Module I - Reverse Microbial Fuel Cell (rMFC)

Overview

Genetical Approach

Reactor system

Measurement System

Mediators

Results

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 are 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 its characteristics cytochromes can be assigned to one of three main groups, 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, 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 electron donor as well as an electron acceptor by changing the redox state of their heme group. The heme group consists of an iron atom surrounded by porphyrin rings and is shown in Figure 3.(Qiao et al., 2010)

Figure 3:heme group of cytochrome c

These 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. To avoid side reactions with gases like oxygen or carbon monoxide the position of the heme group is favourable, because it ensures that it is sufficiently protected from an unwanted reactant. However, it is assumed that the location in the inner core of the protein and the resulting poor accessibility could also be a reason for a limited or blocked direct electron transfer between the electrode and the heme. For that reason the electrode material is key factor and different materials have to be tested to optimize the direct electron transfer via cytochromes.(Qiao et al., 2010)
One of the best known organisms that are 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 caused us to consider it as an potential alternative to realize enhanced electron uptake in E. coli.