Triclosan enriched resistance genes more easily than copper in the presence of environmental tetracycline in aerobic granular sludge system

Triclosan (TCS) and copper (Cu2+) were exposed to aerobic granular sludge (AGS) system treating wastewater containing environmental tetracycline, respectively, to explore the different biochemical responses, more importantly, the fates of resistance genes (RGs) in AGS system. The results showed that TCS and Cu2+ could significantly inhibit the N and P removal in AGS system by reducing several key functional genes, including amoA gene of ammonia-oxidizing bacteria, Nitrospira and phosphorus accumulating organisms 16S rRNA genes. TCS caused higher degree of RGs' enrichment than Cu2+, which made the average total relative abundance of RGs of 1.38 ± 0.73 and 0.78 ± 0.24 in TCS and Cu system, respectively. Cu2+ could induce a wider range of horizontal gene transfer than TCS, leading to the detections of more potential hosts harboring RGs in Cu system. Cu system seemed to have stronger repair, immunity and defense ability than TCS system, which enabled it to have sufficient ability to trigger protection mechanism to realize self-protection, eventually the RGs also were controlled. Integron (intI1 and intI3) and plasmids (trb-C and IncQ) might cooperate with microorganisms and water quality parameters to enhance the enrichment of RGs in TCS system, however this interaction among various environmental factors was not obvious in Cu system, which might be responsible for the lower abundance of RGs. The increasing levels of TCS and Cu2+ in wastewater should be paid more attentions during the treatment of wastewater containing environmental tetracycline by AGS system. Especially for TCS, it had the ability to enrich RGs more easily than Cu2+, which should be prevented from entering wastewater treatment plants as far as possible, to avoid more serious proliferation and dissemination of various RGs.