Bioelectrochemistry increases the risk of resistance genes proliferation and transfer with sulfamethoxazole pressure decreasing in constructed wetlands: An overlooked double-edged effect.

Microbial electrochemical system-enhanced constructed wetland (MES-CW) attracts more attention for antibiotic-contaminated wastewater treatment, but the effect of bioelectrochemistry on resistance genes is still unclear with the removal of antibiotic. In this study, the antibiotic resistance genes (ARGs) dynamics, driving factors and transmission mechanism in effluent and substrate samples were investigated under decreasing sulfamethoxazole (SMX) pressure. Bioelectrochemistry was suitable for reducing the risk of ARGs proliferation and transmission at concentration of 0.1 mg/L SMX. The heterogeneity between effluent and substrate shaped different ARGs occurrence profiles. With SMX concentration decreasing from 10 mg/L to 0.1 mg/L, the ARGs abundance in effluent of MES-CW showed an overall inverted "V" shape (i.e., first up and then down) while that in substrate continued to decrease. Bioelectrochemical microcurrent respectively induced an increase of 61.5 % and 39.3 % in ARGs abundance of effluent and substrate when antibiotic pressure was eliminated. Co-occurrence analysis suggested the rearrangement of ARGs and mobile genetic elements with the decrease of SMX concentrations, which increased the risk of antibiotic resistance transmission. Metagenomic binning disclosed that pathogenic bacteria were enriched in MES-CW and ARGs hosts showed multiple resistance after SMX pressure elimination. Improving voltage to alleviate the proliferation of antibiotic resistant bacteria and resistance genes was preliminarily attempted. This study unveils the double-edged effects of bioelectrochemistry on ARGs proliferation and transfer in CW with the decrease of antibiotic concentration for the first time, and broadens the knowledge of ARGs evolution in MES-CW. These findings provide a guidance for the safe operation of MES-CW for rural livestock wastewater treatment.