Membrane bioreactors (MBRs) were operated on-site for 56 d with different powdered activated carbon (PAC) dosages of 0, 1.5 and 5.0 g/L, to pretreat seawater for reverse osmosis desalination. It was hypothesized that PAC would stimulate adsorption and biological degradation of organic compounds. The microbial communities responsible for biofouling on microfiltration (MF) membranes and biological organic removal in MBR were assessed using terminal restriction fragment length polymorphism fingerprinting and 454-pyrosequencing. The PAC addition improved assimilable organic carbon removal (53-59%), and resulted in reduced biofouling development on MF (>50%) with only a marginal development in trans membrane pressure. Interestingly, the bacterial community composition was significantly differentiated by the PAC addition. Cyanobacterium, Pelagibaca and Maricoccus were dominant in the PAC-free conditions, while Thiothrix and Sphingomonas were presumably responsible for the better reactor performances in PAC-added conditions. In contrast, the archaeal communities were consistent with predominance of Candidatus Nitrosopumilus. These data therefore show that the addition of PAC can improve MBR performance by developing different bacterial species, controlling AOC and associated biofouling on the membranes. (C) 2016 Elsevier B.V. All rights reserved.