High-Resolution Mass Spectrometry Combined with Reactive Oxygen Species Reveals Differences in Photoreactivity of Dissolved Organic Matter from Microplastic Sources in Aqueous Environments.

Microplastics (MPs)-derived dissolved organic matter (MPs-DOM) is becoming a non-negligible source of DOM pools in aquatic systems, but there is limited understanding about the photoreactivity of different MPs-DOM. Herein, MPs-DOM from polystyrene (PS), polyethylene terephthalate (PET), poly(butylene adipate-co-terephthalate) (PBAT), PE, and polypropylene (PP), representing aromatic, biodegradable, and aliphatic plastics, were prepared to examine their photoreactivity. Spectral and high-resolution mass spectrometry analyses revealed that PS/PET/PBAT-DOM contained more unsaturated aromatic components, whereas PE/PP-DOM was richer in saturated aliphatic components. Photodegradation experiments observed that unsaturated aromatic molecules were prone to be degraded compared to saturated aliphatic molecules, leading to a higher degradation of PS/PET/PBAT-DOM than PE/PP-DOM. PS/PET/PBAT-DOM was mainly degraded by hydroxyl (•OH) via attacking unsaturated aromatic structures, whereas PE/PP-DOM by singlet oxygen (1O2) through oxidizing aliphatic side chains. The [•OH]ss was 1.21-1.60 × 10-4 M in PS/PET/PBAT-DOM and 0.97-1.14 × 10-4 M in PE/PP-DOM, while the [1O2]ss was 0.90-1.35 × 10-12 and 0.33-0.44 × 10-12 M, respectively. This contributes to the stronger photoreactivity of PS/PET/PBAT-DOM with a higher unsaturated aromatic degree than PE/PP-DOM. The photodegradation of MPs-DOM reflected a decreasing tendency from aromatic-unsaturated molecules to aliphatic-saturated molecules. Special attention should be paid to the photoreactivity and environmental impacts associated with MPs-DOM containing highly unsaturated aromatic compounds.

Molecular Weights of Dissolved Organic Matter Significantly Affect Photoaging of Microplastics.

The fate of ubiquitous microplastics (MPs) is largely influenced by dissolved organic matter (DOM) in aquatic environments, which has garnered significant attention. The reactivity of DOM is reported to be greatly regulated by molecular weights (MWs), yet little is known about the effects of different MW DOM on MP aging. Here, the aging behavior of polystyrene MPs (PSMPs) in the presence of different MW fulvic acids (FAs) and humic acids (HAs) was systematically investigated. Under ultraviolet (UV) illumination, O/C of PSMPs aged for 96 h surged from 0.008 to 0.146 in the lower MW FA (FA<1kDa) treatment, suggesting significant PSMP aging. However, FA exhibited a stronger effect on facilitating PSMP photoaging than HA, which can be attributed to the fact that FA<1kDa contains more quinone and phenolic moieties, demonstrating a higher redox capacity. Meanwhile, compared to other fractions, FA<1kDa was more actively involved in the increase of different reactive species yields by 50-290%, including •OH, which plays a key role in PSMP photoaging, and contributed to a 25% increase in electron-donating capacity (EDC). This study lays a theoretical foundation for a better understanding of the environmental fate of MPs.

Interactions between Iron Minerals and Dissolved Organic Matter Derived from Microplastics Inhibited the Ferrihydrite Transformation as Revealed at the Molecular Scale.

Microplastic-derived dissolved organic matter (MP-DOM) is an emerging carbon source in the environment. Interactions between MP-DOM and iron minerals alter the transformation of ferrihydrite (Fh) as well as the distribution and fate of MP-DOM. However, these interactions and their effects on both two components are not fully elucidated. In this study, we selected three types of MP-DOM as model substances and utilized Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and extended X-ray absorption fine structure (EXAFS) spectroscopy to characterize the structural features of DOMs and DOM-mineral complexes at the molecular and atomic levels. Our results suggest that carboxyl and hydroxyl groups in MP-DOM increased the Fe-O bond length by 0.02-0.03 Å through interacting with Fe atoms in the first shell, thereby inhibiting the transformation of Fh to hematite (Hm). The most significant inhibition of Fh transformation was found in PS-DOM, followed by PBAT-DOM and PE-DOM. MP-DOM components, such as phenolic compounds and condensed polycyclic aromatics (MW > 360 Da) with high oxygen content and high unsaturation, exhibited stronger mineral adsorption affinity. These findings provide a profound theoretical basis for accurately predicting the behavior and fate of iron minerals as well as MP-DOM in complex natural environments.

Microplastics in Qinghai-Tibet Plateau and Yunnan-Guizhou Plateau lakes, China.

As an emerging environmental pollutant, microplastics (MPs) have received widespread attention. Recently, studies examining microplastic pollution in plateau lakes have been increasing, but few have examined the distributions, sources, and fates of MPs in different plateau areas. In this work, the abundances and characteristics of MPs in surface waters and sediments in lakes of the Qinghai-Tibet Plateau (QTP) and Yunnan-Guizhou Plateau (YGP) were systematically investigated. The abundances of MPs in the lakes of the QTP ranges within 0.05-1.8 n/L in surface waters and 10-2643.7 n/kg in sediments. In the lakes of the YGP, the abundances of MPs ranged within 1.3-10.1 n/L in surface waters and 171.7-4260 n/kg in sediments. The dominant shape, color, and size class of MPs were fiber, transparent, and 0-0.5 mm in plateau lakes, respectively. MPs were mainly composed of polypropylene, polyethylene, and polyethylene terephthalate polymers. The different sources of MPs in the QTP and YGP lakes were mainly due to differences in human activities. The primary sources of microplastic pollution in the lakes of the QTP were tourism and atmospheric transport, while sewage discharge, agriculture, and fishing activities were the main sources of MPs in urban lakes of the YGP. Although the level of microplastic pollution in plateau lakes was relatively low, the sources should be identified and monitored so that the effects and extent of microplastic pollution in these fragile environments can be fully understood. This study provides a valuable dataset and theoretical basis for subsequent research on microplastic pollution in plateau lakes.

Insights into the photoreactivity mechanisms of micro-sized rubber particles with different structure: The crucial role of reactive oxygen species and released dissolved organic matter.

Micro-sized rubber particles (MRPs), as a significant component of tire wear particles (TWPs), increasingly garnered attention due to the potential ecological risks. However, the impact of photoaging of MRPs and the characteristics of the dissolved organic matter (DOM) derived from MRPs on the photoreactivity of co-existing pollutants is remain unclear. To bridge this knowledge gap, this study selected MRPs with different structure including butadiene rubber (BR), styrene butadiene rubber (SBR) and nitrile butadiene rubber (NBR) and took tetracycline (TC) as the target pollutant to firstly study potential effects of structural characteristics and active components of MRPs on TC photodegradation process under simulated sunlight irradiation. The results indicated that BR, NBR and SBR enhanced TC photodegradation to varying extents, with SBR having the most pronounced effect. This effect was attributed mainly to the high electron transport capacity and the generation of more triple excited DOM (3DOM*) of SBR, thereby producing more active species (•OH and 1O2) and significantly promoting TC photodegradation. Additionally, the unsaturated bonds and aromatic groups in MRPs-DOM was identified as another crucial factor influencing their photoreactivity. This study will provide a new perspective for understanding the potential ecological effects between MRPs and co-existing pollutants in the natural environment.

Significant contribution of different sources of particulate organic matter to the photoaging of microplastics.

Particulate organic matter (POM), as an important component of organic matter, can act as a redox mediator and thus intervene in the environmental behavior of microplastics (MPs). However, quantitative information on the role of POM in the photoaging of MPs under ultraviolet (UV) light is still lacking. To raise the knowledge gap, through environmental simulation experiments and qualitative/quantitative experiments of active substances, we found that POM from peat soil has stronger oxidation capacity than POM from sediment, and the involvement of POM at high water content makes the aging of MPs more obvious. This is because the persistent radicals and electron-absorbing groups on the surface of POM indirectly generate reactive oxygen species (ROS) by promoting electron transfer, and the dissolved organic matter (DOM) released from POM under UV light (POM-DOM) is further excited to generate triplet-state photochemistry of DOM (3DOM*) to promote the aging of MPs. Theoretical calculations revealed that the benzene ring, mainly C = C, and C = O in the main chain in the plastic macromolecule structure are more susceptible to ROS attack, and the differences in the vulnerable sites contained in different plastic structures as well as the differences in the energy band gaps lead to differences in their aging processes. This study firstly elucidates the key role and intrinsic mechanism of POM in the photoaging of MPs, providing a theoretical basis for a comprehensive assessment of the effect of POM on MPs in the environment.

Novel insights into the relationship between the functional groups and photoactivity of biochar-derived dissolved organic matter.

Due to the production of a large amount of biochar, highly photoactive biochar-derived dissolved organic matter (BDOM) from different sources is released into surface water. This study investigated the molecular composition of BDOM (sludge, bamboo and stalk BDOM) using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and used tetracycline (TC) as model pollutant to investigate the relationship between molecular composition and BDOM photoactivity, specifically focusing on reactive oxygen species (ROS) production. The results indicate that the fluorescence signal intensity of humic acid-like and aromatic proteins in the plant-derived BDOM are significantly higher than that derived from sewage sludge. FT-ICR MS results also showed that plant-derived BDOM contained more CHO molecular formula. Photodegradation experiments of TC mediated by various BDOM analogues demonstrated the photoactivity is highly correlated with the components and functional groups. The electrochemical experiments and density functional theory (DFT) calculations further verified that the aromatic moiety, sulfydryl group and amino group of BDOM affected the electronic supply and energy transfer. Higher electron and energy transfer favor the reaction of BDOM with the ground state oxygen to generate ROS, thus promoting photodegradation of TC. This study provides a new basis for better assessing the ecological risks of BDOM.

Internalization, physiological responses and molecular mechanisms of lettuce to polystyrene microplastics of different sizes: Validation of simulated soilless culture.

Microplastics (MPs) exists widely in the environment, and the resulting pollution of MPs has become a global environmental problem. Plants can absorb MPs through their roots. However, studies on the mechanism of the effect of root exposure to different size MPs on vegetables are limited. Here, we use Polystyrene (PS) MPs with different particle sizes to investigate the internalization, physiological response and molecular mechanism of lettuce to MPs. MPs may accumulate in large amounts in lettuce roots and migrate to the aboveground part through the vascular bundle, while small particle size MPs (SMPs, 100 nm) have stronger translocation ability than large particle size MPs (LMPs, 500 nm). MPs can cause physiological and biochemical responses and transcriptome changes in lettuce. SMPs and LMPs resulted in reduced biomass (38.27 % and 48.22 % reduction in fresh weight); caused oxidative stress (59.33 % and 47.74 % upregulation of SOD activity in roots) and differential gene expression (605 and 907 DEGs). Signal transduction, membrane transport and alteration of synthetic and metabolic pathways may be the main causes of physiological toxicity of lettuce. Our study provides important information for understanding the behavior and fate of MPs in edible vegetables, especially the physiological toxicity of MPs to edible vegetables, in order to assess the potential threat of MPs to food safety and agricultural sustainable development.

Identification and visualization of environmental microplastics by Raman imaging based on hyperspectral unmixing coupled machine learning.

Microplastics (MPs) are ubiquitous contaminants that have become an emerging pollutant of concern, potentially threatening human health and ecosystem environments. Although current detection methods can accurately identify various types of MPs, it remains necessary to develop non-destructive and rapid methods to meet growing demands for detection. Herein, we combine a hyperspectral unmixing method and machine learning to analyse Raman imaging data of environmental MPs. Five MPs types including poly(butylene adipate-co-terephthalate) (PBAT), poly(butylene succinate) (PBS), p-polyethylene (PE), polystyrene (PS) and polypropylene (PP) were visualized and identified. Individual or mixed pure or aged MPs along with environmental samples were analysed by Raman imaging. Alternating volume maximization (AVmax) combined with unconstrained least squares (UCLS) method estimated end members and abundance maps of each of the MPs in the samples. Pearson correlation coefficients (r) were used as the evaluation index; the results showed that there is a high similarity between the raw spectra and the average spectra calculated by AVmax. This indicates that Raman imaging based on machine learning and hyperspectral unmixing is a novel imaging analysis method that can directly identify and visualize MPs in the environment.

Size-dependent vector effects of microplastics on bioaccumulation of hydrophobic organic contaminants in earthworm: A dual-dosing study.

The potential of microplastics to act as a vector for anthropogenic contaminants is of rising concern. However, directly quantitatively determining the vector effects of microplastics has been rarely studied. Here, we present a dual-dosing method that simulates the chemical bioaccumulation from soil and microplastics simultaneously, wherein unlabeled hydrophobic organic contaminants (HOCs) were spiked in the soil and their respective isotope-labeled reference compounds were spiked on the polyethylene microplastics. The comparison of the bioavailability, i.e., the freely dissolved concentration in soil porewater and bioaccumulation by earthworm, between the unlabeled and isotope-labeled HOCs was carried out. Relatively higher level of bioavailability of the isotope-labeled HOCs was observed compared to the unlabeled HOCs, which may be attributed to the irreversible desorption of HOCs from soil particles. The average relative fractions of bioaccumulated isotope-labeled HOCs in the soil treated with 1 % microplastics ranged from 6.9 % to 46.4 %, which were higher than those in the soil treated with 0.1 % microplastics. Treatments with the smallest microplastic particles were observed to have the highest relative fractions of bioaccumulated isotope-labeled HOCs, with the exception of phenanthrene, suggesting greater vector effects of smaller microplastic particles. Biodynamic model analysis indicated that the contribution of dermal uptake to the bioaccumulation of isotope-labeled HOCs was higher than that for unlabeled HOCs. This proposed method can be used as a tool to assess the prospective vector effects of microplastics in complex environmental conditions and would enhance the comprehensive understanding of the microplastic vector effects for HOC bioaccumulation.

Insight into interactions between microplastics and fulvic acid: Mechanisms affected by microplastics type.

Microplastics (MPs) can interact with dissolved organic matter (DOM), a common component found in the environment. However, the effect of MPs type on its interaction with DOM has not been systematically studied. Therefore, the binding properties of different MPs with fulvic acid (FA) were explored in this study. The results showed that polypropylene (PP) and polyethylene (PE) had higher adsorption affinity for FA than polystyrene (PS) and polyvinyl chloride (PVC). The interaction between MPs and FA conformed to the pseudo-first-order model and Freundlich model (except PS). The interaction mechanisms between various MPs tested in this paper and FA are considered to be different. PP, PE and PS interacted with the aromatic structure of FA and were entrapped in the FA polymers by the carboxyl groups and CO bonds, resulting in a highly conjugated co-polymer, suggesting that oxygen-containing functional groups played a key role. However, it was assumed that the interaction between PVC and FA was more likely to be caused by hydrophobic interaction. This research will help to enhance our comprehension of the environmental behavior of MPs and their interaction with the DOM specifically.

Metagenomic insights into ecological risk of antibiotic resistome and mobilome in riverine plastisphere under impact of urbanization.

Microplastics (MPs) are of increasing concern due to their role as reservoirs for antibiotic resistance genes (ARGs) and pathogens. To date, few studies have explored the influence of anthropogenic activities on ARGs and mobile genetic elements (MGEs) within various riverine MPs, in comparison to their natural counterparts. Here an in-situ incubation was conducted along heavily anthropogenically-impacted Houxi River to characterize the geographical pattern of antibiotic resistome, mobilome and pathogens inhabiting MPs- and leaf-biofilms. The metagenomics result showed a clear urbanization-driven profile in the distribution of ARGs, MGEs and pathogens, with their abundances sharply increasing 4.77 to 19.90 times from sparsely to densely populated regions. The significant correlation between human fecal marker crAssphage and ARG (R2 = 0.67, P=0.003) indicated the influence of anthropogenic activity on ARG proliferation in plastisphere and natural leaf surfaces. And mantel tests and random forest analysis revealed the impact of 17 socio-environmental factors, e.g., population density, antibiotic concentrations, and pore volume of materials, on the dissemination of ARGs. Partial least squares-path modeling further unveiled that intensifying human activities not only directly boosted ARGs abundance but also exerted a comparable indirect impact on ARGs propagation. Furthermore, the polyvinylchloride plastisphere created a pathogen-friendly habitat, harboring higher abundances of ARGs and MGEs, while polylactic acid are not likely to serve as vectors for pathogens in river, with a lower resistome risk score than that in leaf-biofilms. This study highlights the diverse ecological risks associated with the dissemination of ARGs and pathogens in varied MPs, offering insights for the policymaking of usage and control of plastics within urbanization.

Metabolomics reveals how spinach plants reprogram metabolites to cope with intense stress responses induced by photoaged polystyrene nanoplastics (PSNPs).

While land-based sources have been recognized as significant long-term sinks for micro- and nanoplastics, there is limited knowledge about the uptake, translocation, and phytotoxicity of nanoplastics (NPs) in terrestrial environments, especially aged NPs. In this study, we investigated the impact of aged polystyrene nanoplastics (PSNPs) on the uptake, physiology, and metabolism of spinach. Our findings revealed that both pristine and aged PSNPs can accumulate in the roots and subsequently translocate to the aboveground tissues, thereby influencing numerous key growth indicators in spinach plants. A more pronounced impact was observed in the treatment of aged PSNPs, triggering more significant and extensive changes in metabolite levels. Furthermore, alterations in targeted pathways, specifically aminoacyl-tRNA biosynthesis and phenylpropanoid biosynthesis, were induced by aged PSNPs, while pristine PSNPs influenced pathways related to sulfur metabolism, biosynthesis of unsaturated fatty acids, and tryptophan metabolism. Additionally, tissue-specific responses were observed at the metabolomics level in both roots and leaves. These results highlight the existence of diverse and tissue-specific metabolic responses in spinach plants exposed to pristine and aged PSNPs, providing insights into the mechanisms of defense and detoxification against NP-induced stress.

Microbial metabolism influences microplastic perturbation of dissolved organic matter in agricultural soils.

An estimated 258 million tons of plastic enter the soil annually. Joining persistent types of microplastic (MP), there will be an increasing demand for biodegradable plastics. There are still many unknowns about plastic pollution by either type, and one large gap is the fate and composition of dissolved organic matter (DOM) released from MPs as well as how they interact with soil microbiomes in agricultural systems. In this study, polyethylene MPs, photoaged to different degrees, and virgin polylactic acid MPs were added to agricultural soil at different levels and incubated for 100 days to address this knowledge gap. We find that, upon MP addition, labile components of low aromaticity were degraded and transformed, resulting in increased aromaticity and oxidation degree, reduced molecular diversity, and changed nitrogen and sulfur contents of soil DOM. Terephthalate, acetate, oxalate, and L-lactate in DOM released by polylactic acid MPs and 4-nitrophenol, propanoate, and nitrate in DOM released by polyethylene MPs were the major molecules available to the soil microbiomes. The bacteria involved in the metabolism of DOM released by MPs are mainly concentrated in Proteobacteria, Actinobacteriota, and Bacteroidota, and fungi are mainly in Ascomycota and Basidiomycota. Our study provides an in-depth understanding of the microbial transformation of DOM released by MPs and its effects of DOM evolution in agricultural soils.