Response of antibiotic and heavy metal resistance genes to tetracyclines and copper in substrate-free hydroponic microcosms with Myriophyllum aquaticum

Constructed wetlands for antibiotics and heavy metals removal have become important reservoirs of antibiotic resistance genes (ARGs) and heavy metal resistance genes (MRGs), especially in the substrates. Here, substrate-free hydroponic microcosms of Myriophyllum aquaticum were established; tetracyclines (TCs) and Cu(II) were added to evaluate the behaviours of ARGs and MRGs in the microcosms. Several ARGs, MRGs, and mobile genetic elements (MGE) were detected in the biofilms attached to the plants, ranging from 0.5 to 2.3 x 10(8) copies/g dry weight. ARGs and MRGs exhibited higher relative abundances in the effluent suspended solids (SS); however, their absolute amounts were much lower than those in conventionally constructed wetlands. Microcosms with TCs and Cu(II) exhibited a higher level of resistant genes than those with compound added singularly owing to co-selection pressure. The existence of TCs and copper significantly changed the microbial communities in the microcosms. The exogenous input of TC/Cu(II) and microbial community structure were the factors driving the occurrence of ARGs, whereas MRGs were more correlated with the copper addition. Thus, reducing the exogenous inputs of antibiotics/heavy metals and SS of the effluent is suggested for the mitigation of resistant genes in phytoremediation technologies working in the absence of conventional substrates.