Niche differentiation and symbiotic association among ammonia/nitrite oxidizers in a full-scale rotating biological contactor

•The distribution and activity of ammonia/nitrite oxidizers were profiled.•AOB and anammox bacteria collectively contributed to nitrogen conversion at Stage-A.•Abundant and active comammox nitrospira and AOA were observed at Stage-B.•Complex factors shaped niche differentiation and symbiotic association in the RBC.•Phages likely act as a biotic factor mediating ammonia/nitrite oxidizer populations.