Extracellular Electron Transfer of Weak Electricigens in the Presence of a Competing Electron Acceptor

Abstract

To maximise microbial electroactivity in bioelectrochemical systems, soluble electron acceptors are typically omitted as they compete with the electrode. While practical, this approach provides engineered conditions that do not reflect the natural environment of electroactive microorganisms, which may contain both soluble and insoluble electron acceptors. This study investigates the behaviour of weak electricigens, a relatively understudied category of microorganisms whose members switch between non-electroactive and electroactive states. Enrichments were performed in microbial fuel cells containing both an electrode and the soluble alternative fumarate to probe extracellular electron transfer of weak electricigens. Using fluorescence spectroscopy, chromatography and voltammetry, the electron shuttle riboflavin was not found in these conditions but was found in controls in which only the electrode was available to reduce. Despite this dichotomy in ability to perform riboflavin-based mediated electron transfer, communities of weak electricigens were similarly electroactive in each condition (19.36 ± 0.9 mW m−2 vs 20.25 ± 2.0 mW m−2). 16S rRNA gene sequencing revealed similar communities enriched in each condition, but with differing abundance. Understanding extracellular electron transfer in natural environments is of both fundamental and applied interest, as it can inform the design of real-world bioelectrochemical systems whose influents are likely to contain competing electron acceptors.