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Team:UC-Santa Cruz-BioE
From 2014.igem.org
A Little Introduction...
What is a microbial fuel cell?
Microbial fuel cells (MFCs) generate electricity via the anaerobic metabolism of certain types of bacteria. Such electrogenic organisms are able to transfer electrons during the metabolism of organic material. Naturally, it follows that, with a little bit of design, these bacteria can be used to generate a current in a fuel cell. There is definite possibility that MFCs could be used extensively as a means to collect energy from waste water. Therefore, we have set out to create our own MFC, in a project where life and electronics aggregate together.
On Our Bacteria of Choice
While there are several known electrogenic bacteria known to exist, we have chosen Shewanella oneidensis . There were several reason for this:
- Good Availability: It was accessible even with limited financial resources
- Quick Growth: grows faster and easier than Geobactor metallireducens
- Facultative Anaerobe: Shewanella does not die when exposed to air
- Electrogenic: Shewanella oneidensis is electrogenic because it can reduce metals
Design Goals
We have two primary goals in the MFC's design:
- Increase Coulombic Efficiency
- Create more Biofilm Growth
Increasing Coulombic Efficiency
Under normal conditions when Shewanella oneidensis metabolizes carbon it tends to produce acetate from the break down of acetyl-CoA, rather than taking the alternative pathway of the citric acid cycle[2]. Studies of gene expression have demonstrated this tendency, by showing that enzymes involved in the Citric Acid Cycle are downregulated. Acetate production, on the other hand is upregulated[6].
What does this mean for the MFC?
Most of the carbon being taken up by the microbes of the MFC is simply being secreted as acetate, unused, and is not useful for creating a current. We are trying to get around this issue in two ways:
- 1. Deletion of Phosphate Acetyltransferase-when present helps catalyze the reaction from acetyl-Coa to acetate rather than favoring the citric acid cycle
- 2. Express more Citrate Synthase by Adding Expression Vector to Bacterial DNA-this will make the reaction favor the citric acid cycle, more NADH will be produced, more electrons will go through the electron transport chain and thus... More electrons will be transferred to anode
