Concentration threshold for polyethylene microplastics impacting nitrogen transfor-mation in heterotrophic nitrification-aerobic denitrification systems.
Heterotrophic nitrification-aerobic denitrification (HN-AD) enables simultaneous nitrification and denitrification under aerobic conditions. However, the pathways and mechanisms by which microplastics (MPs) affect HN-AD mediated nitrogen transformation remain unclear, limiting the development of remediation strategies for co-polluted aquatic systems. To address this gap, microcosm tests integrated with optical characterization and high-throughput 16S rRNA sequencing were conducted to evaluate polyethylene microplastic (PE-MPs) influences. Results revealed a concentration-dependent dual effect where PE-MPs suppressed nitrite oxidation and denitrification, featuring a distinct inhibition threshold and inducing NO2--N accumulation up to 8.0 times the control level. Mechanistically, PE-MPs acted as physical carriers that promoted microbial colonization and biofilm development, thereby facilitating microbial community dispersion and boosting the transformation. Concurrently, leaching of chemicals from PE-MPs reduced microbial abundance and diversity, depressing nitrite oxidation. Notably, the relative abundance of HN-AD functional taxa increased with rising PE-MPs concentrations. Emergent dominant phyla, Myxococcota and Patescibacteria, were significantly enriched, indicating adaptive. These opposing forces define the critical concentration threshold where the promotion of ammonia oxidation shifts to inhibition and the overall denitrification collapses. Ultimately, the concentration-driven battle between physical and chemical disrupts the HN-AD microbial community, severely hindering nitrogen transformation. This study provides mechanistic insights into the complex interplay between MPs and nitrogen transformation, offering novel perspectives for optimizing bioremediation strategies in MPs and nitrogen co-polluted aquatic systems.