The ins and outs of microorganism–electrode electron transfer reactions

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Abstract

Electron transfer between microorganisms and an electrode — even across long distances — enables the former to live by coupling to an electronic circuit. Such a system integrates biological metabolism with artificial electronics; studying these systems adds to our knowledge of charge transport in the chemical species involved, as well as, perhaps most importantly, to our knowledge of charge transport and chemistry at the cell–electrode interfaces. This understanding may lead to microbial electrochemical systems finding widespread application, particularly in the energy sector. Bioelectrochemical systems have already shown promise for electricity generation, as well as for the production of biochemical and chemical feedstocks, and with improvement are likely to give rise to viable applications.

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Figure 1: Timeline showing recent major achievements in anodic and cathodic microbial electrosynthesis and electrocatalysis research.
Figure 2: The distinct EET mechanisms operative in Geobacter spp. and Shewanella spp.
Figure 3: Probing EET on the microscale.
Figure 4: Microfabricated wells and nanoelectrodes enable in situ EET current measurement of single Geobacter sulfurreducens DL-1 microorganisms.
Figure 5: A schematic image of a fuel cell incorporating a conventional anode (here performing H2O oxidation) coupled to a microbial biocathode.
Figure 6: Three proposed electron transfer pathways by which microorganisms perform extracellular electron uptake.
Figure 7: Functional groups are grafted onto electrode surfaces to give ‘engineered electrodes’.

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Acknowledgements

The authors thank their national and international granting agencies, in particular the ESBCO2 project (PIOF-GA-2011-302964). The work of A.K. is supported by an EU Marie Curie International Outgoing Fellowship for Career Development and D.L., P.K., L.L. and F.B are supported by the Ulysses France–Ireland programme. The authors thank G. Stephanopoulos, S. Glaven and L. Tender for helpful discussions.

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Correspondence to Amit Kumar.

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Glossary

Bioelectrochemical system (BES).

A microbial reactor — a fuel cell or electrolysis cell — that uses a microbial electrocatalyst.

Microbial electrosynthesis (MES).

An electrode reaction that results in the intentional generation of a useful chemical product (for example, hydrogen or butanol).

Microbial electrocatalyst

A microorganism that catalyses an electrode reaction.

Extracellular electron transfer

(EET). The process by which electrons are transferred outside the cell by shuttles or wires (for example, redox proteins, biopolymers and protein filaments) secreted by microorganisms. Transport can occur across distances exceeding 100 μm, such that intracellular metabolic processes (for example, acetate oxidation or O2 reduction) can be interfaced with insoluble extracellular electron acceptors or donors (for example, minerals and electrodes).

Electrogenic microorganism

A microorganism able to catalyse an anodic electrode reaction.

Microbial bioanode

An electrode colonized by microorganisms that catalyse an anodic reaction (for example, acetate oxidation).

Electrotrophic microorganism

A microorganism able to catalyse a cathodic electrode reaction.

Microbial biocathode

An electrode colonized by microorganisms that catalyse a cathodic reaction (for example, nitrate reduction).

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