Investigation of charge transfer mechanisms for electroconductivity and bioremediation in bacterial biofilms
Wang, Victor Bochuan
Date of Issue2014
School of Materials Science and Engineering
This thesis examines extracellular electron transfer (EET) mechanisms and flavins/metabolite exchange in chemically modified biofilms and natural systems to demonstrate syntrophy for energy and bioremediation applications. Recently, chemical modification of bacteria to exhibit enhancement in microbial fuel cells (MFCs), bioremediation of waste products and electrode-driven biosynthesis of by-products has been demonstrated through insertion and self-assembly of unique organic molecules in cellular membranes. The mechanism for enhancement had been attributed to improved charge transfer through the inserted conjugated molecules. Using the novel transmembrane electron transport molecule (TETM), namely, 4, 4’-bis (4’-(N, N-bis (6’’-(N, N, N trimethylammonium)hexyl)amino)-styryl)stilbene tetraiodide, (DSSN+) to chemically modify Escherichia coli, the role of the TETM as a potential charge transfer pathway at the microbe-electrode (biotic-abiotic) interface is studied using MFCs. Significant EET enhancement is observed in this platform and confocal microscopy techniques confirms the incorporation of DSSN+ into the cell membranes of E. coli biofilms formed at the electrodes. To uncover the dominant EET mechanism in DSSN+ incorporated E. coli systems, the bio-reduction process forming gold nanoparticles is monitored. The prevalent mechanism is revealed to be membrane perturbation with the concomitant release of intracellular redox-active components and not enhanced charge transfer as originally proposed. DSSN+ is also unable to restore charge transfer capabilities in the mutant Shewanella oneidensis strain with deleted EET genes, pointing to the absence of charge transfer through the conjugated molecules. In natural systems, EET and flavins/metabolite exchange are investigated by coupling fermentative E. coli and electrochemically active S. oneidensis in a syntrophic community, which significantly affects bioelectricity generation. Notably, the naturally established syntrophic relationship drove preferential colonization in the planktonic/biofilm modes (on the electrode surface), based on the functions of each strain and the metabolite of exchange. In the syntrophic system, EET mechanisms are especially dominated by S. oneidensis. Further exploitation of another simple syntrophy between Pseudomonas putida and S. oneidensis drove concurrent bioelectricity generation and increased rate of bioremediation. In summary, investigation of novel charge transfer mechanisms and flavins/metabolite exchange for electroconductivity and bioremediation in bacterial biofilms was carried out by studying both chemically modified bacteria and natural interactions between cooperative bacterial species.