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• Acinetobacter sp. YS2 was isolated from petrochemical wastewater treatment process. • Nitrifying and denitrifying enzyme genes were amplified successfully. • The proportion of aerobic denitrifiers reached 87.44% in the microbial community. • YS2 has potential application in the SND treatment for petrochemical wastewater. The ecological niche of aerobic denitrifiers in activated sludge is little studied, but this determines the performance of these bacteria in reactors. Acinetobacter sp. YS2 isolated from a treatment process for petrochemical wastewater and showed excellent nitrogen removal performance. In the nitrification and denitrification process, the removal efficiencies of NH 4 +-N and NO 3 −-N were 87.81% and 88.20% at 24 h, respectively, and nitrogen balance analysis indicated that the proportions of nitrogen removal by gaseous nitrogen were 37.58% and 46.45%, respectively. During simultaneous nitrification and denitrification (SND) treatment of synthetic petrochemical wastewater in a sequencing batch reactor (SBR), aerobic denitrifiers became the dominant species and the proportion reached 87.44% in the microbial community. The removal efficiencies of NH 4 +-N and TN were 99.46% and 80.36% over 5 days, respectively. This research has shown that aerobic denitrifiers can become the dominant species in the applications of SND treatment for petrochemical wastewater. [ABSTRACT FROM AUTHOR]