Current production and metal oxide reduction by Shewanella oneidensis MR-1 wild type and mutants

Abstract

Shewanella oneidensis MR-1 is a gram-negative facultative anaerobe capable of utilizing a broad range of electron acceptors, including several solid substrates. S. oneidensis MR-1 can reduce Mn(IV) and Fe(III) oxides and can produce current in microbial fuel cells. The mechanisms that are employed by S. oneidensis MR-1 to execute these processes have not yet been fully elucidated. Several different S. oneidensis MR-1 deletion mutants were generated and tested for current production and metal oxide reduction. The results showed that a few key cytochromes play a role in all of the processes but that their degrees of participation in each process are very different. Overall, these data suggest a very complex picture of electron transfer to solid and soluble substrates by S. oneidensis MR-1.

FIG. 1.

Dual-compartment fuel cell diagram. Bacteria are inoculated into the anode compartment, attach to the graphite felt electrode, and begin transferring electrons. Electrons are conducted through the anode electrode and across the external circuit to the cathode electrode. The cathode electrode is graphite felt that has been electroplated with platinum, the catalyst driving the reduction of oxygen to water. The anode and cathode compartments are physically separated by a proton-conductive membrane that facilitates the transfer of protons from the anode to the cathode, completing the cell reaction. The cell voltage (V) is measured across a 10-Ω resistance (R), and current is calculated as I = V/R.

FIG. 2.

Current density values for MR-1 WT and cytochrome deletion and protein secretion mutants (a) and MR-1 WT and selected cytochrome mutants with their complementations (b). Averages and standard deviations were obtained using the peak current density values corresponding to each lactate injection for triplicate experiments. Maximum current density was determined to be the highest level of current density that remained constant for at least 3 hours. Averages and standard deviations of the maximum current density were calculated using these data values (between 100 and 200 data points were utilized).

FIG. 3.

SEM images of graphite anode fibers used during the MFC evaluations of the MR-1 ΔpilD mutant (a), ΔomcA ΔmtrC mutant (b), and WT (c).

FIG. 4.

Average percentage of Mn(IV) oxide reduced after 24 hours of exposure to MR-1 WT and cytochrome deletion and protein secretion mutants. Error bars indicate standard deviations.

FIG. 5.

Average Fe(II) concentration resulting from solid Fe oxide (HFOM) reduction after 24 hours of exposure to MR-1 WT and cytochrome deletion and protein secretion mutants. Averages and standard deviations were calculated based on the measured Fe(II) concentrations from triplicate experiments.