| Dissimilatory metal reduction mediated by bacteria is an important biogeochemical process in anaerobic environments. Dissimilatory metal reducing bacteria (DMRB) couple the oxidation of organic carbon with the reduction of metals in order to gain energy for growth (Nealson and Myers, 1992). An understanding of how DMRB transfer electrons to solid-phase terminal electron acceptors during respiration is essential if they are to be used in bioremediation. Here we investigate the hypothesis that outer membrane cytochromes, OmcA and MtrC, are capable of direct electron transfer with a hematite working electrode. In order to investigate this idea, we ran cyclic voltammetry on cell suspensions and protein films. Midpoint potentials for the proteins (OmcA and MtrC) match well with other reported values. The OmcA midpoint (-208 mV vs. Ag/AgCl) is more negative than the MtrC midpoint (-168 mV vs. Ag/AgCl). The cathodic peak seen in DeltaMtrC/OmcA is a single contribution from dissolved iron species and, as expected, does not involve contributions from either protein. This supports our conclusion that DeltaOmcA, DeltaMtrC and MR-1 are composites of multiple electrochemical reactions involving proteins in addition to dissolved iron from the hematite electrodes. We were able to make composite voltammograms from the cathodic current attributed to the proteins plus cathodic current attributed to dissolved iron to "recreate" the cathodic peak seen in DeltaMtrC, DeltaOmcA and MR-1 voltammetry data. Our composites did not match the whole cell scans exactly in some places because of differences in residual O2 (oxygen left after purging) and differences in electrode properties. Results indicate that the outer membrane cytochromes OmcA and MtrC are indeed capable of direct electrochemistry with hematite electrodes and thus are consistent with the hypothesis that these cytochromes act as terminal reductases in the Shewanella oneidensis MR-1 anaerobic respiratory pathway. |
