Short summary

In the first module we aim to identify possible mediators that are capable for electron transport. We want to use electricity to chemically reduce these mediators and transport them into the cells. The process takes place in a bioreactor called "reverse microbial fuel cell" (rMFC). One important requirement for a suitable mediator is that its reduction potential is high enough to restore reduction equivalents, like NAD(P)H (nicotinamide adenine dinucleotide (phosphate)). These reduction equivalents enter the respiratory chain where ATP (adenosine triphosphate) is produced which will be used in the next module.

Here you will find the results of the rMFC.

Theory

Electrotrophes

There are several approaches to feed microorganisms with electrons in order to support microbial respiration. One promising feasibility is the direct transfer of electrons to microorganisms. Bacteria that can directly accept electrons from electrodes for the reduction of terminal electron acceptors are called electrotrophes or electrode oxidizing bacteria. The possibility of electron transfer to microorganisms was investigated for the first time by studies with Geobacter species. (Lovley, Derek R., 2011)
Normally the production of multi-carbon organic products relies on organic feedstocks (biomass) as electron donor. The use of biomass has the disadvantages that biomass production competes with food production and that the costs for the required carbon source are a major factor if a production process is profitable or not. That is why the possibility of powering microbial processes with electricity is very attractive. (Lovley, Derek R. & Nevin, Kelly P. 2013)
Microorganisms can be provided with electrons via two major principles: Direct- and indirect electron transfer.

Indirect electron transfer

Direct electron transfer

Design of a electrobiochemical reactor system

To perform our cultivation experiments under well defined conditions it was necessary to design a new bioreactor system. Besides the typically controlled parameters in bioreactors like the oxygen partial pressure, pH-signal, temperature and other parameters, it was indispensable to have the possibility to energize the reactor with a defined current. That is why we decided to build an H-cell reactor. This kind of reactor consits of two compartments which are connected by a glass flange. It is possible to fix a membrane in the middle of the flange connection so that the two compartments are seperated. We used a cation selective Nafion® membrane which allowed the divison of the two compartments into an anode and cathode space.
Figure 1 shows schematically the layout of our design.

The H-cell reactor could be used for batch fermentations and is constructed for the electron transfer via a mediator.
That is why we considered an alternative reactor design. The other reactor concept is named "scalable flow cell reactor" (SFC) and allows an continous mode of operation. In this reactor type the electron transfer must be realized by direct electron transfer. That is possible if the cells stay in direct contact to the electrode material. The electron transfer is carried out by cytochromes in the outer membrane. That is why we focused on different types of mediators and the expression of key type cytochromes. The Layout of the SFC is shown in Figure 2.

Measurement system

For the investigation of electroactive microorganisms highly sensitive analytics are required. That is why we use a Potentiostat which allows it to set and hold a defined electrode potential. Furthermore we tried out different media and buffer compositions and observed the effect of different electrode materials on the mediator oxidation and reduction peaks.

Mediators

Neutral red

Neutral red is a phenazine-based dye which has an suitable redox-potential to function as an electron-shuttle from the electrode to the cells.

Bromphenol blue

Bromphenolblue is a triarylmethane dye that is similar to neutral red and also capable to function as mediator.

References

• Lovley, Derek R., 2011. Powering microbes with electricity: direct electron transfer from electrodes to microbes. In: Environmental Microbiology Reports 3 (1), pp. 27–35.
• Lovley, Derek R. & Nevin, Kelly P., 2013. Electrobiocommodities: powering microbial production of fuels and commodity chemicals from carbon dioxide with electricity. In: Current Opinion in Biotechnology, 24, pp. 385-390.
• Qiao, Yan; Bao, Shu-Juan; Li, Chang Ming (2010): Electrocatalysis in microbial fuel cells—from electrode material to direct electrochemistry. In: Energy Environ. Sci., 3 (5), pp. 544.