Microplastics exhibit accumulation and horizontal transfer of antibiotic resistance genes.

Although the fates of microplastics (0.1-5 mm) in marine environments and freshwater are increasingly studied, little is known about their vector effect in wastewater treatment plants (WWTPs). Previous studies have evaluated the accumulation of antibiotic resistance genes (ARGs) on microplastics, but there is no direct evidence for the selection and horizontal transfer of ARGs on different microplastics in WWTPs. Here, we show biofilm formation as well as bacterial community and ARGs in these biofilms grown on four kinds of microplastics via incubation in the aerobic and anaerobic tanks of a WWTP. Microplastics showed differential capacities for bacteria and ARGs enrichment, differing from those of the culture environment. Furthermore, ARGs in microplastic biofilms were horizontally transferred at frequencies higher than those in water samples in both tanks. Therefore, microplastics in WWTPs can act as substrates for horizontal transfer of ARGs, potentially causing a great harm to the ecological environment and adversely affecting human health.

Do Polystyrene Nanoplastics Have Similar Effects on Duckweed (Lemna minor L.) at Environmentally Relevant and Observed-Effect Concentrations?

Although the biological effects of nanoplastics (<100 nm in size) in aquatic environments have been increasingly investigated, almost all such studies have been performed at observed-effect concentrations (higher than 1 µg/mL). The use of observed-effect concentrations of nanoplastics can provide essential data for evaluating the potential risks, but how these results apply to the effects of concentrations of nanoplastics observed in the environment remains unclear. Here, we show that exposure to both positively and negatively charged nanoplastics at the observed-effect concentration (ranging from 0 to 50 µg/mL) can result in physiological changes of Lemna minor L., a typical flowering aquatic plant species, inducing H2O2 and O2- accumulation and even cell death. However, the nanoplastics at environmentally relevant concentrations (lower than 0.1 µg/mL) had no obvious effects on phenotype of L. minor. Moreover, nanoplastics at both observed-effect and environmentally relevant concentrations were adsorbed onto the roots and fronds of the plants, whereas uptake by the roots and fronds occurred only at the observed-effect concentration. Although no phenotypic changes across 30 generations of cultivation were observed when the plants were exposed to 0.015 µg/mL nanoplastics, the expression of genes related to the response to stimuli and to oxidative and osmotic stress was upregulated under both observed-effect and environmentally relevant concentrations. Our findings suggest that the long-term presence of nanoplastics at environmentally relevant concentrations might induce some variations in the transcription level and have potential threat to floating microphytes and aquatic ecosystems.

Nitrogen Forms Regulate the Response of Microcystis aeruginosa to Nanoplastics at Environmentally Relevant Nitrogen Concentrations.

As essential primary producers, cyanobacteria play a major role in global carbon and nitrogen cycles. Though the influence of nanoplastics on the carbon metabolism of cyanobacteria is well-studied, little is known about how nanoplastics affect their nitrogen metabolism, especially under environmentally relevant nitrogen concentrations. Here, we show that nitrogen forms regulated growth inhibition, nitrogen consumption, and the synthesis and release of microcystin (MC) in Microcystis aeruginosa exposed to 10 µg/mL amino-modified polystyrene nanoplastics (PS-NH2) with a particle size of 50 nm under environmentally relevant nitrogen concentrations of nitrate, ammonium, and urea. We demonstrate that PS-NH2 inhibit M. aeruginosa differently in nitrate, urea, and ammonium, with inhibition rates of 51.87, 39.70, and 36.69%, respectively. It is caused through the differences in impairing cell membrane integrity, disrupting redox homeostasis, and varying nitrogen transport pathways under different nitrogen forms. M. aeruginosa respond to exposure of PS-NH2 by utilizing additional nitrogen to boost the production of amino acids, thereby enhancing the synthesis of MC, extracellular polymeric substances, and membrane phospholipids. Our results found that the threat of nanoplastics on primary producers can be regulated by the nitrogen forms in freshwater ecosystems, contributing to a better understanding of nanoplastic risks under environmentally relevant conditions.

Using colloidal AFM probe technique and XDLVO theory to predict the transport of nanoplastics in porous media.

The plastic concentration in terrestrial systems is orders of magnitude higher than that found in marine ecosystems, which has raised global concerns about their potential risk to agricultural sustainability. Previous research on the transport of nanoplastics in soil relied heavily on the qualitative prediction of the mean-field extended Derjaguin-Landau-Verwey-Overbeek theory (XDLVO), but direct and quantitative measurements of the interfacial forces between single nanoplastics and porous media are lacking. In this study, we conducted multiscale investigations ranging from column transport experiments to single particle measurements. The maximum effluent concentration (C/C0) of amino-modified nanoplastics (PS-NH2) was 0.94, whereas that of the carboxyl-modified nanoplastics (PS-COOH) was only 0.33, indicating PS-NH2 were more mobile than PS-COOH at different ionic strengths (1-50 mM) and pH values (5-9). This phenomenon was mainly attributed to the homogeneous aggregation of PS-COOH. In addition, the transport of PS-NH2 in the quartz sand column was inhibited with the increase of ionic strength and pH, and pH was the major factor governing their mobility. The transport of PS-COOH was inhibited with increasing ionic strength and decreasing pH. Hydrophilicity/hydrophobicity-mediated interactions and particle heterogeneity strongly interfered with interfacial forces, leading to the qualitative prediction of XDLVO, contrary to experimental observations. Through the combination of XDLVO and colloidal atomic force microscopy, accurate and quantitative interfacial forces can provide compelling insight into the fate of nanoparticles in the soil environment.

The surface groups of polystyrene nanoparticles control their interaction with the methanogenic archaeon Methanosarcina acetivorans.

A better understanding of the interaction between nanoplastics and archaea is crucial to fill the knowledge gaps regarding the ecological safety of nanoplastics. As a vital source for global methane emissions, methanogenic archaea have unique cell membranes that are distinctly different from those in all other forms of life, little is known about their interaction with nanoplastics. Here, we show that polystyrene nanoparticles functionalized with sulfonic acid (PS-SO3H) and amino (PS-NH2) interact with this methanogenic archaeon in distinct ways. Although both of them have no significant phenotype effects on Methanosarcina acetivorans C2A, these nanoparticles could affect DNA-mediated transposition of this methanogenic archaeon, and PS-SO3H also downregulated nitrogen fixation, nitrogen cycle metabolic process, oxidoreductase activity, etc. In addition, both nanoplastics decreased the protein contents in the extracellular polymer substances (EPS), with distinct binding sequences to the functional groups of the EPS. The single particle atomic force microscopy revealed that the force between the amino group and the M. acetivorans C2A was greater than that of sulfonic acid group. Our results exhibit that the surface groups of polystyrene nanoparticles control their risk on the methanogenic archaea, and these effects might influence their contribution on global methane emission.

Insights on the inhibition of anaerobic digestion performances under short-term exposure of metal-doped nanoplastics via Methanosarcina acetivorans.

Anaerobic digestion is an attractive waste treatment technology, achieving both pollution control and energy recovery. Though the inhibition of polystyrene nanoplastics in anaerobic granular sludge is well studied, no direct evidence has been found on the interaction of methanogens and nanoplastics. In this study, to characterize the location of nanoplastics, Pd-doped polystyrene nanoplastics (Pd-PS) were used to explore the inhibition mechanism of anaerobic sludge through short-term exposure to Methanosarcina acetivorans C2A. The results showed that Pd-PS inhibited the methanogenesis of the anaerobic sludge, and the methane production decreased as the Pd-PS increased, with a 14.29% reduction at the Pd-PS concentration of 2.36 × 1010 particles/mL. Also, Pd-PS interacted with the protein in the extracellular polymeric substances (EPS). Furthermore, Pd-PS inhibited the methanogenesis of M. acetivorans C2A without exhibiting an evident reduction in the growth. The inhibition of Pd-PS on methane was due to the inhibition of methane production related genes, MtaA and mcrA. These results provide potential explication for the inhibition of nanoplastics on the methanogens, which will fulfill the knowledge on the stability of methanogens under the short-term exposure of nanoplastics.

Differentially charged nanoplastics demonstrate distinct accumulation in Arabidopsis thaliana.

Although the fates of microplastics (0.1-5 mm in size) and nanoplastics (<100 nm) in marine environments are being increasingly well studied1,2, little is known about the behaviour of nanoplastics in terrestrial environments3-6, especially agricultural soils7. Previous studies have evaluated the consequences of nanoplastic accumulation in aquatic plants, but there is no direct evidence for the internalization of nanoplastics in terrestrial plants. Here, we show that both positively and negatively charged nanoplastics can accumulate in Arabidopsis thaliana. The aggregation promoted by the growth medium and root exudates limited the uptake of amino-modified polystyrene nanoplastics with positive surface charges. Thus, positively charged nanoplastics accumulated at relatively low levels in the root tips, but these nanoplastics induced a higher accumulation of reactive oxygen species and inhibited plant growth and seedling development more strongly than negatively charged sulfonic-acid-modified nanoplastics. By contrast, the negatively charged nanoplastics were observed frequently in the apoplast and xylem. Our findings provide direct evidence that nanoplastics can accumulate in plants, depending on their surface charge. Plant accumulation of nanoplastics can have both direct ecological effects and implications for agricultural sustainability and food safety.

Behavior of tetracycline and polystyrene nanoparticles in estuaries and their joint toxicity on marine microalgae Skeletonema costatum.

Polystyrene nanoplastics (PS NPs), which are newly emerging as particulate pollutants, are one of the most abundant plastic types in marine debris. Although there has been extensive research on microplastics, the sorption behavior of PS NPs in surface waters remains unknown. In addition, in the previous joint toxicity studies, the concentration of organic pollutant in the joint system was based on the EC50 of this pollutant, rather than the actually amount of this pollutant adsorbed on nanoplastics (NPs). In this study, the sorption behavior of PS NPs with different surface charges in the surface water of estuaries and joint toxicity of that absorbed tetracycline antibiotic in equilibrium were investigated for the first time. Because of the electrostatic repulsion, salting-out effect, and partition function, the sorption capacity of tetracycline antibiotic by differently charged PS NPs was enhanced with increasing salinity. The biological effects of exposure to tetracycline-saturated PS NPs were complicated, which can be attributed to the surface characteristics of mixtures such as hydrophobicity and charges. Thus, the role of NPs in the natural environment as a carrier of antibiotics may provide an alternative for antibiotic inputs from inland water to coastal marine water, which would not only change the environmental fate and ecotoxicology of antibiotics and NPs, but also pose challenges to the safety of coastal aquaculture and marine ecosystem.

Role of extracellular polymeric substances in the acute inhibition of activated sludge by polystyrene nanoparticles.

Microplastics and nanoplastics in aquatic systems have become a global concern because of their persistence and adverse consequences to ecosystems and potentially human health. Though wastewater treatment plants (WWTPs) are considered a potential source of microplastics in the environment, the role of extracellular polymeric substances (EPS) of activated sludge on the fate of nanoplastics is not clear. In this study, the role of EPS in the influence of polystyrene nanoparticles (PS-NPs) on the endogenous respiration of activated sludge was investigated for the first time. The results showed that the acute inhibition of activated sludge by PS-NPs was enhanced with increasing PS-NPs concentration. X-ray photoelectron spectroscopy (XPS) results indicate that the functional groups involved in the interactions between PS-NPs and EPS were carbonyl and amide groups and the side chains of lipids or amino acids. Furthermore, the Fourier transform infrared (FTIR) spectroscopy results show that the protein secondary structures in EPS were changed by PS-NPs and lead to the bioflocculation of activated sludge, which provides a better understanding on the fate of nanoplastics in WWTPs.