Electricity production by Geobacter sulfurreducens attached to electrodes

Abstract

Previous studies have suggested that members of the Geobacteraceae can use electrodes as electron acceptors for 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 electron acceptor and acetate or hydrogen was the electron donor. The electron-accepting electrodes were maintained at oxidizing potentials by connecting them to similar electrodes in oxygenated medium (fuel cells) or to potentiostats that poised electrodes at +0.2 V versus an Ag/AgCl reference electrode (poised potential). When a small inoculum of G. sulfurreducens was introduced into electrode-containing chambers, electrical current production was dependent upon oxidation of acetate to carbon dioxide and increased exponentially, indicating for the first time that electrode reduction supported the growth of this organism. When the medium was replaced with an anaerobic buffer lacking nutrients required for growth, acetate-dependent electrical current production was unaffected and cells attached to these electrodes continued to generate electrical current for weeks. This represents the first report of microbial electricity production solely by cells attached to an electrode. Electrode-attached cells completely oxidized acetate to levels below detection (<10 micro M), and hydrogen was metabolized to a threshold of 3 Pa. The rates of electron transfer to electrodes (0.21 to 1.2 micro mol of electrons/mg of protein/min) were similar to those observed for respiration with Fe(III) citrate as the electron acceptor (E(o)' =+0.37 V). The production of current in microbial fuel cell (65 mA/m(2) of electrode surface) or poised-potential (163 to 1,143 mA/m(2)) mode was greater than what has been reported for other microbial systems, even those that employed higher cell densities and electron-shuttling compounds. Since acetate was completely oxidized, the efficiency of conversion of organic electron donor to electricity was significantly higher than in previously described microbial fuel cells. These results suggest that the effectiveness of microbial fuel cells can be increased with organisms such as G. sulfurreducens that can attach to electrodes and remain viable for long periods of time while completely oxidizing organic substrates with quantitative transfer of electrons to an electrode.

FIG. 1.

Current production by G. sulfurreducens in a microbial fuel cell. Cells were inoculated into an anaerobic chamber containing growth medium (5 mM acetate) and a graphite electrode connected to another electrode in an aerobic chamber. At the indicated times, medium was removed and replaced with sterile, anaerobic salts buffer plus 5 mM acetate.

FIG. 2.

Current-voltage and current-power (watts = amperes × volts) relationships for fuel cells containing G. sulfurreducens shown in Fig. 1. Open symbols represent the current produced from the oxidation of acetate in a fuel cell after the initial growth of cells in the electrode chamber. Closed symbols represent current produced from the oxidation of acetate in the fuel cell after the medium was replaced the second time.

FIG. 3.

Growth and current production by G. sulfurreducens inoculated into a chamber containing a graphite electrode poised at +200 mV versus an Ag/AgCl reference. Acetate (1 mM) was provided with the initial inoculum, and pulses of 1 mM acetate were given at the times indicated to demonstrate acetate-dependent growth. Inset gives data for current on a semilogarithmic scale to show exponential growth.

FIG. 4.

Growth and current production by G. sulfurreducens in a chamber containing a graphite electrode poised at +200 mV versus an Ag/AgCl reference and effect of removing the growth medium and replacing it with anaerobic buffer plus acetate as the electron donor. Acetate (0.5 mM) was provided after replacing the medium, followed by a second pulse of 1 mM.

FIG. 5.

SEM image of an electrode surface following growth of G. sulfurreducens with acetate as an electron donor (2 mM) under poised-potential conditions. Over 75% of viewed fields at this magnification had no exposed electrode; however, this image was chosen to provide an example of electrode surface characteristics and show individual bacterial attachment.

FIG. 6.

Hydrogen-dependent current production by attached populations of G. sulfurreducens in two different chambers (open and closed symbols) containing graphite electrodes poised at +200 mV versus an Ag/AgCl reference electrode. Hydrogen-carbon dioxide (80:20) and nitrogen-carbon dioxide (80:20) gas mixtures were bubbled directly into the electrode chambers where indicated.

FIG. 7.

Response of the graphite electrode potential to an attached population of G. sulfurreducens metabolizing acetate as the electron donor, after electrode control (poising by potentiostat) was switched off.