A wide diversity of microorganisms can electrochemically interact with electrodes, directly donating or accepting electrons from electrode surfaces (17
). In most previous studies, the microorganisms donated electrons to the anodes of microbial fuel cells, which served as an electron acceptor. However, when an electrode is poised at a sufficiently low potential, Geobacter sulfurreducens
and Geobacter metallireducens
can accept electrons from graphite electrodes, using the electrons for the reduction of electrochemically more positive electron acceptors, such as fumarate, nitrate, or U(VI) (10
). Other, as-yet-undefined microorganisms may function in a similar manner (23
Previous studies have suggested that Geobacter
species directly transfer electrons to electrode surfaces without a requirement for soluble electron carriers (7
; K. P. Nevin, B.-C. Kim, R. H. Glaven, J. P. Johnson, T. L. Woodard, B. A. Methé, R. J. DiDonato, Jr., S. F. Covalla, A. E. Franks, A. Liu, and D. R. Lovley, submitted for publication), and it is expected that electron transfer is also direct when electrons flow from an electrode to Geobacter
). This contrasts with observations made with a number of organisms, including Shewanella
), and Geothrix
) species, that produce electron shuttles to promote electrode-microbe electron transfer.
Tetrachloroethene (PCE) and trichloroethene (TCE) are prevalent groundwater contaminants due to their widespread commercial, industrial, and military use (20
). PCE and TCE form dense non-aqueous-phase liquids (DNAPLs), and such sources can feed dissolved-phase contaminant plumes for decades (1
). Bioremediation through stimulation of microbial reductive dechlorination of PCE and TCE to the nontoxic end product ethene can be achieved by addition of fermentable organic substrates to indirectly provide the fermentation products, hydrogen and acetate, as electron donors to dechlorinating microorganisms (12
). However, this approach also stimulates the growth of unwanted, nondechlorinating microorganisms and the production of methane, a potent greenhouse gas. Furthermore, delivery and sustained supply of the electron donor(s) to DNAPL source zones are engineering challenges (16
) and are considered major limitations for achieving microbially enhanced DNAPL dissolution (4
It was previously suggested (10
) that just as electrodes may serve as the electron donor for microbial reduction of the groundwater contaminant U(VI) (11
), electrodes might be a suitable electron donor to promote microbially catalyzed reductive dechlorination. Initial studies evaluated this possibility using a mixed culture capable of dechlorinating TCE (5
). Unfortunately, the electrode did not serve as an electron donor for dechlorination, even though it was poised at a very low potential (−500 mV) versus a standard hydrogen electrode (5
). However, when the electron shuttle methyl viologen was added, TCE was dechlorinated, primarily to cis
). There was negligible reductive dechlorination of TCE with the poised electrode in the presence of methyl viologen in the absence of the mixed culture. These and other results suggested that the mixed culture was capable of accepting electrons from electrode-reduced methyl viologen for reductive dechlorination.
In an attempt to promote direct electron transfer from the electrode to the dechlorinating microorganisms, methyl viologen was adsorbed onto a glassy carbon electrode (5
). TCE dechlorination began immediately. This contrasts with the lag period that would be expected if the cells had to first attach to the electrode surface in order to utilize it as an electron donor. Although it was suggested that there was “negligible” dissolution of methyl viologen from the electrode and into the culture (5
), such dissolution was not directly verified, and it seems likely that methyl viologen would leach from such a surface. Furthermore, it was not determined whether cells attached to the electrode or were planktonic. Thus, definitive evidence that there was direct electron transfer from the electrode to the dechlorinating microorganisms was not obtained. Moreover, employing methyl viologen, a highly toxic compound, as a mediator for bioremediation is untenable.
In order to evaluate the possibility that there is direct electron transfer from electrodes to dechlorinating microorganisms under more defined and environmentally friendly conditions, we performed experiments with Geobacter lovleyi
, which reductively dechlorinates PCE and TCE to cis
-DCE with acetate as the electron donor (29
). It was hypothesized that G. lovleyi
might directly interact with electrodes in a manner similar to that observed for previously investigated Geobacter
species. We report here that G. lovleyi
can both donate electrons to and accept electrons from graphite electrodes and that an electrode is as effective an electron donor as acetate for PCE dechlorination.