Enhancing extracellular electron transfer of Shewanella to improve the performance of microbial fuel cells
Date of Issue2016-08-11
School of Chemical and Biomedical Engineering
Microbial electrochemical technologies, also known as bioelectrochemical systems (BESs), including microbial fuel cells (MFCs), microbial electrolysis cells (MECs), microbial electrosynthesis (MES) etc., serve as a diverse platform that combines waste treatment and energy or chemical production utilizing microbial catalytic reactions. MFCs, as a green and sustainable technology enabling simultaneous wastewater treatment and bioelectricity harvest, have attracted extensive attention in recent decades. Different strategies have been implemented to enhance extracellular electron transfer of the well-known electroactive strain, Shewanella oneidensis MR-1, to improve the performance of MFCs. Electroactive biofilms play essential roles in determining the power output of MFCs. To engineer the electroactive biofilm formation of Shewanella oneidensis MR-1, a c-di-GMP biosynthesis gene ydeH was heterologously overexpressed in S. oneidensis MR-1, generating an engineered strain in which the expression of ydeH is under the control of IPTG-inducible promoter, and a strain in which ydeH is under the control of a constitutive promoter. Such engineered Shewanella strains had significantly enhanced biofilm formation and bioelectricity generation. The MFCs inoculated with these engineered strains accomplished a maximum power density of 167.6 ± 3.6 mW/m2, which was ~2.8 times of that achieved by the wild-type MR-1 (61.0 ± 1.9 mW/m2). In addition, a synthetic microbial consortium containing exoelectrogen Shewanella oneidensis MR-1 and riboflavin producing strain, Bacillus subtilis RH33, was rationally designed and successfully constructed, enabling a stable, multiple cycles of MFC operations for more than 500 hours. The maximum power density of MFCs with this synthetic microbial consortium was 277.4 mW/m2, which was 4.9 times of that with MR-1 (56.9 mW/m2) and 40.2 times of RH33 (6.9 mW/m2), separately. At the same time, the Coulombic efficiency of the synthetic microbial consortium (5.6%) was higher than MR-1 (4.1%) and RH33 (2.3%). In the synthetic microbial consortium, it was found that both mediated and direct electron transfer efficiency were enhanced in mixed-culture. By exchanging the anolyte of MR-1 and RH33, it was confirmed that the improved MFC performance with the synthetic microbial consortium was because MR-1 could efficiently utilize the high concentration of riboflavin produced by RH33. Furthermore, external resistance is one of the important factors that affect the performance of MFCs. Bioelectrochemical and biofilm characterization was conducted for Shewanella oneidensis MR-1 inoculated MFCs with 250 Ω, 500 Ω, 2 kΩ, 6 kΩ, and 22 kΩ resistors. In overall, smaller external resistance resulted in higher maximum power density and more riboflavin secretion. Maximum power density of 136.8 ± 3.1 mW/m2 was achieved when MFCs were operated with 500 Ω resistor, which was 3.7 times of that with 22 kΩ resistor. Electrochemical impedance spectra analysis verified an increased internal resistance along with higher external resistance. Meanwhile more biofilm mass and extracellular polymer substances were confirmed on MFC anode with higher external resistance.