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Project Description

Shewanella oneidensis MR-1 is an important microorganism in bioremediation due to its diverse respiratory capabilities. It is a dissmilatory metal reducing bacterium. Being facultative organism, it has the ability to adapt to both aerobic and anaerobic environments, as well as utilise a number of toxic compounds such as manganese and uranium. It does this by accepting electrons, which gives Shewanella oneidensis the potential of producing electricity. An electron transfer pathway, known as the Mtr pathway, facilitates the extracellular translocation of electrons that are gained during the reduction of a carbon source.

The Mtr pathway consists of the following five genes:

OmcA - an outer membrane decahaeme cytochrome c

MtrC - an outer membrane decahaeme cytochrome c

MtrA - a periplasmic dechaeme cytochrome c

MtrB - a transmembrane porin which stabilises interaction between MtrA and C

CymA - an inner membrane tetrahaeme cytochrome c (Figure 1).

We have been working on introducing the Mtr pathway into Escherichia coli, in order to increase the electrical output in a microbial fuel cell (MFC). Shewanella oneidensis is capable of transferring these electrons through extensions known as nanowires. However, E.col have a far greater genetic tool set at their disposal, enabling easy genetic manipulation and modification. Further more, they are a very robust microorganism that, when compared to Shewanella oneidensis, are better equipped to deal with changes in a range of environmental variables, e.g. temperature and pH. The carbon source for our modified bacteria will be wastewater.

We are exploring the possibility of electron transfer through the use of flagella found in E.coli K-12 derivative, DH5-α. E.coli should act as an anode. Formation of biofilm has also been noted in this strain of E.coli. The efficiency of the MFC is down to the biofilm, which is formed when cells adhere to a surface and stick to each other. The composition of these biofilms is what determines the amount of electricity produced as it acts like a conductive matrix, enabling higher kinetics rates of electron transfer along nanowires.

This technology has a multitude of application. It could be used as an electrical source for hard to reach biosensors such as a deep-water biosensors within a sediment microbial fuel cell. Furthermore, MFSc could play a major role in bioremediation, which is removal of organic pollutants from environments to produce clean water and excess electrical energy.

What is Microbial Fuel Cell?

MFC (Figure 1) is a device that utilises micro-organisms ability to catalyse an oxidation and reduction reaction at an anode and cathode electrode, respectively, and can produce electricity when connected to a load/resistor via an external circuit, whilst producing water at the cathode.

One major concern facing the modern world is the gradual depletion of fossil fuels and rising greenhouse gas emission. A MFC is a device that holds great promise for the sustainable production of renewable energy in the form of electricity, alongside bioremediation of organic waste from Industrial wastewater. It also offers potential to extract useful chemicals from wastewater, such as hydrogen and bioplastics. However, the performance of this system is currently poor, typically <10% of what is theoretically possible. In addition, energy recovery by MFCs from treatment of Industrial wastewater including brewery wastewater, azo-dye wastewater, municipal wastewater and other sources is still poor typically less than 150W/m3 of the anode volume and for potential sustainable operation, energy recovery need to reach 1000W/m3.

Principle of operation

Conventional MFCs are made up of two compartments: the biological anode and abiotic anode. In the anode chamber, anaerobic bacteria biologically oxidize organic waste in order to generate adenosine triphosphate by a series of redox reaction, and finally electrons and protons are produced. The electrons generated during microbial oxidation are transferred to the anodic electrode and subsequently conducted through an external wire over a load (bulb) or resistor to the cathode. While protons that are produced pass through a cation exchange membrane and combine with electrons and oxygen at the cathode to form water. The flow of electrons and the positive potential difference between the electrodes (between the cathode which should be at higher potential and the anode at a lower potential) give rise to the generation of electric power. The electricity producing microorganisms are referred to as anode respiring bacteria (ARB). The growth rate of the microorganisms in the MFCs depends on the difference between redox potential of the electron donor and the potential of the anode. Theoretically the higher the anode potential is determined to yield higher energy gain for the growth of the ARB.

Figure 1: Animated working model of a microbial fuel cell

Adapted from https://www.youtube.com/watch?v=DVI6tMP-rOY/