G.metallireducens strain GS-15 (DSM 7210) was purchased from the Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH (DSMZ), Germany. Microorganisms were cultivated in DSMZ Geobacter medium 579 with 1 mL L−1DSMZ trace element solution (SL10) and 0.5 mL L−1DSMZ 7 vitamins solution. Cultivation of G. metallireducens in retentostats was undertaken in three replicate runs with 2.5 mM acetate plus 0.7 mM benzoate as mixed carbon sources, and in two runs with 5 mM acetate as a single carbon source. The retentostat equipment was designed and built by the electronics and mechanics workshop of the Faculty of Earth and Life Sciences, VU University Amsterdam, the Netherlands, and details of the design can be found in . The fermenter vessel had a maximum capacity of 2 L with a working volume of 1.5 L,and it was operated under anoxic conditions. A N2/CO2 mixture(90%:10%) was passed through a Titanium(III) citrate solution in order to remove traces of oxygen, prior to entering the reactor at a constant rate of 2 L per hour. The fermenter was stirred at200 rpm, and medium was pumped out through a retention unit incorporating a 0.22 m pore size filter to retain biomass. The pH of the culture was maintained at 6.8 by the addition of 2 M HCl or 2 M NaOH, and the temperature was maintained at 30◦C. The gas outlet was connected to a bottle filled with water, which kept the fermenter at slight overpressure in order to avoid oxygen leakage into the system. The fermenter and medium reservoir were kept dark by wrapping in aluminium foil. The fermenter was inoculated at 10% v/v with an exponentially growing pre-culture, and cultivated under batch conditions until the Fe2+concentration reached 30–40 mM, which indicated complete carbon source consumption.Then, the peristaltic feed pump was switched on (medium supply rate of 50 mL h−1) and the fermenter was operated in retentostat mode.