Research PaperMicroplastics affect the ammonia oxidation performance of aerobic granular sludge and enrich the intracellular and extracellular antibiotic resistance genes
Graphical Abstract
Introduction
People have paid attention to the ecological risks of microplastics (MPs) which are regarded as plastics particles smaller than 5 mm (Thompson et al., 2004). Among them, polyvinyl chloride (PVC), polyamide (PA), polystyrene (PS) and polyethylene (PE) are widely applied in packaging films, protective packaging, bottles, and pharmaceutical and personal care products, etc. (Cheung and Fok, 2017, Ziajahromi et al., 2017), and are frequently detected in aquatic systems (Rezania et al., 2018) such as marine (ranged from 3.4 × 10−4 to 6.237 mg/L (approximately 8.5 particles/m3 to 1490.8 particles/L)) and freshwater (1.74 × 10−4 to 1.8 × 10−2 mg/L (approximately 43–4650 particles/m3)) (Wong et al., 2020), especially wastewater treatment plants (WWTPs) (0.004–1.812 mg/L (approximately 1–447 particles/L)) (Sun et al., 2019). A recent study (Murphy et al., 2016) indicated that WWTPs are a source and sink of MPs. Therefore, it was vital to investigate the impacts of MPs on biological sewage treatment. Previous studies (Li et al., 2020b, Wang et al., 2020) showed that MPs inhibit the nitrification of activated sludge. Aerobic granular sludge (AGS), which was aggregated by variety of microorganisms and had strong resistance to toxic substance due to its special spherical structure, was a promising sewage bio-technique. Therefore, the impacts of MPs on AGS and activated sludge might be different. Dai et al. (2020) discovered that MPs (PVC) inhibit the abundance of ammonia-oxidizing bacteria (AOB) and promote nitrite-oxidizing bacteria (NOB) of AGS. To date, no study focused on the effects of different types of MPs (PVC, PA, PS and PE) on nitrification in AGS system.
Antibiotics are frequently detected in the influent of WWTPs (Michael et al., 2013), which causes a new hazard to the sewage treatment process and leads to the contamination of antibiotic resistance genes (ARGs). Tetracycline (TC) was one of the most generally used antibiotics in the world, meanwhile, environmental TC concentration (0–0.1 mg/L) was reported to enrich TC resistance genes (Zhang et al., 2019). A recent study (Zhao et al., 2021) found MPs could carry TC and promote the propagation of TC resistance genes. tetW and tetE were predominant TC resistance genes and extensively existed in sewage and sludge (Xu et al., 2016, Zhu et al., 2017). Mobile genetic elements (MGEs) might promote the spread of ARGs through the horizontal gene transfer (HGT) mechanism and pose a threat to various environments. Moreover, intI1 was strongly related to the dissemination of ARGs and was one of the most detected MGEs. Thus, it was essential to investigate the impacts of MPs on tetW, tetE and intI in AGS system.
In addition, there are two types of ARGs: intracellular ARGs (iARGs) and extracellular ARGs (eARGs) (Zhou et al., 2019, Li et al., 2020a). It was worth noting that both intracellular DNA (iDNA) and extracellular DNA (eDNA) could promote the spread of iARGs and eARGs through HGT (Zhao et al., 2019). In addition, iARGs would be released after the cells rupture, and iARGs might convert into eARGs. eARGs could exist in the external environment for a long time and continue to propagate (Dong et al., 2019). Hence, the impacts of MPs on iARGs and eARGs in AGS system should be paid more attention to. And the potential impacts of MPs on intracellular and extracellular microbial communities in AGS remain unknown. Notably, potential pathogenic bacteria and ARGs were closely associated (Wu et al., 2019), thus potential pathogenic bacteria might be the hosts of ARGs, and this would lead to the resistance of pathogenic bacteria, posing a greater threat to humans. Amaral-Zettler et al. (2020) revealed that MPs biofilms could support pathogenic bacteria. Thus, this study focused on the migration and reproduction of iARGs and eARGs in iDNA and eDNA of host bacteria especially potential pathogenic bacteria in AGS system and on MPs biofilms.
The objectives of this paper were: (I) to investigate the effects of four representative MPs (PVC, PA, PS and PA) on nitrification, oxidative stress and extracellular polymeric substances (EPS) in AGS under TC stress; (II) to evaluate the shifts in the abundance of iARGs and eARGs in AGS system and on MPs biofilms; (III) to compare the variations of bacterial community structure in iDNA and eDNA of AGS with MPs dosed; (4) to reveal the microbial hosts of eARGs and iARGs in AGS system and on MPs biofilms. This study would provide a new perspective to explore the impacts of MPs on AGS.
Section snippets
Reactor set-up
AGS was collected from an 18 L sequencing batch reactor (SBR) (mixed liquor suspended solids (MLSS): 4120 ± 52 mg/L), which adapted the influence of 0.1 mg/L TC during 28 days (Text S1). The AGS adapted to TC selective pressure was equally divided into five SBRs, and the initial MLSS concentrations of five SBRs were 4008 ± 25 mg/L. Five identical SBRs, which were column-type unit (14 cm internal diameter and 21 cm effective height) with working volume of 3 L, were set up to investigate the
Performance of nitrification in the AGS system under MPs stress
The influences of MPs on the conversion performance of NH4+-N in the AGS system were compared. As shown in Fig. 1b, stable ammonia removal efficiency (ARE) (99.60%) was observed in SBR-CK, and the main removal product of NH4+-N was NO2−-N. Compared with SBR-CK, it was found that adding 1 mg/L of MPs did not affect the removal efficiency of NH4+-N (Fig. 1c–f) and COD (Fig. S1c–f). However, compared with SBR-CK, the dose of PVC and PA caused the NO2−-N accumulation rate (NAR) drop from 89.71% to
Conclusions
PVC, PA, PS and PE led partial nitrification to losing stability. Furthermore, PVC, PA and PS facilitated AGS to produce more EPS and ROS, and led to AGS bulking. However, PE had little effect on the performance of AGS. 1, 10 and 100 mg/L MPs promoted the propagation of iARGs and eARGs in AGS, especially tetE in eDNA. Potential pathogenic bacteria might be the hosts of eARGs and iARGS in AGS system. Moreover, the introduction of PVC, PA and PS reduced the stability of ecological function in
CRediT authorship contribution statement
All authors have seen the manuscript and approved to submit to your journal.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We would like to thank the Beijing Science and Technology Planning Project (Z181100005518002), Natural Science Foundation of Beijing Municipality (8202006), National Science and Technology Major Project (2017ZX07103-003) and China Postdoctoral Science Foundation (2020M680279) for the financial supports of this study.
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