB performance.
Our research group has developed microscale tools to directly measure the chemical and electrochemical gradients formed within EABs. It was found that pH was not always limiting EAB performance. We have also demonstrated the usefulness of redox potential measurements within EABs, and we have defined and measured the local biofilm potential for the first time. We measured diffusion coefficients, porosity, and electron donor concentration inside EABs using nuclear magnetic resonance imaging techniques. This presentation discusses the reactor configurations used in bioelectrochemical systems research and demonstrate how to use selected voltammetric techniques to study extracellular electron transfer in bioelectrochemical systems. Finally, we address some critical concerns with the proposed electron transfer mechanisms in biofilms and the prospects of bioelectrochemical systems as energy-converting and energy-harvesting devices.

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