Homo-acetogens in the anode of a microbial electrolysis cell (MEC) fed with H(2) as sole electron donor allowed current densities similar to acetate-fed biofilm anodes (similar to 10 A/m(2)). Evidence for homo-acetogens included accumulation of acetate at high concentrations (up to 18 mM) in the anode compartment: detection of formate, a known intermediate during reductive acetogenesis by the acetyl-CoA pathway: and detection of formyl tetrahydrofolate synthetase (FTHFS) genes by quantitative real-time PCR. Current production and acetate accumulation increased in parallel in batch and continuous mode, while both values decreased simultaneously at short hydraulic retention times (1 h) in the anode compartment, which limited suspended homo-acetogens. Acetate produced by homo-acetogens accounted for about 88% of the current density of 10 A/m(2), but the current density was sustained at 4 A/m(2) at short hydraulic retention time because of a robust partnership of homo-acetogens and anode respiring bacteria (ARB) in the biofilm anode. (C) 2010 Elsevier Ltd. All rights reserved.