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

Indirect electron transfer is mediated by soluble redox mediators that are freely moving in the media. There naturally expressed mediators like flavins, quinones and phenanzines, but structurally similar synthetic molecules are also suitable to serve as electron shuttles for numerous oxidation-reduction cycles. (Harnisch, F. & Freguia, S., 2012)

Figure 1: Electron flow into GRAM-negative bacteria cells.

Direct electron transfer

Direct-electron transfer occurs through direct contact of the electrode with outer membrane cytochromes. (Harnisch, F. & Freguia, S., 2012)

Figure 2: Principle of direct electron transfer mediated by outer membrane cytochromes.

One special case of direct electron transfer is realized by conductive pili called microbial nanowires.
The current situation is that there has been a lot of research on microbial fuel cells (mfc) where the aim is to produce electricity and convey electrons to an electrode. The mechanisms for the electron uptake has been investigated, but is not understood yet.

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.
• Harnisch, F. & Freguia, S., 2012. A Basic Tutorial on Cyclic Voltammetry for the investigation of Electroactive Microbial Biofilms. In: Chemistry – An Asian Journal, 7 (3), pp. 466–475.