Entry pathways determined the effects of MPs on sludge anaerobic digestion system: The views of methane production and antibiotic resistance genes fates.

Sludge is one of the primary reservoirs of microplastics (MPs), and the effects of MPs on subsequent sludge treatment raised attention. Given the entry pathways, MPs would exhibit different properties, but the entry pathway-dependent effect of MPs on sludge treatment performance and the fates of antibiotic resistance genes (ARGs), another high-risk emerging contaminant, were seldom documented. Herein, MPs with two predominant entry pathways, including wastewater-derived (WW-derived) and anaerobic digestion-introduced (AD-introduced), were used to investigate the effects on AD performance and ARGs abundances. The results indicated that WW-derived MPs, namely the MPs accumulated in sludge during the wastewater treatment process, exhibited significant inhibition on methane production by 22.8%-71.6%, while the AD-introduced MPs, being introduced in the sludge AD process, slightly increased the methane yield by 4.7%-17.1%. Meanwhile, MPs were responsible for promoting transmission of target ARGs, and polyethylene terephthalate MPs (PET-MPs) showed a greater promotion effect (0.0154-0.0936) than polyamide MPs (PA-MPs) (0.0013-0.0724). Compared to size, entry pathways and types played more vital roles on MPs influences. Investigation on mechanisms based on microbial community structure revealed characteristics (aging degree and types) of MPs determined the differences of AD performance and ARGs fates. WW-derived MPs with longer aging period and higher aging degree would release toxics and decrease the activities of microorganisms, resulting in the negative impact on AD performance. However, AD-introduced MPs with short aging period exhibited marginal impacts on AD performance. Furthermore, the co-occurrent network analysis suggested that the variations of potential host bacteria induced by MPs with different types and aging degree attributed to the dissemination of ARGs. Distinctively from most previous studies, the MPs with different sizes did not show remarkable effects on AD performance and ARGs fates. Our findings benefited the understanding of realistic environmental behavior and effect of MPs with different sources.

Exposure modes determined the effects of nanomaterials on antibiotic resistance genes: The different roles of oxidative stress and quorum sensing.

The effects of co-occurrent pollutants on antibiotic resistance genes (ARGs) in wastewater treatment plants (WWTPs) have raised attentions. However, how the different realistic exposure scenarios determining the effects of nanomaterials (NMs) on ARGs, was still unknown. Herein, the effects of NMs on ARGs under two realistic scenarios was investigated by short-term and long-term exposure modes. The presence of NMs with two different exposure modes could both promote the dissemination of ARGs, and the results were dose-, type- and duration-dependent. Compared to short-term exposure, the long-term exposure increased the abundances of ARGs with a greater extent except nano-ZnO. The long-term exposure increased the overall abundances of target ARGs by 2.9%-20.4%, while shot-term exposure caused the 3.4%-10.5% increment. The mechanisms of ARGs fates driven by NMs exposure were further investigated from the levels of microbial community shift, intracellular oxidative stress, and gene abundance. The variations of several potential bacterial hosts did not contribute to the difference in the ARGs transmission with different exposure modes because NMs types played more vital roles in the shift of microbial community compared to the exposure modes. For the short-term exposure, NMs were capable of triggering the QS by upregulating relevant genes, and further activated the production of surfactin and increased membrane permeability, resulting in the facilitation of ARGs transfer. However, NMs under long-term exposure scenario preferentially stimulated oxidative stress by generating more ROS, which then enhanced ARGs dissemination. Therefore, the exposure mode of NMs was one of the pivotal factors determining the ARGs fates by different triggering mechanisms. This study highlighted the importance of exposure scenario of co-occurrent pollutants on ARGs spread, which will benefit the comprehensive understanding of the actual environmental fates of ARGs.