Improving Electrochemical Techniques for Studying Dissimilatory Metal Reducing Bacteria

Abstract

Advancements in the field of biotechnology over the past few decades have provided solutions to many research problems and have paved the way to new scientific discoveries. From development of different applications of microbial fuel cells and discovery and characterization of new classes of microorganisms to progress in whole-genome sequencing and annotation methods, the world of biotechnology is expanding every day. Currently, the world is dependent on fossil fuels to meet its energy demand but for future, we have to invent new strategies to fulfill global energy needs. One strategy is the use of biofuels produced by genetically engineered microbes. The dissimilatory metal reducing bacteria like Shewanella oneidensis utilize metals including Fe(III), Mn(IV), and U(VI) as terminal electron acceptors for anaerobic respiration and possess the ability of extracellular electron transfer. This ability of extracellular electron transfer can be useful to many biotechnological applications, however there are three major technological challenges that must be addressed: scalability, control over operation process, and defined biological pathways. The dissimilatory metal reducing bacterium (DMRB) Shewanella oneidensis MR-1 is used as a model organism for studies in microbial fuel cells and bioelectrochemical reactors. In this study that follow, aspects related to increasing control of bioelectrochemical reactors were addressed. Work was done to improve the coulombic efficiency of S. oneidensis in reactors by improving design of anoxic bioreactors. The new reactors were found to have < 3 ppm oxygen and showed no planktonic growth. The current density was also higher in new reactors (23 µA/cm2) compared to 10 µA/cm2 in old bioreactors. The coulombic efficiency of S. oneidensis in new reactors was measured to be 86.4 ± 10.37%, a vast improvement over alternative electrochemical systems. Hydrogen metabolism in S. oneidensis biofilms was also studied in these new better controlled reactors, and the role of hydrogenases in electron transfer from S. oneidensis to electrodes in a bioreactor was studied. It was found that deletion of the hydrogenases in S. oneidensis bioreactors funneled the electron transfer to electrodes and improved the coulombic efficiency by 30% indicating their role in extracellular electron transfer. In summary, the improved bioelectrochemical system described herein will be useful in studying phenotypes of various mutants of S. oneidensis and other metal reducing bacteria under anaerobic conditions on electrodes of defined redox potential. This well-defined and robust bioelectrochemical system will aid the study of extracellular electron transfer and may provide a platform for the design of microbial fuel cells and the production of alternative fuels.