G. sulfurreducens strain DL-1 was cultivated in the anode of an MFC with graphite block electrodes. An H-cell (Adams & Chittenden Scientific Glass) was used as the reactor. The volume of each chamber of the H-cell is 100 ml. A Nafion N117 membrane (Chemours) was boiled in deionized (DI) water and then inserted between the two chambers. A 100-ml DL-1 cell culture was grown to mid-log phase with fumarate, centrifuged, and used to inoculate the anode chamber. To obtain the MFC strain well adapted for reducing insoluble electron acceptors (fig. S2), DL-1 cells were maintained on the growth medium amended with 100 to 120 mM poorly crystalline Fe(III) oxide as the electron acceptor and then transferred (10% inoculum) three times in medium containing 40 mM fumarate before inoculation into the MFC (4). The anode chamber contained 100 ml of the growth medium without fumarate. The cathode contained 100 ml of the growth medium lacking fumarate or acetate but included 50 mM potassium ferricyanide as the electron acceptor. The graphite block electrodes were connected by a titanium wire through a 1-kilohm resistor. Current and open circuit voltages of the MFC were monitored periodically using an EX430 MultiMeter (Extech Instruments). The bacterial cells were harvested from the anode (with a 16.6 cm2 surface area) inside the anaerobic chamber when the MFC approached a current density higher than 45 mA/m2. The cells were scrapped off the anode surface using a cell scraper (Thermo Fisher Scientific) and suspended in 1.5 ml of their native growth medium.

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