Antagonistic interactions between polyethylene microplastics and oxytetracycline mitigate toxicity of the anammox process and the proliferation of ARGs

Emerging pollutants such as microplastics and antibiotics pose potentially serious risks to the metabolism and ecology of microorganisms. To explore the short-term stress mechanism of polyethylene microplastics (PE MPs) and oxytetracycline (OTC) on the anaerobic ammonia oxidation (anammox) process, single and combined stress experiments of with 5 mg/L PE MPs and 5 mg/L OTC on anammox were conducted based on the gradient experiments. The results showed that OTC had a more significant impact on the nitrogen removal performance of anammox process compared to PE MPs alone and that of the combined stress, and the combined stress exhibited an antagonistic effect. OTC stress can alter the secretion and structural composition of extracellular polymeric substances (EPS) of anammox bacteria, reduce the size of anammox granular sludge (AnGS), and decrease the stability of AnGS. However, PE MPs had a positive effect on the EPS of anammox bacteria. The relative abundance of Planctomycetes decreased in all stress groups, with the greatest decrease observed in the OTC group. The relative abundances of OTC resistance genes, tetG and tetX, decreased in the PE MPs group, while they increased in the OTC group and the combined stress group. PE MPs had an inhibitory effect on the spread of antibiotic resistance genes (ARGs), and the combined stress of PE MPs and OTC may mainly affect the amplification of the efflux pump gene tetG.