Microbial community and carbon–nitrogen metabolism pathways in integrated vertical flow constructed wetlands treating wastewater containing antibiotics

[Display omitted] •Co-metabolism of SMX and COD resulted in highly efficient SMX removal exceeding 98%.•SMX regulated diverse microbial community and metabolic function response pathways.•SMX promoted simultaneous heterotrophic nitrification and aerobic denitrification.•SMX reduced abundance and diversity but enriched resistant denitrifying bacteria. This study demonstrates effects of sulfamethoxazole (SMX) on carbon–nitrogen transformation pathways and microbial community and metabolic function response mechanisms in constructed wetlands. Findings showed co-metabolism of SMX with organic pollutants resulted in high removal of 98.92 ± 0.25% at influent concentrations of 103.08 ± 13.70 μg/L (SMX) and 601.92 ± 22.69 mg/L (COD), and 2 d hydraulic retention. Microbial community, co-occurrence networks, and metabolic pathways analyses showed SMX promoted enrichment of COD and SMX co-metabolizing bacteria like Mycobacterium, Chryseobacterium and Comamonas. Relative abundances of co-metabolic pathways like Amino acid, carbohydrate, and Xenobiotics biodegradation and metabolism were elevated. SMX also increased relative abundances of the resistant heterotrophic nitrification–aerobic denitrification bacteria Paracoccus and Comamonas and functional genes nxrA, narI, norC and nosZ involved in simultaneous heterotrophic nitrification-aerobic denitrification. Consequently, denitrification rate increased by 1.30 mg/(L∙d). However, insufficient reaction substrate and accumulation of 15.29 ± 2.30 mg/L NO3–-N exacerbate inhibitory effects of SMX on expression of some denitrification genes.