Biofilm Characterization in Microfluidic Microbial Fuel Cells

Abstract

Microbial fuel cell (MFC) flow rate experiments, utilizing Shewanella oneidensis MR-1, were previously conducted by Daniele Vigolo, Talal Al-Housseiny, and colleagues. These experiments established a relationship between flow rate and voltage production, pinpointing maximum voltage production in a flow rate range of approximately 15-20 μL/min for their device geometry. The objective of this study is to measure biofilm formation and development in a microbial fuel cell (MFC) as a function of flow rate. In order to understand the biofilm behavior on the heterogeneous carbon paper surface, this study first seeks to understand biofilm behavior on a homogeneous surface, a glass slide. Since the fluorescent Shewanella varieties did not reliably fluoresce, this study then strived to understand biofilm behavior on homogeneous and heterogeneous surfaces for a similar bacterial system, Pseudomonas aeruginosa. The homogeneous surface experiments utilized serpentine or straight channel half-cell devices, no carbon paper, and wild type Shewanella or wild type Pseudomonas aeruginosa. The heterogeneous surface experiments utilized serpentine or straight channel half-cell devices, carbon paper, and Shewanella strains pBAD GFP and pBAD BS2 or GFP-tagged Pseudomonas aeruginosa. The wild type Shewanella results show relatively high levels of fractional surface coverage (area of cells per unit area of the channel surface) and biofilm formation for the 5 μL/min and 25 μL/min trials. However, the 10 μL/min and 20 μL/min trials display relatively lower levels of fractional surface coverage and biofilm formation. Therefore, this result indicates that maximum voltage production in the 15-20 μL/min range corresponds to relatively lower levels of fractional surface coverage and biofilm formation. At flow rates greater than 25 μL/min, biofilm formation and fractional surface coverage slightly decrease. The results display a cyclical fractional surface coverage pattern with most of the trials tending towards coverage around 40-50%. Meanwhile, for wild type Pseudomonas, the fractional surface coverage decreases from the 45 μL/min trials to the 25 μL/min trials; fractional surface coverage increases from the 25 μL/min trials to the 5 μL/min trials. These results also present a cyclical fractional surface coverage pattern, but the Pseudomonas trials tend towards coverage around 4- 5%. Unfortunately, the experiments utilizing fluorescent bacteria varieties could not be completed. The fluorescent Shewanella varieties had only recently been synthesized in the lab by colleagues at Georgia Tech. These fluorescent Shewanella varieties could not be effectively and reliably visualized under the microscope. In the experiments with GFP-tagged Pseudomonas aeruginosa, the bacteria filled channel in the carbon paper region could not be localized under the microscope. Since the MFC flow rate experiments were not conducted with Pseudomonas aeruginosa, no definitive relationship can be established between fractional surface coverage, flow rate, and voltage production. Conclusively, since wild type Shewanella and wild type Pseudomonas display different relationships between flow rate and biofilm behavior, it is likely that the two bacterial species produce maximum voltage in different optimum flow rate ranges.