Long-term nitrogen and phosphorus removal, shifts of functional bacteria and fate of resistance genes in bioretention systems under sulfamethoxazole stress

To understand the long-term performance of bioretention systems under sulfamethoxazole (SMX) stress, an unplanted bioretention system (BRS) and two modified BRSs with coconut-shell activated carbon (CAC) and CAC/zero-valent-iron (Fe⁰) granules (CAC-BRS and Fe/CAC-BRS) were established. Both CAC-BRS and Fe/CAC-BRS significantly outperformed BRS in removing total nitrogen (TN) (CAC-BRS: 82.48%; Fe/CAC-BRS: 78.08%; BRS: 47.51%), total phosphorous (TP) (CAC-BRS: 79.36%; Fe/CAC-BRS: 98.26%; BRS: 41.99%), and SMX (CAC-BRS: 99.74%, Fe/CAC-BRS: 99.80%; BRS: 23.05%) under the long-term SMX exposure (0.8 mg/L, 205 days). High-throughput sequencing revealed that the microbial community structures of the three BRSs shifted greatly in upper zones after SMX exposure. Key functional genera, dominantly Nitrospira, Rhodoplanes, Desulfomicrobium, Geobacter, were identified by combining the functional prediction by the FAPROTAX database with the dominant genera. The higher abundance of nitrogen functional genes (nirK, nirS and nosZ) in CAC-BRS and Fe/CAC-BRS might explain the more efficient TN removal in these two systems. Furthermore, the relative abundance of antibiotic-resistant genes (ARGs) sulI and sulII increased in all BRSs along with SMX exposure, suggesting the selection of bacteria containing sul genes. Substrates tended to become reservoirs of sul genes. Also, co-occurrence network analysis revealed distinct potential host genera of ARGs between upper and lower zones. Notably, Fe/CAC-BRS succeeded to reduce the effluent sul genes by 1-2 orders of magnitude, followed by CAC-BRS after 205-day exposure. This study demonstrated that substrate modification was crucial to maintain highly efficient nutrients and SMX removals, and ultimately extend the service life of BRSs in treating SMX wastewater.