Removal of antibiotic resistance genes by composite constructed wetlands of Tianjin Lingang in Winter

Drug-resistant bacteria and antibiotic resistance genes (ARGs) exist widely in the environment due to the extensive application of antibiotics, which affect the therapeutic effect of antibiotics on diseases and pose a great threat to human health and ecological security. Studies have shown that constructed wetlands (CWs) can effectively remove ARGs, but the effect of composite CWs on ARGs removal in northern China in winter is still unclear. Tianjin Lingang CWs, a compound of regulating pond, horizontal subsurface flow wetland and surface flow wetland, was used as the research object to study the removal effect of ARGs in winter. Water samples were collected from different functional zones, and 16S rRNA genes, ARGs, mobile genetic elements (MGEs) and bacterial population composition in water were detected by high-throughput quantitative PCR. The removal effect of ARGs was comprehensively analyzed, and the key factors affecting the removal effect during winter operations were discussed. The results showed that the absolute abundance of 16S rRNA gene was 2.70×10⁴-1.41×10⁵ copies/mL. The total detection rate of ARGs was 72.5%, in which floR and sul2 did not originate from influent water. The abundance of ARGs in different functional zones was significantly different, and the removal effect of ARGs in different functional zones was also significantly different. Overall, CWs in Tianjin Lingang had the best removal effect on aminoglycoside resistance genes and multi-drug resistance genes, with total absolute abundance removal rates of 85.59% and 47.78%, and total relative abundance removal rates of 97.09% and 89.44%, respectively. The removal efficiency of β-lactam resistance genes was the worst, and the total absolute abundance and relative abundance removal rates were −404.40% and −2.01%, respectively. The removal rates of total absolute abundance of ARGs were 38.05%, −7.78% and −2.41% in regulating pond, horizontal subsurface flow wetland and surface flow wetland, and the total relative abundance removal rates were 75.02%, −45.60% and −7.75%, respectively. The removal effects of different functional zones were as follows: the regulating pond > surface flow wetland > horizontal subsurface flow wetland, in which the regulating pond had a better removal effect for the absolute abundance of other ARGs except tetracycline resistance genes, the horizontal subsurface flow wetland had a better removal effect for sulfonamides resistance genes, and the surface flow wetland had a certain removal effect for macrolide resistance genes. Low temperature, MGEs, functional zones of different process types and operation time were the key factors affecting the removal effect of ARGs. The non-selectivity of ARGs to bacterial hosts promoted the rapid spread of ARGS among various bacterial groups in Tianjin Lingang CWs system. It is suggested to strengthen the optimization technology research to improve the removal effect of new pollutant ARGs by CWs.