Microbial electrolysis cells (MECs) can generate methane by fixing carbon dioxide without using expensive catalysts, but the impact of acclimation procedures on subsequent performance has not been investigated. Granular activated carbon (GAC) was used to pre-enrich electrotrophic methanogenic communities, as GAC has been shown to stimulate direct transfer of electrons between different microbial species. MEC startup" times using pre-acclimated GAC were improved compared to controls (without pre-acclimation or without GAC), and after three fed batch cycles methane generation rates were similar (P> 0.4) for GAC acclimated to hydrogen (22 +/- 9.3 nmol cm(-3) d(-1)), methanol (25 +/- 9.7 nmol cm(-3) d(-1)), and a volatile fatty acid (VFA) mix (22 +/- 11 nmol cm(-3) d(-1)). However, MECs started with GAC but no pre-acclimation had lower methane generation rates (13 +/- 4.1 nmol cm(-3) d(-1))and MECs without GAC had the lowest rates (0.7 +/- 0.8 nmol cm(-3) d(-1) after cycle 2). Microbes previously found in methanogenic MECs, or previously shown to be capable of exocellular electron transfer, were enriched on the GAC. Pre-acclimation using GAC is therefore a simple approach to enrich electroactive communities, improve methane generation rates, and decrease startup times in MECs. (C) 2016 Elsevier B.V. All rights reserved.