Power densities and oxidation-reduction potentials (ORPs) of MFCs containing a pure culture of Shewanella oneidensis MR-1 were compared to mixed cultures (wastewater inoculum) in cube shaped, 1-, 2-, and 3-bottle batch-fed MFC reactor configurations. The reactor architecture influenced the relative power produced by the different inocula, with the mixed culture generating 68-480% more power than MR-1 in each MFC configuration. The mixed culture produced the maximum power density of 858 +/- 9 mW m(-2) in the cubic MFC, while MR-1 produced 148 +/- 20 mW m(-2). The higher power by the mixed culture was primarily a result of lower internal resistances than those produced by the pure culture. Power was a direct function of ohmic resistance for the mixed culture, but not for strain MR-1. ORP of the anode compartment varied with reactor configuration and inoculum, and it was always negative during maximum power production but it did not vary in proportion to power output. The ORP varied primarily at the end of the cycle when substrate was depleted, with a change from a reductive environment during maximum power production (approximately -175 mV for mixed and approximately -210 mV for MR-1 in cubic MFCs), to an oxidative environment at the end of the batch cycle ( approximately 250 mV for mixed and approximately 300 mV for MR-1). Mixed cultures produced more power than MR-1 MFCs even though their redox potential was less negative. These results demonstrate that differences between power densities produced by pure and mixed cultures depend on the MFC architecture. 2009 Wiley Periodicals, Inc.