Systems Biology Approaches to Elucidating Electron Transfer Mechanisms and Optimizing Power Output of Microbial Fuel Cells

Even with the recent engineering innovations in fuel cell design that have substantially increased power densities, applications of microbial fuel cells are limited because of their low power outputs. One reason for this is that there are certain properties of the microorganisms that are donating electrons to anodes that are less than optimum for high-density current production. A better understanding of electron transfer mechanisms and metabolism with anode biofilms may aid the rational design of improved anode materials, other fuel cell attributes, or the organisms themselves. Geobacter sulfurreducens was chosen as a model organism because it: produces current densities as high as any known pure culture; completely oxidizes fuels, resulting in high coulombic efficiencies; and does not use electron shuttles for electron transfer to anodes. Furthermore, a full suite of genomic and genetic tools, including a genome-scale in silico model of metabolism, is available. Gene expression, proteomic, and genetic studies demonstrated that, in addition to the electrically conductive pili of this organism, a novel c-type cytochrome is specifically required for high-density power production. Several adaptive evolution strategies yielded strains that can produce substantially higher power densities and/or have enhanced substrate ranges. With new genome resequencing technologies it has been possible to identify the mutations associated with these improvements. The adaptive evolution approach as proven to be an important basic discovery tool for investigating the physiological factors limiting current production and is helpful for developing strategies for further enhancing the power output of microbial fuel cells.