Solar pMFC
Microbial Fuel Cells (MFCs) are bio-electrochemical systems that drive current by using bacteria, that are isolated often times from waste waters or soil. Modeling the metabolism and electron transfer strategies of the bacteria living in waste waters through a controlled system based on a single species can help to optimize and enhance the MFCs technological landscape. Our idea is to optimize MFC’s platform using an engineered E. coli that exploits sunlight to live better under stressful conditions and that has an increased electron production.
A typical MFC is composed of two separate chambers, the anode and the cathode, separated by a proton exchange membrane (PEM). Bacteria are grown in the anode under anaerobic conditions. The electrons are the product of the bacteria metabolism. The lack of oxygen as acceptor enables the electrons to be transferred to the electrode. In the cathode, electrons combine with oxygen and protons to form water.
How it works
Life in the anode
In the anode there is no oxygen and bacteria must survive under anaerobic conditions. Our system uses E. coli as the model organism. E. coli is a facultative anaerobic bacterium, able to live without oxygen undergoing fermentation. In these conditions, the bacterial metabolism is slowed down and thus affecting the electrons production. Our idea is to increase E. coli viability in the anaerobic anode, in order to optimize electricity production.
Exploiting sunlight power: Proteorhodopsin
Proteorhodopsin (PR) is a light-powered proton pump that belongs to the rhodopsin family. This 7-transmembrane protein exploits light to create an outward proton gradient, increasing the proton motive force (pmf) across the membrane. The generated pmf can subsequently power cellular processes. In particular, PR supports a light-driven ATP synthesis as proton reenter the cell through the H+-ATP synthetase complex. Therefore PR should increase the lifespan of E. coli and the electron flow under anaerobic conditions. BBa_K1604010 is an improvement of the proteorhodopsin part that we extracted from the registry (BBa_K773002). We fully characterized our part to demonstrate that the proton pump does work when the bacteria are light exposed.
Retinal-producer: blh
Proteorhodopsin needs retinal as chromophore. In the MFC, PR-engineered E. coli can be supplemented with all-trans-retinal. A cheaper solution is to engineer a retinal-producer E. coli with β-carotene 15,15’-dioxygenase (encoded by the gene blh), an enzyme that splits one molecule of β-carotene into two molecules of retinal. We also engineered E. coli with enzymes required for β-carotene production.
How can electrons be stolen?
Electrons can be stolen by exogenous mediators or by expressing heterologous-cytochrome in E. coli. Shewanella odenensis Mtr electrons transport pathway transfers metabolic electrons across the double membrane. Electrons are transported from CymA to MtrA and from MtrA to MtrC through the MtrCAB complex. The electrons coming out from MtrC are in direct contact with the electrodes. Alternatively, electrons can be transferred to the electrodes by exogenous chemical redox molecules called mediators (e.g. neutral red and methylen blue).
pncB: an electron producer booster
We planned to increase electrons production by over-expressing pncB. This gene encodes for the enzyme NAPRTase (nicotinic acid phosphorbosyl-transferase) that catalyzes the formation of nicotinate mono-nucleotide, a direct precursor of NAD+, starting from NA. The presence of higher levels of NAD+ should push the cell to produce more electron carriers molecules (NADH), thus increasing electricity.
This figure shows how our bacteria work inside a MFC. Sunlight hits retinal, which is synthetised with the blh-βcarotene pathway, making Proteorhodopsin able to produce an outward proton gradient. Protons re-entry the cells through the ATP synthetase increases cell viability. Meanwhile pncB boosts the electrons production. Chemical mediators steal electrons from the cell and bring them to the electrode. Charges travel from the anode to the catode, producing an electric current.
A glance into the future
MFCs provide new opportunities for the sustainable energy production, a rapidly evolving technology. In particular, our controlled and self-sustainable platform could have many future applications. We envision that our Solar pMFCs will become a valid cheaper and greener alternative to modern photovoltaic panels. Solar energy activates the system, that provides electricity to sustain domestic needs.