Bioelectrochemical systems (BESs) take advantage of the ability of organisms to catalyze electrochemical reactions for the production of electricity, biofuels and chemicals, or to perform valuable services such as wastewater treatment or the bioremediation toxic compounds. While many of these reactions occur naturally, increased reaction rates and product yields can be obtained by growing the organisms in BESs where a voltage difference between two electrodes provides additional thermodynamic impetus for the reactions by either accepting or donating electrons. Research to date has focused on the application of microbial fuel cells (MFCs) in which organisms oxidize organic matter and produce electricity, and on microbial electrolysis cells (MECs) in which a combination of organic matter oxidation and additional voltage inputs between the two electrodes catalyzes the production of H2 for use as a biofuel. Many microorganisms have been shown to be capable of electron transfer reactions in BESs. Understanding these organisms will help to develop better-performing strains and is a major avenue of research for improving the performance of BESs. Geobacter sulfurreducens is a dissimilatory iron-reducing bacterium that forms electrochemically active biofilms on anode electrodes of BESs and efficiently couples the oxidation of acetate to electricity production. I investigated additional electron donors for G. sulfurreducens current production. Formate and lactate were able to support the growth and electroactivity of G. sulfurreducens with electrodes as a terminal electron acceptor, though with reduced efficiency compared with acetate. The anode biofilms grown with these substrates had different structural characteristics than those grown with acetate, which was suggestive of metabolic limitations. The addition of small amounts of acetate promoted formate carbon assimilation and improved current production, and pre-growing the biofilms with acetate enabled lactate to be oxidized as an electron donor.Acetate, formate and lactate are often byproducts of ethanol fermentation so I sought to identify organisms capable of fermenting renewable substrates into ethanol and producing the electron donors for G. sulfurreducens. Cellulomonas uda was identified as a consolidated bioprocessing organism capable of degrading ammonia fiber expansion-pretreated corn stover. The two organisms were able to syntrophically interact, resulting in the simultaneous production of ethanol and H2 in a MEC. Optimization of C. uda fermentation by nitrogen supplementation resulted in energy recoveries of ca. 56% from ethanologenesis, and the co-generation of H2 in the MEC further increased the energy recoveries to ca. 73%.Clostridium cellobioparum was identified as a species capable of fermenting glycerol, a major waste product of the biodiesel industry, into ethanol, 1,3-propanediol and electron donors for G. sulfurreducens. The two strains were improved by adaptive evolution for glycerol tolerance so growth could be sustained at 100 g/L glycerol loadings. Optimization of the coculture growth in MECs increased the consumption of glycerol to ca. 50 g/L. Decreases in pH due to the large amounts of fermentation products presented a challenge to MEC operation and opened avenues of further research for improvements of the BES configurations.
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