Module I - Reverse Microbial Fuel Cell (rMFC)

Overview

Genetical Approach

Reactor system

Measurement System

Mediators

Neutral Red

Neutral red (3-Amino-7-dimethylamino-2-methylphenazine hydrochloride) is a phenazine-based dye which is normally used as pH-indicator due to the fact that it changes its colour from red (pH 6.8) to yellow (pH 8.0).
Besides that it could be shown that reduced neutral red is also capable as the sole source of reducing power for growth and metabolism of H2 consuming bacteria cultures.(Park, D. H. & Zeikus, J. G. 2000)
The chemical structure is shown in figure 1.

Figure 1: Chemical structure of the triphenylmethane dye neutral red.

That indicates that based on its redox-potential it can function as an electron-shuttle from the electrode to the cells getting reduced by an one step two electron transfer.(Azariah, A. N. et al., 1998) In this case the literature value for the redox potential of neutral red accounts to -325 mV vs. NHE.(Futz, M. L. & Durst, R. A., 1982)
There is also evidence that electrically reduced neutral red can bind to the cell membrane and chemically reduces NAD. Furthermore it is not toxic to the cells and can replace H₂ which is the natural electron shuttle for some bacteria species. (Park et al., 1999)

Bromphenol Blue

Bromphenol blue is a triarylmethane dye that is similar to neutral red and is normally used as a biological stain or as a pH indicator.(NCBI:PubChem Compound) Having a redox potential of -739 mV vs. SCE it is also capable to function as a mediator.(Strehlitz et al., 1994)
Figure 2 shows the chemical structure of bromphenol blue.

Figure 2: Chemical structure of the triphenylmethane dye bromphenol blue.

Cytochromes

Apart from the synthetic mediators like neutral red or bromphenol blue there are naturally occuring mediators like cytochromes. Cytochromes are hemoproteins which act as a part of electron transport in the respiratory chain and therefore they play a role in direct electron transfer from an electrode to the cells. Depending on the characteristics of the cytochrome it can be assigned to one of three main groups, called cytochrome a, b and c.(Spektrum, 2001)
Members of the cytochrome a class are the reduction/oxidation units of the cytochrome-oxidase which catalyzes the electron transport from cytochrome c to oxygen as the final electron acceptor. B-type cytochromes are known to have the lowest redoxpotential of the respiratory chain and are located between ubichinon and cytochrome c.(Spektrum, 2001)
However c-type cytochromes are the best studied and most abundant group of cytochromes. For metal-reducing bacteria it has been already shown that several of these c-type cytochromes are essential for electron transfer between an electrode and the cell. Therefore they can function as an electrondonor as well as an acceptor by changing the redox state of their heme group consisting of an iron atom surrounded by porphyrin rings.(Qiao et al., 2010)
The heme group of cytochrome c is shown in figure 3.

Figure 3:heme group of cytochrome c

Those heme groups are located in the inner core of the protein which is an advantage for reduction/oxidation reactions as well as a problem at the same time. For the avoidance of side reactions with gases like oxygen or carbon monoxide the position of the heme group is favourable because it ensures that this domain is sufficiently protected from unwanted reactant. However it is assumed that this structure could also be a reason why the direct electron transfer between the electrode and the heme group could be limited or blocked because the heme group is poorly accessible. For that reason the electrode material is essential and different ones have to be tested for optimization of direct electron transfer via cytochromes.(Qiao et al., 2010)
One of the best known organisms that is able to accept electrons from an electrode is Geobacter sulfurreducens. It has been demonstrated that current-consuming biofilms of this organism highly express a periplasmic key-type cytochrome c with one heme group. The deletion of the corresponding gene GSU 3274 leads to a total inhibition of electron uptake.(Strycharz et al., 2011)
This indicates the importance of this c-type cytochrome for current-consuming biofilms and induces us to consider it as an alternative of realizing enhanced electron uptake in E. coli.