Microbial Community Restructuring Under Zinc Oxide Nanoparticle Stress in Membrane Bioreactors

Zinc oxide nanoparticles (ZnO NPs) are increasingly released into aquatic environments, raising concerns regarding their impacts on biological wastewater treatment systems. This study investigated the long‐term effects of ZnO NP exposure on microbial community dynamics and extracellular responses in submerged anoxic–aerobic membrane bioreactors (MBRs) operated continuously for 144 days. Two identical MBRs were operated under control and ZnO NP exposure conditions (1–50 mg L −1 ). Reactor performance, extracellular polymeric substances (EPS), soluble microbial products (SMPs), molecular weight (MW) distribution, fluorescence characteristics and bacterial community structure were evaluated using high‐performance liquid chromatography–size exclusion chromatography (HPLC–SEC), fluorescence excitation‐emission matrix (FEEM) spectroscopy and 16S ribosomal RNA (16S rRNA) sequencing. ZnO NP exposure significantly reduced ammonium removal efficiency from 85.0% ± 2.5% to 65.2% ± 5.7% at 50 mg L −1 , indicating inhibition of nitrification. Both LB–EPS and TB–EPS increased substantially under nanoparticle stress, while HPLC–SEC and FEEM analyses revealed accumulation of high‐molecular‐weight, protein‐like and fulvic‐like SMP fractions. Long‐term ZnO NP exposure restructured microbial community composition and was associated with reduced taxonomic and phylogenetic richness and substantial reductions in important nitrogen‐transforming genera such as Nitrospira and Dechloromonas . Overall, long‐term nanoparticle exposure promoted microbial community restructuring and enhanced extracellular stress responses, while simultaneously impairing important biological treatment functions.