Hydrogen‐fed biofilm reactors reducing selenate and sulfate: Community structure and capture of elemental selenium within the biofilm

Remediation of selenate (SeO 4 2− ) contamination through microbial reduction is often challenging due to the presence of sulfate (SO 4 2− ), which can lead to competition for the electron donor and the co‐production of toxic H 2 S. Microbial reduction of SeO 4 2− in the presence of SO 4 2− was studied in two hydrogen‐based membrane biofilm reactors (MBfRs). One MBfR was initiated with SO 4 2− ‐reducing conditions and gradually shifted to SeO 4 2− reduction. The second MBfR was developed with a SeO 4 2− ‐reducing biofilm, followed by SO 4 2− introduction. Biofilms within both MBfRs achieved greater than 90% SeO 4 2− reduction, even though the SeO 4 2− concentration ranged from 1,000–11,000 μg/L, more than 20–200 times the maximum contaminant level for drinking water (50 μg/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 SeO 4 2− loading. Specifically, high SeO 4 2− 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 ). SeO 4 2− was reduced to elemental selenium (Se°), 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 SeO 4 2− or SO 4 2− ‐reducing conditions attained high SeO 4 2− removal rates even though their microbial community composition was quite distinct. Biotechnol. Bioeng. 2016;113: 1736–1744. © 2016 Wiley Periodicals, Inc.