Toxicity of substituted p-phenylenediamine antioxidants and their derived novel quinones on aquatic bacterium: Acute effects and mechanistic insights

A Nationwide Investigation of Substituted p-Phenylenediamines (PPDs) and PPD-Quinones in the Riverine Waters of China

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) has been identified as the cause of the "urban runoff mortality syndrome." Thus, the ecological risks of substituted p-phenylenediamines (PPDs) and their quinone derivatives (PPD-Qs) in water have gained global attention. However, large-scale observation of their pollution characteristics in surface water is still lacking. Herein, a nationwide investigation revealed the pervasive occurrence of PPDs and PPD-Qs in riverine waters across China, with the mean concentration of ∑5PPD-Qs being 4.9 times higher than their parent ∑5PPDs. Notably, the 6PPD-Q concentrations at eight sampling sites exceeded the median lethal concentration for coho salmon. National annual riverine fluxes were estimated at 113.1 and 276.2 tonnes/year for PPDs and PPD-Qs, respectively, with the Yangtze River contributing more than one-third of the total fluxes. The transformation of PPDs to PPD-Qs was dependent on atmospheric hydroxylation rates and the half-lives of PPDs. A combined multimedia exposure assessment revealed that water exposure accounted for 82.5% of human exposure to PPDs and PPD-Qs, surpassing the contributions from dust and air exposure. This study provides a comprehensive spatial picture of PPDs and PPD-Qs in China. The national atlas highlights their potential ecological risks and implies that targeted actions should be taken to mitigate potential exposure to PPDs and PPD-Qs.

"Tire plastisphere" in aquatic ecosystems: Biofilms colonizing on tire particles exhibiting a distinct community structure and assembly compared to conventional plastisphere

Tire particles (TPs) significantly contribute to microplastics in aquatic ecosystems, which has recently attracted ecological concerns worldwide. Numerous studies have shown that biofilms on microplastics harbor unique species and harmful functions, but it remains unclear whether TPs could offer distinct niches for biofilms compared to conventional microplastics (CP). This study investigated the succession and assembly of biofilms on TPs compared with CP over 60 days. Our results showed the community structures of biofilms on TPs and CP were distinct. Intriguingly, a greater structural dissimilarity was observed between TPs-associated communities and natural biofilms compared to that between CP-associated communities and natural biofilms. This dissimilarity became more pronounced as biofilms progressed through succession. Furthermore, the bacterial community on the TPs exhibits a network of greater complexity, more stable structure, and higher activity than that on the CP, but the pattern was reversed in the eukaryotic community. Deterministic processes had a more critical impact on bacterial communities on TPs, whereas distinct stochastic processes controlled eukaryotic communities on TPs (dispersal limitation) and CP (undominated processes). Altogether, this study tentatively introduced the term "tire plastisphere" (i.e., TP-attached biofilms), emphasizing TPs could serve as more artificial microbial habitats and pose potential risks in disturbing aquatic ecology.

Insights into the impact of 6PPD-Q and 6PPD on nitrogen metabolism and microbial community in the anammox system

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is an antioxidant commonly used in tire manufacturing, and its release into the environment has significantly increased due to rapid urbanization. When subjected to ozonation, 6PPD converts into the harmful pollutant 6PPD quinone (6PPDQ). These substances enter wastewater treatment plants (WWTPs) via stormwater runoff and pipelines, posing significant risks to the functional microorganisms. Anammox, a strictly controlled and sensitive microbial nitrogen removal process, is especially susceptible to the effects of the pollutants. This study investigates the comprehensive impact of 6PPD-Q and 6PPD on anammox communities based on characterization analysis and metagenomics. At environmental concentrations, 6PPD-Q at 200 ng/L-1000 ng/L led to the disintegration of anammox granules. Extended exposure to both 6PPD-Q and 6PPD significantly reduces the population of anammox bacteria (AnAOB). By utilizing organic matter from dead cells and incoming carbonate as a carbon source, the system evolved into a nitrogen metabolism network primarily focused on denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This transformation was accompanied by a reshuffling of the microbial community and associated genes, resulting in an accumulation of NH4+-N. These findings underscore the toxicity of 6PPD-Q and 6PPD to anammox and stress the importance of incorporating 6PPD into regulatory and preventive strategies.

Development and Application of a Comprehensive Non-targeted Screening Strategy for Fentanyl Analogues

The growing abuse of fentanyl and its analogues (FTNs) presents a substantial public health threat, prompting the introduction of regulatory controls by government authorities. Nevertheless, existing screening strategies for FTNs are primarily based on targeted or non-targeted approaches that utilize a limited set of mass spectrometry fragmentation data, which are far from meeting the needs of class scheduling. In this study, a comprehensive non-targeted screening strategy for FTNs was developed. Firstly, existing and potential unknown structures were obtained and a database was created by combining the substitution patterns and the characteristic fragmentation routes (CFRs). Following this, a preliminary screening was executed by matching entries from the database with empirical data to identify CFRs and suggest candidate structures. The likelihood of an analyte being an FTNs increased with the identification of multiple CFRs. After that, a final judgment was made based on the similarity of the isotope distribution of the candidate structures and the spectra obtained, along with the evaluation of chromatographic peak profile. This proposed strategy could achieve an extensive coverage of 1.25 × 1015. To facilitate the establishment and implementation of the strategy, a computer-aided program named FTNs_Finder was developed. To validate this method, false negative and false positive assessments were performed using the MS/MS data from 29 FTNs and 5180 datasets from MassBank. In addition, it was further applied to the presence screening of FTNs in environmental water samples. Overall, this work could effectively promote the class scheduling of FTNs and provide a reference for the design and implementation of screening strategies for other hazardous substances.

Hydrolysis of p-Phenylenediamine Antioxidants: The Reaction Mechanism, Prediction Model, and Potential Impact on Aquatic Toxicity

While p-phenylenediamine antioxidants (PPDs) pose potential risks to aquatic ecosystems, their environmental persistence and transformation remain ambiguous due to the undefined nature of PPD C-N bond hydrolysis. Here, we investigated the hydrolysis patterns of PPDs by analyzing their hydrolysis half-lives, hydrolysis products around neutral pH (pH 6.0-7.7), and the role of atoms within the C-N bonds in PPDs. Hydrolysis preferentially targets the aromatic secondary amine N with the strongest proton affinity and the C atom of C-N with the highest nucleophilic-attack reactivity. The hydrolysis half-life (t1/2) shortens when the maximum proton affinity of N increases. These results are supported by theoretical calculations, demonstrating a hydrolysis reaction propelled by proton transfer from water to N and complemented by aromatic nucleophilic substitution of N in C-N by water hydroxyl. With the experimental results and the atom reactivity-based predictive model, the t1/2 around neutral pH for 60 PPDs (monitored in environment, commercially available, or under investigation) is determined, showing variations ranging from 2.2 h to 47 days. The model prediction of primary C-N hydrolysis is confirmed through typical PPDs. With the elucidated mechanism and developed model, this research provides new insights into PPD hydrolysis, underscoring its significance in delineating environmental impacts.

Tracking the biogeochemical behavior of tire wear particles in the environment - A review

The environmental fate and risks associated with tire wear particles (TWPs) are closely linked to their biogeochemical behaviors. However, reviews that focus on TWPs from this perspective remain scarce, hindering our understanding of their environmental fate and cascading effects on ecosystems. In this review, we summarize the existing knowledge on TWPs by addressing five key areas: (i) the generation and size-dependent distribution of TWPs; (ii) the release and transformation of TWP-leachates; (iii) methodologies for the quantification of TWPs; (iv) the toxicity of TWPs; and (v) interactions of TWPs with other environmental processes. It has been established that the size distribution of TWPs significantly influences their transport and occurrence in different matrices, leading to the release and transformation of specific TWP-chemicals that can be toxic to organisms. By highlighting the challenges and knowledge gaps in this field, we propose critical issues that need to be addressed to enhance the risk assessment of TWPs. This review aims to provide a comprehensive framework for evaluating the environmental behavior of TWPs.

Characterizing the Metabolism of Tire Rubber-Derived p-Phenylenediamine Quinones to Identify Potential Exposure Biomarkers in Humans

There is growing evidence of the frequent detection of tire rubber-derived contaminants p-phenylenediamine-derived quinones (PPD-Qs) (e.g., highly toxic N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q)) in the environment and biota and the adverse impact on organisms. Hence, a better understanding of their biotransformation/metabolism in humans is essential. However, relevant data are lacking owing to recent discoveries. Herein, the biotransformation patterns of 6PPD-Q and other five commonly detected PPD-Qs were characterized via combined in vitro assay and maternal cord blood screening monitoring. Rapid metabolism was found for each PPD-Q incubated with human liver S9 fraction and microsomes, resulting in the formation of abundant phase I and phase II metabolites. The subsequent screening for potential PPD-Q metabolites in blood samples showed the presence of suspect metabolites. Three detected metabolites were confirmed by matching the mass spectra and retention times of in vitro metabolites. N-Dealkylated, carboxy, carbonyl, and reductive metabolites and glucose, cysteine, and methionine conjugates were observed for the first time. The semiquantitative concentrations of metabolites were higher than those of the parent PPD-Qs, and several metabolites such as carboxy products were proposed as candidate biomarkers of PPD-Q exposure to humans. 6PPD-Q and N,N'-diphenyl-p-phenylenediamine quinone were detected in maternal and/or cord whole blood samples for the first time. This study holds great importance in elucidating the potential risks and health effects of PPD-Qs.

Analysis of factors affecting microbial degradation of cyanobacterial toxins based on theoretical calculations

Cyanobacterial toxins are the most common algal toxins, which are highly toxic and can persist in the aquatic environment without easy degradation, posing risks to the ecosystem and human health that cannot be ignored. Although microbiological methods for the removal of cyanobacterial toxins from aqueous environments are highly efficient, their degradation efficiency is susceptible to many abiotic environmental factors. In this paper, Microcystin-LR (MC-LR) and its microbial degrading enzymes were selected to study the effects of common environmental factors (temperature (T), NO3-, NH4+, Cu2+, Zn2+) and their levels during microbial degradation of cyanobacterial toxins in aqueous environments by using molecular docking, molecular dynamics simulation, analytical factor design, and the combined toxicokinetics of TOPKAT (toxicity prediction). It was found that the addition of T, NO3- and Cu2+ to the aqueous environment promoted the microbial degradation of MC-LR, while the addition of NH4+ and Zn2+ inhibited the degradation; The level effect study showed that the microbial degradation of MC-LR was promoted by increasing levels of added T and NO3- in the aqueous environment, whereas it was inhibited and then promoted by increasing levels of NH4+, Cu2+ and Zn2+. In addition, the predicted toxicity of common Microcystins (MCs) showed that MC-LR, Microcystin-RR (MC-RR) and Microcystin-YR (MC-YR) were not carcinogenic, developmentally toxic, mutagenic or ocular irritants in humans. MC-LR and MC-RR are mild skin irritants and MC-YR is not a skin irritant. MC-YR has a higher chronic and acute toxicity in humans than MC-LR and MC-RR. Acute/chronic toxicity intensity for aquatic animals: MC-YR > MC-LR > MC-RR and for aquatic plants: MC-LR > MC-YR > MC-RR. This suggests that MC-YR also has a high environmental health risk. This paper provides theoretical support for optimizing the environmental conditions for microbial degradation of cyanobacterial toxins by studying the effects of common environmental factors and their level effects in the aquatic environment.

Nanospray Laser-Induced Plasma Ionization Mass Spectrometry for Sensitive and High-Coverage Analysis of Broad Polarity Organic Pollutants

Accurate, sensitive, and high-coverage analysis of various organic pollutants in complex samples and single cells is significant in investigating their environmental behaviors and toxic effects. Here we proposed a nanospray laser-induced plasma ionization mass spectrometry (nLIPI-MS) method for sensitive and high-coverage analysis of broad polarity organic pollutants under ambient and open-air conditions. The nLIPI-MS method combines nanospray with laser-induced plasma ionization, enabling direct analysis without sophisticated sample pretreatment. For the analysis of nonpolar and very weakly polar organic pollutants such as polycyclic aromatic hydrocarbons (PAHs) and alkyl-PAHs, the nLIPI-MS method showed desirable analytical performance, with limits of detection of as low as the μg/L level. For moderately polar organic pollutants such as p-phenylenediamine quinones (PPD-Qs), nLIPI-MS displayed significant enhanced sensitivity by 1 to 2 orders of magnitude in comparison to nanoelectrospray ionization (nESI)-MS. For more polar organic pollutants such as per- and polyfluoroalkyl substances (PFASs), nLIPI-MS also showed good analytical performance, with a sensitivity improvement of 1.3- to 2.7-fold over that of nESI-MS. The sensitivity of nLIPI-MS for the analysis of PPD-Qs and PFASs reached as low as the ng/L level, enabling the analysis of ultratrace levels of these pollutants in complex samples and even single cells. Our experimental results demonstrated that nLIPI-MS was an extensive ambient MS method, showing desirable analytical performance for the analysis of organic pollutants with broad polarity ranges.

Distinct Species-Specific and Toxigenic Metabolic Profiles for 6PPD and 6PPD Quinone by P450 Enzymes: Insights from In Vitro and In Silico Studies

The tire rubber antioxidant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone product (6PPDQ) are prevalent emerging contaminants, yet their biotransformation profiles remain poorly understood, hampering the assessment of environmental and health risks. This study investigated the phase-I metabolism of 6PPD and 6PPDQ across aquatic and mammalian species through in vitro liver microsome (LM) incubations and in silico simulations. A total of 40 metabolites from seven pathways were identified using the highly sensitive nano-electrospray ionization mass spectrometry. Notably, 6PPDQ was consistently detected as a 6PPD metabolite with an approximate 2% yield, highlighting biotransformation as a neglected indirect exposure pathway for 6PPDQ in organisms. 6PPDQ was calculated to form through a facile two-step phenyl hydroxylation of 6PPD, catalyzed by cytochrome P450 enzymes. Distinct species-specific metabolic kinetics were observed, with fish LM demonstrating retarded biotransformation rates for 6PPD and 6PPDQ compared to mammalian LM, suggesting the vulnerability of aquatic vertebrates to these contaminants. Intriguingly, two novel coupled metabolites were identified for 6PPD, which were predicted to exhibit elevated toxicity compared to 6PPDQ and result from C-N oxidative coupling by P450s. These unveiled metabolic profiles offer valuable insights for the risk assessment of 6PPD and 6PPDQ, which may inform future studies and regulatory actions.

Neurotoxicity and accumulation of CPPD quinone at environmentally relevant concentrations in Caenorhabditis elegans

CPPD quinone (CPPDQ) is a member of PPDQs, which was widely distributed in different environments. Using Caenorhabditis elegans as an animal model, we here examined neurotoxicity and accumulation of CPPDQ and the underlying mechanism. After exposure to 0.01-10 μg/L CPPDQ, obvious body accumulation of CPDDQ was detected. Meanwhile, exposure to CPPDQ (0.01-10 μg/L) decreased head thrash, body bend, and forward turn, and increased backward turn. Nevertheless, only exposure to 10 μg/L CPPDQ induced neurodegeneration in GABAergic system. Exposure to CPPDQ (0.01-10 μg/L) further decreased expressions of daf-7 encoding TGF-β ligand, jnk-1 encoding JNK MAPK, and mpk-1 encoding ERK MAPK. Additionally, among examined G protein-coupled receptor (GPCR) genes, exposure to CPPDQ (0.01-10 μg/L) decreased dcar-1 expression and increased npr-8 expression. RNAi of daf-7, jnk-1, mpk-1, and dcar-1 resulted in susceptibility, and nhr-8 RNAi caused resistance to CPPDQ neurotoxicity and accumulation. Moreover, in CPPDQ exposed nematodes, RNAi of dcar-1 decreased jnk-1 and mpk-1 expressions, and RNAi of npr-8 increased mpk-1 expression. Therefore, exposure to CPPDQ potentially resulted in neurotoxicity by inhibiting TGF-β, JNK MAPK, and ERK MAPK signals. The inhibition in JNK MAPK and ERK MAPKs signals in CPPDQ exposed nematodes was further related to alteration in GPCRs of DCAR-1 and NHR-8 in nematodes.

Fragmentation Pattern-Based Screening Strategy Combining Diagnostic Ion and Neutral Loss Uncovered Novel para-Phenylenediamine Quinone Contaminants in the Environment

Identifying transformed emerging contaminants in complex environmental compartments is a challenging but meaningful task. Substituted para-phenylenediamine quinones (PPD-quinones) are emerging contaminants originating from rubber antioxidants and have been proven to be toxic to the aquatic species, especially salmonids. The emergence of multiple PPD-quinones in various environmental matrices and evidence of their specific hazards underscore the need to understand their environmental occurrences. Here, we introduce a fragmentation pattern-based nontargeted screening strategy combining full MS/All ion fragmentation/neutral loss-ddMS2 scans to identify potential unknown PPD-quinones in different environmental matrices. Using diagnostic fragments of m/z 170.0600, 139.0502, and characteristic neutral losses of 199.0633, 138.0429 Da, six known and three novel PPD-quinones were recognized in air particulates, surface soil, and tire tissue. Their specific structures were confirmed, and their environmental concentration and composition profiles were clarified with self-synthesized standards. N-(1-methylheptyl)-N'-phenyl-1,4-benzenediamine quinone (8PPD-Q) and N,N'-di(1,3-dimethylbutyl)-p-phenylenediamine quinone (66PD-Q) were identified and quantified for the first time, with their median concentrations found to be 0.02-0.21 μg·g-1 in tire tissue, 0.40-2.76 pg·m-3 in air particles, and 0.23-1.02 ng·g-1 in surface soil. This work provides new evidence for the presence of unknown PPD-quinones in the environment, showcasing a potential strategy for screening emerging transformed contaminants in the environment.