Bioelectrode Membrane-free bioelectrochemical systems ( MFBESs) have been developed for the degradation of nitroaromatic contaminants, but the microbial communities that are involved have not been comprehensively investigated. In this study, the microbial communities were evaluated and compared for treating different structures of nitrophenols ( NPs), i. e., o-nitrophenol ( ONP), m-nitrophenol ( MNP) and p-nitrophenol ( PNP), in the MFBES. The results demonstrated that NPs reduction in the MFBES decreased in efficiency in the following order: ONP > MNP > PNP. Illumina MiSeq sequencing results showed that richness and diversity of bacterial species in the anodic and cathodic communities decreased when fed different NPs. Though remarkable differences in community composition were found between anodic and cathodic biofilms in the MFBES, three core genera-Treponema, Desulfovibrio and Geobacter-were dominant in the anodic or cathodic biofilm, regardless of various NPs. Other functional genera in the anodic or cathodic biofilm were selectively enriched in the MFBES treating the three NPs with different structures. (C) 2016 Elsevier Ltd. All rights reserved.