APPLIED AND ENVIRONMENTAL MICROBIOLOGY, Mar. 2003, p. 1548–1555 Vol. 69, No. 30099-2240/03/$08.00⫹0 DOI: 10.1128/AEM.69.3.1548–1555.2003Copyright © 2003, American Society for Microbiology. All Rights Reserved.Electricity Production by Geobacter sulfurreducens Attachedto ElectrodesDaniel R. Bondand Derek R. Lovley*Department of Microbiology, University of Massachusetts, Amherst, Massachusetts 01003Received 29 August 2002/Accepted 10 December 2002Previous studies have suggested that members of the Geobacteraceae can use electrodes as electron acceptorsfor anaerobic respiration. In order to better understand this electron transfer process for energy production,Geobacter sulfurreducens was inoculated into chambers in which a graphite electrode served as the sole electronacceptor and acetate or hydrogen was the electron donor. The electron-accepting electrodes were maintainedat oxidizing potentials by connecting them to similar electrodes in oxygenated medium (fuel cells) or topotentiostats that poised electrodes at ⴙ0.2 V versus an Ag/AgCl reference electrode (poised potential). Whena small inoculum of G. sulfurreducens was introduced into electrode-containing chambers, electrical currentproduction was dependent upon oxidation of acetate to carbon dioxide and increased exponentially, indicatingfor the first time that electrode reduction supported the growth of this organism. When the medium wasreplaced with an anaerobic buffer lacking nutrients required for growth, acetate-dependent electrical currentproduction was unaffected and cells attached to these electrodes continued to generate electrical current forweeks. This represents the first report of microbial electricity production solely by cells attached to anelectrode. Electrode-attached cells completely oxidized acetate to levels below detection (<10 ␮M), andhydrogen was metabolized to a threshold of 3 Pa. The rates of electron transfer to electrodes (0.21 to 1.2 ␮molof electrons/mg of protein/min) were similar to those observed for respiration with Fe(III) citrate as theelectron acceptor (Eoⴕ ⴝⴙ0.37 V). The production of current in microbial fuel cell (65 mA/m2of electrodesurface) or poised-potential (163 to 1,143 mA/m2) mode was greater than what has been reported for othermicrobial systems, even those that employed higher cell densities and electron-shuttling compounds. Sinceacetate was completely oxidized, the efficiency of conversion of organic electron donor to electricity wassignificantly higher than in previously described microbial fuel cells. These results suggest that the effective-ness of microbial fuel cells can be increased with organisms such as G. sulfurreducens that can attach toelectrodes and remain viable for long periods of time while completely oxidizing organic substrates withquantitative transfer of electrons to an electrode.Fuel cells theoretically bypass the inefficiencies of internalcombustion engines by directly oxidizing and reducing com-pounds at electrode surfaces, the most common example beingthe hydrogen fuel cell, which oxidizes hydrogen at the anodesurface and passes electrons to a cathode, where they are usedto reduce molecular oxygen (10). So-called microbial fuel cellsseek to add the diversity of microbial catalytic abilities to thishigh-efficiency design, allowing organic compounds, from sim-ple carbohydrates to waste organic matter, to be converted intoelectricity (28).Previous studies on the microbial conversion of organic mat-ter to electricity have noted several problems. A major short-coming is that most microorganisms studied to date only par-tially oxidize their organic substrates and transfer a portion ofthese electrons to electrodes (8, 9, 11, 12, 24, 25). Furthermore,electrical current was generated only when soluble mediatorcompounds were added to these microbial cultures to facilitateelectron transfer from the bacteria to the electrodes. Examplesof mediators include potassium ferricyanide (9), anthraqui-none 2,6-disulfonic acid, cobalt sepulchrate (7), thionine (13),neutral red (24), and azure A (3). The requirement for medi-ators is problematic, because many of these mediators are toxicand cannot be used when electrical energy is harvested fromorganic matter in an open environment.A new concept in the construction of microbial fuel cellsresulted from the observation (26) that if graphite or platinumelectrodes were placed into anoxic marine sediments and con-nected to similar electrodes in the overlying oxic water, sus-tained electrical power could be harvested (on the order of0.01 W/m2of electrode). Analysis of the microbial communityfirmly attached to anodes harvesting electricity from a varietyof sediments demonstrated that microorganisms in the familyGeobacteraceae were highly enriched on these anodes (1, 27;D. Holmes, D. Bond, L. M. Tender, and D. R. Lovley, sub-mitted for publication). Given the ability of Geobacteraceae totransfer electrons to other insoluble electron acceptors, such asFe(III) oxides (14), these results suggested that the electrodesurfaces were serving as terminal electron acceptors forGeobacteraceae.Studies with Desulfuromonas acetoxidans, a marine repre-sentative of the Geobacteraceae, demonstrated that suspen-sions of this organism could oxidize acetate in a two-electrodefuel cell that simulated the marine sediment fuel cells, with noadded mediator compounds (1). Furthermore, a freshwaterrepresentative of the Geobacteraceae, Geobacter metalliredu-cens, oxidized aromatic compounds, such as benzoate and tol-uene, to carbon dioxide in a three-electrode poised-potentialsystem, where an electrode maintained at ⫹0.2 V (versus an* Corresponding author. Mailing address: Department of Microbi-ology, University of Massachusetts, Amherst, MA 01003. Phone: (413)545-9651. Fax: (413) 545-1578. E-mail: [email protected]/AgCl reference electrode) served as the sole electron ac-ceptor (1). Since Geobacteraceae are not known to produce anysoluble electron shuttles (20), it was hypothesized that theseGeobacteraceae were directly transferring electrons to the elec-trode surface. However, the mechanisms for this electrontransfer, the possibility that this form of electron transportcould support cell growth, and the role of attached versusunattached cells have not been previously investigated.The study of Geobacteraceae has been facilitated by theavailability of the genome sequence of Geobacter sulfurredu-cens and the development of a