Electrochemical evidence of direct electrode reduction by a thermophilic Gram-positive bacterium, Thermincola ferriacetica

Microbial fuel cells (MFCs) are bioelectrochemical devices capable of converting chemical energy to electrical energy using bacteria as the catalysts. Mechanisms of microbial electron transfer to solid electrode surfaces are not well defined in most electrochemically-active microorganisms, particularly for Gram-positive bacteria. In this study, we investigated the electrochemical characteristics of the Gram-positive, thermophilic bacterium Thermincola ferriacetica strain Z-0001. This organism was capable of transferring electrons from acetate to the anode of an MFC to generate an electric current. T. ferriacetica exhibited rapid recovery of current following medium exchanges, recovering to near-maximum current output in less than three hours. The recovery of electrons from acetate was 97% in air-cathode MFCs inoculated with T. ferriacetica. Further insights into the anode reduction by these biofilms were gained through cyclic voltammetry (CV). A continuous steady-state current was reached above -0.1 V vs. Ag/AgCl reference electrode in CV scans of an established T. ferriacetica biofilm. A catalytic wave with a midpoint potential consistently near -0.28 V indicated a continuous electron-transporting interface between the attached microbial biofilm and the electrode surface. Additionally, no significant peaks were observed when scanning cell-free spent medium from active MFCs. These data suggest that T. ferriacetica directly transfers electrons to an electrode through a mechanism that is tightly associated with the biofilm that forms on the electrode. This is the first mechanistic insight into how Gram-positive extracellular electron transfer might occur without the addition of soluble electron shuttling mediators. These mechanistic evaluations will be essential for the improvement and application of such biocatalysts in microbial fuel cells and other bioelectrochemical systems.