Heavy metals at environmentally relevant concentrations enhance antibiotic and ammonia removal in constructed wetlands.

Constructed wetlands (CWs) as the last barrier for ensuring water quality often face the simultaneous occurrence of multiple pollutants such as metals, antibiotics and nutrients. However, the mechanisms by which environmentally relevant concentrations of heavy metals (HMs) influence pollutant removal processes remain poorly understood. In this study, HMs (Zn(II), As(V), and Pb(II)) at environmentally relevant concentrations significantly enhanced doxycycline (DOX) removal and promoted ammonia oxidation in CWs, indicating functional activation rather than inhibition. Mass balance and functional gene analyses revealed that this enhancement was driven by microbial adaptation, characterized by shifts in community composition and the enrichment of functional taxa. Under low-dose metal stress, microbes carrying DOX degradation genes (tetX1, tetX2) and ammonia-oxidizing genes were enriched. Concurrently, the increased abundance of metal resistance genes (MRGs) and plasmid-mediated horizontal gene transfer (HGT) facilitated the coexistence of resistance and metabolic traits. Importantly, the consistent responses observed across different metals indicate that this adaptive activation is largely independent of metal identity. These findings expand understanding of the ecological roles of HMs at environmentally relevant concentrations and underscore their potential to modulate microbial functionality, offering valuable implications for optimizing CW performance under complex co-contamination scenarios.