The idea of using microorganisms to unlock the energy present in waste products has been around since the 1980s, but its application has been held back by problems such as the inefficiency of the bacteria used at converting waste sugars to electricity. Now, all that could change in the light of work published in Nature Biotechnology describing an efficient biofuel cell that uses the recently isolated bacterium Rhodoferax ferrireducens.
Biofuel cells harness the ability of bacteria to remove electrons from organic compounds and transfer them to an electron acceptor. In the case of biofuel cells, the acceptor is a graphite anode and the electrons that are transferred produce an electric current that can power a so-called 'bacterial battery'. But the bacteria tested so far for this application have had drawbacks, such as inefficiency in using the electrons available from their substrates or a requirement for unstable electron-shuttling compounds to transfer electrons to the anode, reducing the useful life of the fuel cell. Now, Derek Lovley and his colleague Swades Chaudhuri have overcome these problems using an R. ferrireducens biofuel cell with glucose as a substrate. They obtained an electron-transfer efficiency of over 80% — a substantial improvement on the average efficiency of ∼10% using other bacteria. R. ferrireducens can also transfer electrons directly to the anode, bypassing the need for electron-shuttling compounds, which cuts costs and enables a longer-term use of the fuel cell.
Another advantage of using R. ferrireducens in biofuel cells is its ability to produce electricity from a range of substrates — as well as glucose, it can strip electrons from other carbohydrates such as xylose, which is generated in large quantities in the production of paper.
The amount of current that Lovley and Chaudhuri generated from the R. ferrireducens fuel cell was too small to have a practical use at the moment, and modifications to the design are needed to improve on this. Steps such as changing the material used to make the anode and increasing its surface area should increase the power generated, bringing us one step closer to using bacterial batteries as an efficient way of disposing of waste biomass.
ORIGINAL RESEARCH PAPER
Chaudhuri, S. K. & Lovley, D. R. Electricity generation by direct oxidation of glucose in mediatorless microbial fuel cells. Nature Biotechnol. 21, 1229–1232 (2003).
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Flintoft, L. Boost for bacterial batteries. Nat Rev Microbiol 1, 88 (2003). https://doi.org/10.1038/nrmicro758