Quantitative response of nitrifying and denitrifying communities to environmental variables in a full-scale membrane bioreactor

•A full-scale MBR treating urban wastewater was operated for nine months.•Gene markers specific of AOB, NOB and denitrifiers were quantified by qPCR.•Both abundance (DNA) and transcription level (cDNA) of gene markers were analyzed.•MDS was used to link population abundances to changes in operation parameters.•N-cycle bacterial groups showed varying trends of response to operation parameters. The abundance and transcription levels of specific gene markers of total bacteria, ammonia-oxidizing Betaproteobacteria, nitrite-oxidizing bacteria (Nitrospira-like) and denitrifiers (N2O-reducers) were analyzed using quantitative PCR (qPCR) and reverse-transcription qPCR during 9months in a full-scale membrane bioreactor treating urban wastewater. A stable community of N-removal key players was developed; however, the abundance of active populations experienced sharper shifts, demonstrating their fast adaptation to changing conditions. Despite constituting a small percentage of the total bacterial community, the larger abundances of active populations of nitrifiers explained the high N-removal accomplished by the MBR. Multivariate analyses revealed that temperature, accumulation of volatile suspended solids in the sludge, BOD5, NH4+ concentration and C/N ratio of the wastewater contributed significantly (23–38%) to explain changes in the abundance of nitrifiers and denitrifiers. However, each targeted group showed different responses to shifts in these parameters, evidencing the complexity of the balance among them for successful biological N-removal.