Microbial community structure associated with treatment of azo dye in a start-up anaerobic sequenced batch reactor

• More than 85% of COD and 75% of MO was removed during the start-up period.
• Successful competition of MO reduction over sulfate reduction and methanogenesis was observed.
• Soluble Fe(II) enhanced azo-reduction by redox reaction.

The start-up efficiency of an anaerobic sequencing batch reactor (ASBR) fed with increasing methyl orange (MO) concentrations (from 25 mg l−1 to 500 mg l−1) were investigated in this study. The relationship between MO decolorization capacity and the microbial community structure was evaluated. More than 85% of COD and 75% of MO were removed during the whole operation period. However, little methane was generated, due to the successful competition of azo reduction over sulfate reduction and methanogenesis. The scanning electron microscopy and denaturing gradient gel electrophoresis analysis showed a significant variation of the microbial community composition with MO concentration increase. The thresholds of structural and functional disturbances were similar, suggesting a good correlation between degradation performance and community structure. Phylogenetic analysis indicated that Sulfuricurvum and Anaerolineaceae family were the most abundant microorganisms in ASBR responsible for MO decolorization. Batch experiments indicated that acclimated sludge could also use quinoide and Fe(III) as the electron acceptors, thus allowing an efficient reduction of azo dye.