Remediation of selenate (SeO42-) contamination through microbial reduction is often challenging due to the presence of sulfate (SO42-), which can lead to competition for the electron donor and the co-production of toxic H2S. Microbial reduction of SeO42- in the presence of SO42- was studied in two hydrogen-based membrane biofilm reactors (MBfRs). One MBfR was initiated with SO42--reducing conditions and gradually shifted to SeO42- reduction. The second MBfR was developed with a SeO42--reducing biofilm, followed by SO42- introduction. Biofilms within both MBfRs achieved greater than 90% SeO42- reduction, even though the SeO42- concentration ranged from 1,000-11,000mg/L, more than 20-200 times the maximum contaminant level for drinking water (50mg/L). Biofilm microbial community composition, assessed by 16S rRNA gene-based amplicon pyrosequencing, was distinct between the two MBfRs and was framed by alterations in SeO42- loading. Specifically, high SeO42- loading resulted in communities mainly composed of denitrifying bacteria (e.g., Denitratisoma and Dechloromonas). In contrast, low loading led to mostly sulfate-reducing bacteria (i.e., Desulfovibrio) and sulfur-oxidizing bacteria (i.e., Sulfuricurvum and Sulfurovum). SeO42- was reduced to elemental selenium (Se degrees), which was visualized within the biofilm as crystalloid aggregates, with its fate corresponding to that of biofilm solids. In conclusion, microbial biofilm communities initiated under either SeO42- or SO42--reducing conditions attained high SeO42- removal rates even though their microbial community composition was quite distinct. (C) 2016 Wiley Periodicals, Inc.