Biological treatment of sulfate-laden wastewater consists of two separate reactors to reduce sulfate to sulfide by sulfate-reducing bacteria (SRB) and to oxidize sulfide to sulfur (S-0) by sulfide oxidation bacteria (SOB). To have SRB + SOB in a single reactor faced difficulty of low S-0 conversion. This study for the first time revealed that dissolved oxygen (DO) level can be used to manipulate SRB + SOB reactions in a single reactor. This work demonstrated successful operation of an integrated SRB + SOB reactor under microaerobic condition. At DO = 0.10-0.12 mg l(-1), since the activities of SOB were enhanced by limited oxygen, the removal efficiency for sulfate reached 81.5% and the recovery of S-0 peaked at 71.8%, higher than those reported in literature. At increased DO, chemical oxidation of sulfide with molecular oxygen competed with SOB so conversion of S-0 started to decline. At DO > 0.30 mg l(-1) activities of SRB were inhibited, leading to failure of the SRB + SOB reactor. (C) 2012 Elsevier Ltd. All rights reserved.