The effects of dimethyl phthalate on sludge anaerobic digestion unveiling the potential contribution of plastic chemical additive to spread of antibiotic resistance genes

Abstract

The ubiquitous microplastics become the spread hotspot of antibiotic resistance genes (ARGs), which indicates the nature inherent to microplastics is conducive to the spread of ARGs. To date, the influence of plastic chemical additives (one of the critical toxic contributors of microplastics) on the enrichment of ARGs is an open question. Here, the integral effects of dimethyl phthalate (DMP, a typical plastic chemical additive) at different levels (0.1 mg/L as the environmental level, or 10.0 mg/L as the potential high level), on the performance of sludge anaerobic digestion and the profile of ARGs and mobile genetic elements (MGEs) during batch anaerobic digestion were assessed in turn. The presence of 0.1 mg/L of DMP had no significant impact on methane production. In stark contrast, 10.0 mg/L of DMP promoted methanogenesis by facilitating sludge disintegration. The presence of 10.0 mg/L of DMP favored the development of methanogens, and such a shaping force did not change the ARGs profile dominated by the multidrug resistance gene. Meanwhile, the presence of 10.0 mg/L increased the concentrations of several dominating MGEs, which implied that DMP could increase ARGs' spread potential. While DMP at 0.1 mg/L did not affect the concentrations of ARGs and MGEs. Finally, the co-exposure of DMP and antibiotics further demonstrated that 10.0 mg/L of DMP contributed to the spread of ARGs in the presence of antibiotic stress. This work suggests that the DMP does not accelerate the dissemination of ARGs directly, but it can act as an essential driver to the spread of ARGs.