Reversible bioelectrodes were designed by alternating acetate and oxygen supply. It was demonstrated that the protons produced and accumulated inside the biofilm during the anodic phase greatly favored the oxygen reduction reaction when the electrode was switched to become the biocathode. Protons accumulation, which hindered the bioanode operation, thus became an advantage for the biocathode. The bioanodes, formed from garden compost leachate under constant polarization at -0.2 V vs. SCE, were able to support long exposure to forced aeration, with only a slight alteration of their anodic efficiency. They produced a current density of 16 +/- 1.7 A/m(2) for acetate oxidation and up to similar to 0.4 A/m(2) for oxygen reduction. Analysis of the microbial communities by 16S rRNA pyrosequencing revealed strong selection of Chloroflexi (49 +/- 1%), which was not observed for conventional bioanodes not exposed to oxygen. Chloroflexi were found as the dominant phylum of electroactive biofilms for the first time. (C) 2014 Elsevier Ltd. All rights reserved.