The presence of Ni and Cu ions promote the spread of ARGs and MRGs in activated sludge

Antibiotic resistance genes (ARGs) are considered as emerging environmental pollutants. At present, the co-occurrence of ARGs and metal resistance genes (MRGs) with the presence of heavy metals in sewage treatment systems remains to be further explored. Therefore, a laboratory-scale sequencing batch reactor (SBR) was constructed to investigate the effects of Ni2+ and Cu2+ on the co-occurrence of ARGs and MRGs in sludge. The impact of low (0.2-3 mg/L) and high (10 mg/L) concentrations of Ni2+ as well as high (10 mg/L) concentrations of Cu2+ on the structure of microbial communities, antibiotic resistance genes (ARGs), and heavy metal resistance genes (MRGs) in aerobic activated sludge from a sewage treatment system was investigated. Additionally, the coexistence and correlation between ARGs and MRGs were examined. In the reactors with added heavy metals, the highest total abundance of ARGs and MRGs were 2.5 and 2.7 times that of the raw sludge, respectively. The co-occurrence rates of ARGs and MRGs in the reactors with low concentrations of Ni2+ were slightly higher than those in the control group and the original sludge by approximately 8.3%. However, in the reactor with high concentrations of Ni2+ and Cu2+, the co-occurrence rates were approximately 1.02 times higher than those in the control group and blank group. The long-term exposure to different concentrations of heavy metals resulted in a substantial dynamic change in species composition within the activated sludge, with the abundance of certain MRGs and ARGs being associated with the dominant genus present in the activated sludge. In this paper, it was verified that Ni2+ also promoted the proliferation and propagation of ARGs and MRGs at low concentrations, and the abundance of resistance genes gradually increased with the increase of metal concentration. The effects of resistance gene composition and bacterial community structure on sterile, unsterilized and synthetic wastewater were discussed. This study reveals the promoting effect of heavy metal pollution on the spread of resistance genes in activated sludge, deepening our understanding of the relationship between heavy metals and resistance gene diffusion, and providing important theoretical insights for wastewater treatment and the control of resistance gene proliferation.