Acute Toxicity of 6PPD-Quinone to Early Life Stage Juvenile Chinook (Oncorhynchus tshawytscha) and Coho (Oncorhynchus kisutch) Salmon

P-phenylenediamine antioxidants and their quinone derivatives: A review of their environmental occurrence, accessibility, potential toxicity, and human exposure

Substituted p-phenylenediamines (PPDs), a class of antioxidants, have been widely used to extend the lifespan of rubber products, such as tires and pipes. During use, PPDs will generate their quinone derivatives (PPD-Qs). In recent years, PPDs and PPD-Qs have been detected in the global environment. Among them, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-Q), the oxidation product of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD), has been identified as highly toxic to coho salmon, with the lethal concentration of 50 % (LC50) being 95 ng/L, highlighting it as an emerging pollutant of great concern. This review summarizes the physicochemical properties, global environmental distribution, bioaccessibility, potential toxicity, human exposure risk, and green measures of PPDs and PPD-Qs. These chemicals exhibit lipophilicity, bioaccumulation potential, and poor aqueous stability. They have been found in water, air, dust, soil, and sediment worldwide, indicating their significance as emerging pollutants. Notably, current studies have identified electronic waste (e-waste), such as discarded wires and cables, as a non-negligible source of PPDs and PPD-Qs, in addition to tire wear. PPDs and PPD-Qs exhibit strong bioaccumulation in aquatic organisms and mammals, with a tendency for biomagnification within the food web, posing health threats to humans. Available toxicity data indicate that PPDs and PPD-Qs have negative effects on aquatic organisms, mammals, and invertebrates. Acute exposure leads to death and acute damage, and long-term exposure can cause a series of adverse effects, including growth and development toxicity, reproductive toxicity, neurotoxicity, intestinal toxicity, and multi-organ damage. This paper discusses current research gaps and offers recommendations to understand better the occurrence, behavior, toxicity, and environmental exposure risks of PPDs and PPD-Qs.

Exposure to 6-PPD quinone enhances glycogen accumulation in Caenorhabditiselegans

Glycogen metabolism is an important biological process for organisms. In Caenorhabditis elegans, effect of 6-PPD quinone (6-PPDQ) on glycogen accumulation and underlying mechanism were examined. Exposure to 6-PPDQ (1 and 10 μg/L) increased glycogen accumulation. Meanwhile, exposure to 6-PPDQ (1 and 10 μg/L) increased expression of gsy-1 encoding glycogen synthase and decreased expression of pygl-1 encoding glycogen phosphorylase. In 6-PPDQ exposed animals, glycogen content and glycogen accumulation were inhibited by RNAi of gsy-1 and enhanced by RNAi of pygl-1. RNAi of gsy-1 increased pygl-1 expression, and RNAi of pygl-1 increased gsy-1 expression after 6-PPDQ exposure. In 6-PPDQ exposed nematodes, daf-16 and aak-2 expressions were decreased and glycogen accumulation was suppressed by RNAi of daf-16 and aak-2, suggesting alteration in daf-16 and aak-2 expressions did not mediate glycogen accumulation. Moreover, resistance to 6-PPDQ toxicity on locomotion and brood size was observed in gsy-1(RNAi) animals, and susceptibility to 6-PPDQ toxicity was found in pygl-1(RNAi) animals. Therefore, glycogen accumulation could be enhanced by exposure to 6-PPDQ in nematodes. In addition, alteration in expressions of gsy-1 and pygl-1 governing this enhancement in glycogen accumulation mediated induction of 6-PPDQ toxicity.

Storms mobilize organophosphate esters, bisphenols, PFASs, and vehicle-derived contaminants to San Francisco Bay watersheds

In urban to peri-urban watersheds such as those surrounding San Francisco Bay, stormwater runoff is a major pathway by which contaminants enter aquatic ecosystems. We evaluated the occurrence of 154 organic contaminants via liquid chromatography coupled to tandem mass spectrometry, including organophosphate esters (OPEs), bisphenols, per- and polyfluoroalkyl substances (PFASs), and a suite of novel urban stormwater tracers (SWCECs; i.e., vehicle-derived chemicals, pesticides, pharmaceuticals/personal care products, benzothiazoles/benzotriazoles). Time-averaged composite sampling focused on storms in highly developed watersheds over four wet seasons, with complementary sampling in less-urban reference watersheds, near-shore estuarine sites, and the open Bay. Of the targeted contaminants, 68 (21 SWCECs, 29 OPEs, 3 bisphenols, 15 PFASs) were detected in ≥10 of 26 urban stormwater samples. Median concentrations exceeded 500 ng L-1 for 1,3-diphenylguanidine, hexa(methoxymethyl)melamine, and caffeine, and exceeded 300 ng L-1 for 2-hydroxy-benzothiazole, 5-methyl-1H-benzotriazole, pentachlorophenol, and tris(2-butoxyethyl) phosphate. Median individual PFAS concentrations were <10 ng L-1, with highest concentrations for PFHxA (180 ng L-1), PFOA (110 ng L-1), and PFOS (81 ng L-1). In six of eight urban stormwater samples analyzed for 6PPD-quinone (a tire rubber-derived transformation product), concentrations exceeded coho salmon acute toxicity thresholds, suggesting (sub)lethal impacts for sensitive species. Observed concentrations were generally significantly higher in highly developed watersheds relative to reference watersheds, but not statistically different in near-shore estuarine sites, suggesting substantial transient exposure potential at stormwater outfalls or creek outflows. Results emphasized the role of stormwater in contaminant transport, the importance of vehicles/roadways as contaminant sources, and the value of monitoring broad multi-analyte contaminant suites to enable comprehensive source and toxicity evaluations.

6-PPD quinone at environmentally relevant concentrations induced damage on longevity in C. elegans: Mechanistic insight from inhibition in mitochondrial UPR response

6-PPD quinone (6-PPDQ) exists widely in water environment media, causing acute lethality to some aquatic species. Long-term exposure to 6-PPDQ reduced the lifespan of Caenorhabditis elegans. However, the molecular basis for mitochondrial control of 6-PPDQ toxicity remains largely unclear. Using HSP-6 as marker of mitochondrial unfolded protein response (mt UPR), we observed activation of mt UPR by 0.1 and 1 μg/L 6-PPDQ and inhibition in mt UPR by 10 μg/L 6-PPDQ. Additionally, increased atfs-1, ubl-5, and dve-1 expressions were caused by 0.1 and 1 μg/L 6-PPDQ and decreased expressions of these genes were induced by 10 μg/L 6-PPDQ. Neuronal and intestinal RNA interference (RNAi) of hsp-6 caused susceptibility to 6-PPDQ toxicity on longevity, and atfs-1, ubl-5, and dve-1 acted in neurons and intestine to modulate mt UPR and 6-PPDQ toxicity on longevity. Meanwhile, 6-PPDQ (1 and 10 μg/L) increased expressions of histone methyltransferase genes met-2 and set-6, and decreased expressions of histone demethylase genes jmjd-1.2 and jmjd-3.1. Neuronal RNAi of set-6 and intestinal RNAi of met-2 accelerated hsp-6, atfs-1, ubl-5, and dve-1 expressions and extended lifespan of 6-PPDQ exposed nematodes. In contrast, neuronal RNAi of jmjd-1.2 and jmjd-3.1 and intestinal RNAi of jmjd-1.2 suppressed these 4 gene expressions and reduced lifespan of 6-PPDQ exposed nematodes o. In nematodes, RNAi of hsp-6 could also enhance mitochondrial dysfunction and mitochondrial reactive oxygen species (ROS) induced by 6-PPDQ. Therefore, 6-PPDQ caused damage on longevity was associated with suppression in mt UPR, which was under regulation of certain histone methylation related signals.

Neurotoxicity from long-term exposure to 6-PPDQ: Recent advances

The recent acceleration of industrialization and urbanization has brought significant attention to N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ), an emerging environmental pollutant from tire wear, due to its long-term effects on the environment and organisms. Recent studies suggest that 6-PPDQ can disrupt neurotransmitter synthesis and release, impact receptor function, and alter signaling pathways, potentially causing oxidative stress, inflammation, and apoptosis. This review investigates the potential neurotoxic effects of prolonged 6-PPDQ exposure, the mechanisms underlying its cytotoxicity, and the associated health risks. We emphasize the need for future research, including precise exposure assessments, identification of individual differences, and development of risk assessments and intervention strategies. This article provides a comprehensive overview of 6-PPDQ's behavior, impact, and neurotoxicity in the environment, highlighting key areas and challenges for future research.

Tire-derived contaminants 6PPD and 6PPD-Q: Analysis, sample handling, and reconnaissance of United States stream exposures

The environmental ubiquity of tire and road wear particles (TRWP) underscores the need to understand the occurrence, persistence, and environmental effects of tire-related chemicals in aquatic ecosystems. One such chemical is 6PPD-quinone (6PPD-Q), a transformation product of the tire antioxidant 6PPD. In urban stormwater runoff 6PPD-Q can exceed acute toxicity thresholds for several salmonid species and is being implicated in significant coho salmon losses in the Pacific Northwest. There is a critical need to understand the prevalence of 6PPD-Q across watersheds to identify habitats heavily affected by TRWPs. We conducted a reconnaissance of 6PPD and 6PPD-Q in surface waters across the United States from sites (N = 94) with varying land use (urban, agricultural, and forested) and streamflow to better understand stream exposures. A rapid, low-volume direct-inject, liquid chromatography mass spectrometry method was developed for the quantitation of 6PPD-Q and screening for 6PPD. Laboratory holding times, bottle material, headspace, and filter materials were investigated to inform best practices for 6PPD-Q sampling and analysis. Glass bottles with PTFE-lined caps minimized sorption and borosilicate glass fiber filters provided the highest recovery. 6PPD-Q was stable for at least 5 months in pure laboratory solutions and for 75 days at 5 °C with minimal headspace in the investigated surface water and stormwaters. Results also indicated samples can be frozen to extend holding times. 6PPD was not detected in any of the 526 analyzed samples and there were no detections of 6PPD-Q at agricultural or forested sites. 6PPD-Q was frequently detected in stormwater (57%, N = 90) and from urban impacted sites (45%, N = 276) with concentrations ranging from 0.002 to 0.29 μg/L. The highest concentrations, above the lethal level for coho salmon, occurred during stormwater runoff events. This highlights the importance of capturing episodic runoff events in urban areas near ecologically relevant habitat or nursery grounds for sensitive species.

Acute toxicity testing of 6PPD-quinone on the estuarine-dependent sport fish, Sciaenops ocellatus

Recently, large-scale fish kills in the Pacific Northwest were linked to tire wear particles (TWPs) left on roadways, with the lethality attributed to 6PPD-quinone. which has a median lethal concentration of <1 µg/L for selected salmonids. However, there remains a paucity of 6PPD-quinone toxicity values developed for estuarine fish species, which is particularly significant because estuaries receiving inflows from highly urbanized watersheds are especially vulnerable to TWP contamination. Therefore, the present study aimed to determine the toxicity of 6PPD-quinone to an economically and ecologically important estuarine-dependent fish-red drum (Sciaenops ocellatus). Here, we examined the relative sensitivities of three early life stages within red drum: embryonic, larval, and post-settlement for 24-72 hours, depending on the life stage. Exposure concentrations ranged from 10 μg/L to 500 μg/L. We also assessed the sub-lethal impacts of 6PPD-quinone exposure on development during embryonic and larval stages, including body and organ sizes. Our results indicate that red drum are not acutely sensitive to 6PPD-quinone at each early life stage tested. We also found that yolk-sac larvae did not exhibit sub-lethal morphological impacts in a dose-dependent manner, regardless of exposure during embryonic and larval stages. These data are the first to assess the impacts of 6PPD-quinone on estuarine-dependent non-model fishes.

Effects of environmental concentrations of 6PPD and its quinone metabolite on the growth and reproduction of freshwater cladoceran

The widespread occurrence and accumulation of N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) and its quinone metabolite, 6PPD quinone (6PPD-Q), have been globally recognized as a critical environmental issue. However, knowledge on the adverse effects of 6PPD and 6PPD-Q on freshwater invertebrates is limited. This study investigated the effects of 6PPD and its oxidative byproduct, 6PPD-Q, on the growth and reproduction of Daphnia pulex. Through 21-day exposure experiments, we measured the uptake of 0.1, 1, and 10 μg/L 6PPD and 6PPD-Q by D. pulex and assessed the effects on growth and fecundity of D. pulex. While 6PPD and 6PPD-Q did not affect the mortality rate of D. pulex, 6PPD-Q exposure inhibited the growth of D. pulex, indicating potential ecological risks. In particular, the reproductive capacity of D. pulex remained unaffected across the tested concentrations of 6PPD and 6PPD-Q, suggesting specific toxicological pathways that warrant further investigation. This study underscored the importance of evaluating the sublethal effects of emerging contaminants such as 6PPD and 6PPD-Q on aquatic invertebrates, and highlighted the need for comprehensive risk assessments to better understand their environmental impacts.

The lethal and sublethal impacts of two tire rubber-derived chemicals on brook trout (Salvelinus fontinalis) fry and fingerlings

Recent toxicity studies of stormwater runoff implicated N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) as the contaminant responsible for the mass mortality of coho salmon (Oncorhynchus kisutch). In the wake of this discovery, 6PPD-quinone has been measured in waterways around urban centers, along with other tire wear leachates like hexamethoxymethylmelamine (HMMM). The limited data available for 6PPD-quinone have shown toxicity can vary depending on the species. In this study we compared the acute toxicity of 6PPD-quinone and HMMM to Brook trout (Salvelinus fontinalis) fry and fingerlings. Our results show that fry are ∼3 times more sensitive to 6PPD-quinone than fingerlings. Exposure to HMMM ≤6.6 mg/L had no impact on fry survival. These results highlight the importance of conducting toxicity tests on multiple life stages of fish species, and that relying on fingerling life stages for species-based risk assessment may underestimate the impacts of exposure. 6PPD-quinone also had many sublethal effects on Brook trout fingerlings, such as increased interlamellar cell mass (ILCM) size, hematocrit, blood glucose, total CO2, and decreased blood sodium and chloride concentrations. Linear relationships between ILCM size and select blood parameters support the conclusion that 6PPD-quinone toxicity is an outcome of osmorespiratory challenges imposed by gill impairment.

Exposure to 6-PPD quinone causes ferroptosis activation associated with induction of reproductive toxicity in Caenorhabditis elegans

Exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) caused toxicity on Caenorhabditis elegans, including reproductive toxicity. However, the underlying mechanisms for this induced reproductive toxicity by 6-PPDQ remain largely unclear. We examined possible association of ferroptosis activation with reproductive toxicity of 6-PPDQ. In 1-100 μg/L 6-PPDQ exposed nematodes, Fe2+ content was increased, which was accompanied with enhanced lipid peroxidation, increased malonydialdehyde (MDA) content, and decreased L-glutathione (GSH) content. Exposure to 1-100 μg/L 6-PPDQ decreased expressions of ftn-1 encoding ferritin, ads-1 encoding AGPS, and gpx-6 encoding GPX4 and increased expression of bli-3 encoding dual oxidase. After 6-PPDQ exposure, RNAi of ftn-1 decreased ads-1 and gpx-6 expressions and increased bli-3 expression. RNAi of ftn-1, ads-1, and gpx-6 strengthened alterations in ferroptosis related indicators, and RNAi of bli-3 suppressed changes of ferroptosis related indicators in 6-PPDQ exposed nematodes. Meanwhile, RNAi of ftn-1, ads-1, and gpx-6 induced susceptibility, and RNAi of bli-3 caused resistance to 6-PPDQ reproductive toxicity. Moreover, expressions of DNA damage checkpoint genes (clk-2, mrt-2, and hus-1) could be increased by RNAi of ftn-1, ads-1, and gpx-6 in 6-PPDQ exposed nematodes. Therefore, our results demonstrated activation of ferroptosis in nematodes exposed to 6-PPDQ at environmentally relevant concentrations, and this ferroptosis activation was related to reproductive toxicity of 6-PPDQ.

Navigating the environmental dynamics, toxicity to aquatic organisms and human associated risks of an emerging tire wear contaminant 6PPD quinone

N-1,3-Dimethylbutyl-N'-phenyl-p-quinone diamine (6PPDQ) is a derivative of 6PPD, a synthetic antioxidant used in tire manufacturing to control the degradation caused by oxidation and heat aging. Its discovery in 2020 has raised important environmental concern, particularly regarding its association with acute mortality in coho salmon, prompting surge in research on its occurrence, fate, and transport in aquatic ecosystems. Despite this attention, there remain notable gaps in grasping the knowledge, demanding an in depth overview. Thus, this review consolidates recent studies to offer a thorough investigation of 6PPDQ's environmental dynamics, pathways into aquatic ecosystems, toxicity to aquatic organisms, and human health implications. Various aquatic species exhibit differential susceptibility to 6PPDQ toxicity, manifesting in acute mortalities, disruption of metabolic pathways, oxidative stress, behavioral responses, and developmental abnormalities. Whereas, understanding the species-specific responses, molecular mechanisms, and broader ecological implications requires further investigation across disciplines such as ecotoxicology, molecular biology, and environmental chemistry. Integration of findings emphasizes the complexity of 6PPDQ toxicity and its potential risks to human health. However, urgent priorities should be given to the measures like long-term monitoring studies to evaluate the chronic effects on aquatic ecosystems and the establishment of standardized toxicity testing protocols to ensure the result comparability and reproducibility. This review serves as a vital resource for researchers, policymakers, and environmental professionals seeking appraisals into the impacts of 6PPDQ contamination on aquatic ecosystems and human health.

Acute Toxicity Testing of Pink Salmon (Oncorhynchus gorbuscha) with the Tire Rubber-Derived Chemical 6PPD-Quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-quinone) is a widespread contaminant of emerging concern resulting from oxidation of 6PPD, which is an antidegradant substance added to tires. The recent identification of 6PPD-quinone as the cause of acute mortality in coho salmon has quickly motivated studies on 6PPD-quinone toxicity to other species. Subsequent findings have shown that 6PPD-quinone toxicity is highly species specific. Closely related species can differ widely in response to 6PPD-quinone from extremely sensitive to tolerant. Hence toxicity testing is currently the only way to establish whether a species exhibits 6PPD-quinone toxicity. We investigated the acute toxicity of 6PPD-quinone in pink salmon alevins (sac fry). This species has is the only Pacific salmon that so far has not been tested for 6PPD-quinone sensitivity. Fish were exposed in static water in eight treatments with initial concentrations ranging from 0.1 to 12.8 μg/L. Fish were observed for 48 h, and changes in concentrations of 6PPD-quinone were monitored throughout the experiment. No mortalities or substantial changes in behavior were recorded. Environ Toxicol Chem 2024;43:1332-1338. © 2024 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

Where the rubber meets the road: Emerging environmental impacts of tire wear particles and their chemical cocktails

About 3 billion new tires are produced each year and about 800 million tires become waste annually. Global dependence upon tires produced from natural rubber and petroleum-based compounds represents a persistent and complex environmental problem with only partial and often-times, ineffective solutions. Tire emissions may be in the form of whole tires, tire particles, and chemical compounds, each of which is transported through various atmospheric, terrestrial, and aquatic routes in the natural and built environments. Production and use of tires generates multiple heavy metals, plastics, PAH's, and other compounds that can be toxic alone or as chemical cocktails. Used tires require storage space, are energy intensive to recycle, and generally have few post-wear uses that are not also potential sources of pollutants (e.g., crumb rubber, pavements, burning). Tire particles emitted during use are a major component of microplastics in urban runoff and a source of unique and highly potent toxic substances. Thus, tires represent a ubiquitous and complex pollutant that requires a comprehensive examination to develop effective management and remediation. We approach the issue of tire pollution holistically by examining the life cycle of tires across production, emissions, recycling, and disposal. In this paper, we synthesize recent research and data about the environmental and human health risks associated with the production, use, and disposal of tires and discuss gaps in our knowledge about fate and transport, as well as the toxicology of tire particles and chemical leachates. We examine potential management and remediation approaches for addressing exposure risks across the life cycle of tires. We consider tires as pollutants across three levels: tires in their whole state, as particulates, and as a mixture of chemical cocktails. Finally, we discuss information gaps in our understanding of tires as a pollutant and outline key questions to improve our knowledge and ability to manage and remediate tire pollution.

A Review of N-(1,3-Dimethylbutyl)-N'-phenyl-p-Phenylenediamine (6PPD) and Its Derivative 6PPD-Quinone in the Environment

As an antioxidant and antiozonant, N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is predominantly used in the rubber industry to prevent degradation. However, 6PPD can be ozonated to generate a highly toxic transformation product called N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6PPD-quinone), which is toxic to aquatic and terrestrial organisms. Thus, 6PPD and 6PPD-quinone, two emerging contaminants, have attracted extensive attention recently. This review discussed the levels and distribution of 6PPD and 6PPD-quinone in the environment and investigated their toxic effects on a series of organisms. 6PPD and 6PPD-quinone have been widely found in air, water, and dust, while data on soil, sediment, and biota are scarce. 6PPD-quinone can cause teratogenic, developmental, reproductive, neuronal, and genetic toxicity for organisms, at environmentally relevant concentrations. Future research should pay more attention to the bioaccumulation, biomagnification, transformation, and toxic mechanisms of 6PPD and 6PPD-quinone.

Environmental profiles, hazard identification, and toxicological hallmarks of emerging tire rubber-related contaminants 6PPD and 6PPD-quinone

N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine (6PPD) is commonly used in rubber compounds as antioxidants to protect against degradation from heat, oxygen, and ozone exposure. This practice extends the lifespan of rubber products, including tires, by preventing cracking, aging, and deterioration. However, the environmental consequences of waste generated during rubber product use, particularly the formation of 6PPD-quinone (6PPD-Q) through the reaction of 6PPD with ozone, have raised significant concerns due to their detrimental effects on ecosystems. Extensive research has revealed the widespread occurrence of 6PPD and its derivate 6PPD-Q in various environmental compartments, including air, water, and soil. The emerging substance of 6PPD-Q has been shown to pose acute mortality and long-term hazards to aquatic and terrestrial organisms at concentrations below environmentally relevant levels. Studies have demonstrated toxic effects of 6PPD-Q on a range of organisms, including zebrafish, nematodes, and mammals. These effects include neurobehavioral changes, reproductive dysfunction, and digestive damage through various exposure pathways. Mechanistic insights suggest that mitochondrial stress, DNA adduct formation, and disruption of lipid metabolism contribute to the toxicity induced by 6PPD-Q. Recent findings of 6PPD-Q in human samples, such as blood, urine, and cerebrospinal fluid, underscore the importance of further research on the public health and toxicological implications of these compounds. The distribution, fate, biological effects, and underlying mechanisms of 6PPD-Q in the environment highlight the urgent need for additional research to understand and address the environmental and health impacts of these compounds.

Mixture toxicity of 6PPD-quinone and polystyrene nanoplastics in zebrafish

Plastic pollution, including micro- and nanoplastics, is a growing concern. Tyre-wear particles (TWPs) are the second largest source of microplastics in the ocean following abrasion of synthetic fibres. In addition to the particles themselves, TWPs contain many harmful chemicals, including 6PPD. This chemical reacts with atmospheric ozone and forms the toxic compound 6PPD-quinone (6PPDq), which poses a danger to aquatic life. There is a knowledge gap in understanding risks associated with the combined toxicity of nanoplastics (NPs) and 6PPDq. The present study aimed to investigate the toxicity of NPs and 6PPDq on adult zebrafish using phenotypic (behaviour, histology) and transcriptomic endpoints. Zebrafish were exposed to four treatments: control (contaminant-free), 50 μg/L 6PPDq, 3 mg/L polystyrene (PS)-NPs, and a combination of 50 μg/L 6PPDq and 3 mg/L PS-NPs. We did not observe locomotory dysregulation in zebrafish exposed to NPs. However, we found significant hyperlocomotion in zebrafish exposed to 6PPDq and this effect was even more substantial after co-exposure with PS-NPs. This study explores the molecular mechanisms behind these effects, identifying genes associated with neurotransmitters and fatty acid metabolism that were dysregulated by the co-exposure. Transcriptomic analysis further showed that both 6PPDq and PS-NPs impacted cellular processes associated with sterol biosynthesis, cholesterol metabolism, and muscle tissue development. The effects on these mechanisms were stronger in co-exposed zebrafish, indicating a heightened risk to cellular integrity and mitochondrial dysfunction. These results highlight the significance of mixture toxicity when studying the effects of NPs and associated chemicals like 6PPDq.

Bioretention Design Modifications Increase the Simulated Capture of Hydrophobic and Hydrophilic Trace Organic Compounds

Stormwater rapidly moves trace organic contaminants (TrOCs) from the built environment to the aquatic environment. Bioretention cells reduce loadings of some TrOCs, but they struggle with hydrophilic compounds. Herein, we assessed the potential to enhance TrOC removal via changes in bioretention system design by simulating the fate of seven high-priority stormwater TrOCs (e.g., PFOA, 6PPD-quinone, PAHs) with log KOC values between -1.5 and 6.74 in a bioretention cell. We evaluated eight design and management interventions for three illustrative use cases representing a highway, a residential area, and an airport. We suggest two metrics of performance: mass advected to the sewer network, which poses an acute risk to aquatic ecosystems, and total mass advected from the system, which poses a longer-term risk for persistent compounds. The optimized designs for each use case reduced effluent loadings of all but the most polar compound (PFOA) to <5% of influent mass. Our results suggest that having the largest possible system area allowed bioretention systems to provide benefits during larger events, which improved performance for all compounds. To improve performance for the most hydrophilic TrOCs, an amendment like biochar was necessary; field-scale research is needed to confirm this result. Our results showed that changing the design of bioretention systems can allow them to effectively capture TrOCs with a wide range of physicochemical properties, protecting human health and aquatic species from chemical impacts.

Implications of biotic factors for toxicity testing in laboratory studies

There is an emerging call from scientists globally to advance the environmental relevance of laboratory studies, particularly within the field of ecotoxicology. To answer this call, we must carefully examine and elucidate the shortcomings of standardized toxicity testing methods that are used in the derivation of toxicity values and regulatory criteria. As a consequence of rapidly accelerating climate change, the inclusion of abiotic co-stressors are increasingly being incorporated into toxicity studies, with the goal of improving the representativeness of laboratory-derived toxicity values used in ecological risk assessments. However, much less attention has been paid to the influence of biotic factors that may just as meaningfully impact our capacity to evaluate and predict risks within impacted ecosystems. Therefore, the overarching goal is to highlight key biotic factors that should be taken into consideration during the experimental design and model selection phase. SYNOPSIS: Scientists are increasingly finding that lab reared results in toxicology might not be reflective of the external wild environment, we highlight in this review some key considerations when working between the lab and field.

A mini review on 6PPD quinone: A new threat to aquaculture and fisheries

Numerous toxic substances are directly and indirectly discharged by humans into water bodies, causing distress to the organisms living on it. 6PPD, an amino antioxidant from tires reacts with ozone to form 6PPD-Q, which has garnered global attention due to its lethal nature to various organisms. This review aims to provide an understanding of the sources, transformation, and fate of 6PPD-Q in water and the current knowledge on its effects on aquatic organisms. Furthermore, we discuss research gaps pertaining to the mechanisms by which 6PPD-Q acts within fish bodies. Previous studies have demonstrated the ubiquitous presence of 6PPD-Q in the environment, including air, water, and soil. Moreover, this compound has shown high lethality to certain fish species while not affecting others. Toxicological studies have revealed its impact on the nervous system, intestinal barrier function, cardiac function, equilibrium loss, and oxidative stress in various fish species. Additionally, exposure to 6PPD-Q has led to organ injury, lipid accumulation, and cytokine production in C. elegans and mice. Despite studies elucidating the lethal dose and effects of 6PPD-Q in fish species, the underlying mechanisms behind these symptoms remain unclear. Future studies should prioritize investigating the mechanisms underlying the lethality of 6PPD-Q in fish species to gain a better understanding of its potential effects on different organisms.

Interspecies Differences in 6PPD-Quinone Toxicity Across Seven Fish Species: Metabolite Identification and Semiquantification

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone (6PPD-Q) is a recently identified contaminant that originates from the oxidation of the tire antidegradant 6PPD. 6PPD-Q is acutely toxic to select salmonids at environmentally relevant concentrations, while other fish species display tolerance to concentrations that surpass those measured in the environment. The reasons for these marked differences in sensitivity are presently unknown. The objective of this research was to explore potential toxicokinetic drivers of species sensitivity by characterizing biliary metabolites of 6PPD-Q in sensitive and tolerant fishes. For the first time, we identified an O-glucuronide metabolite of 6PPD-Q using high-resolution mass spectrometry. The semiquantified levels of this metabolite in tolerant species or life stages, including white sturgeon (Acipenser transmontanus), chinook salmon (Oncorhynchus tshawytscha), westslope cutthroat trout (Oncorhynchus clarkii lewisi), and nonfry life stages of Atlantic salmon (Salmo salar), were greater than those in sensitive species, including coho salmon (Oncorhynchus kisutch), brook trout (Salvelinus fontinalis), and rainbow trout (Oncorhynchus mykiss), suggesting that tolerant species might detoxify 6PPD-Q more effectively. Thus, we hypothesize that differences in species sensitivity are a result of differences in basal expression of biotransformation enzyme across various fish species. Moreover, the semiquantification of 6PPD-Q metabolites in bile extracted from wild-caught fish might be a useful biomarker of exposure to 6PPD-Q, thereby being valuable to environmental monitoring and risk assessment.

Bioretention filtration prevents acute mortality and reduces chronic toxicity for early life stage coho salmon (Oncorhynchus kisutch) episodically exposed to urban stormwater runoff

As the human population of western North America continues to expand, widespread patterns of urban growth pose increasingly existential threats to certain wild stocks of Pacific salmon and steelhead (Oncorhynchus sp.). Rainfall previously absorbed into the soils of forests and grasslands falls instead on pavement and other hardened surfaces. This creates stormwater runoff that carries toxic metals, oil, and many other contaminants into salmon-bearing habitats. These include freshwater streams where coho salmon (O. kisutch) spawn in gravel beds. Coho salmon embryos develop within a thick eggshell (chorion) for weeks to months before hatching as alevins and ultimately emerging from the gravel as fry. Untreated urban runoff is highly toxic to older coho salmon (freshwater-resident juveniles and adult spawners), but the vulnerability of the earliest life stages remains poorly understood. To address this uncertainty, we fertilized eggs and raised them under an episodic stormwater exposure regimen, using runoff collected from a high-traffic arterial roadway from 15 discrete storm events. We monitored survival and morphological development, as well as molecular markers for contaminant exposure and cardiovascular stress. We also evaluated the benefit of treating runoff with green infrastructure (bioretention filtration) on coho salmon health and survival. Untreated runoff caused subtle sublethal toxicity in pre-hatch embryos with no mortality, followed by high rates of mortality from exposure at hatch. Bioretention filtration removed most measured contaminants (bacteria, dissolved metals, and polycyclic aromatic hydrocarbons), and the treated effluent was considerably less toxic - notably preventing mortality at the alevin stage. Our findings indicate that untreated urban runoff poses an important threat to early life stage coho salmon, in terms of both acute and delayed-in-time mortality. Moreover, while inexpensive management strategies involving bioinfiltration are promising, future green infrastructure effectiveness research should emphasize sublethal metrics for contaminant exposure and adverse health outcomes in salmonids.

Disruption of dopamine metabolism by exposure to 6-PPD quinone in Caenorhabditis elegans

Caenorhabditis elegans is a useful model for examining metabolic processes and related mechanisms. We here examined the effect of exposure to N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) on dopamine metabolism and underling molecular basis in nematodes. The dopamine content was reduced by 6-PPDQ (1 and 10 μg/L). Meanwhile, dopamine related behaviors (basal slowing response and area restricted searching) were changed by 6-PPDQ (1 and 10 μg/L). Exposure to 6-PPDQ (1 and 10 μg/L) decreased expressions of genes (cat-2 and bas-1) encoding enzymes governing dopamine synthesis and cat-1 encoding dopamine transporter. Development of dopaminergic neurons was also affected by 10 μg/L 6-PPDQ as reflected by decrease in fluorescence intensity, neuronal loss, and defect in dendrite development. Exposure to 6-PPDQ (1 and 10 μg/L) altered expressions of ast-1 and rcat-1 encoding upregulators of cat-2 and bas-1. The dopamine content and expressions of cat-2 and bas-1 were inhibited by RNAi of ast-1 and increased by RNAi of rcat-1 in 6-PPDQ exposed nematodes. Using endpoints of locomotion behavior and brood size, in 6-PPDQ exposed nematodes, the susceptibility to toxicity was caused by RNAi of ast-1, cat-2, bas-1, and cat-1, and the resistance to toxicity was induced by RNAi of rcat-1. Therefore, 6-PPDQ exposure disrupted dopamine metabolism and the altered molecular basis for dopamine metabolism was associated with 6-PPDQ toxicity induction. Moreover, the defects in dopamine related behaviors and toxicity on locomotion and reproduction could be rescued by treatment with 0.1 mM dopamine.

Toxicity of 6PPD-quinone to four freshwater invertebrate species

The antioxidant N-(1,3-Dimethylbutyl)-N'-phenyl-p- phenylenediamine (6PPD) is used to protect the rubber in tires from oxidation, which extends the life of the tire. When oxidized, 6PPD is transformed into 6PPD-quinone (6PPDQ). 6PPDQ, along with other tire ingredients, can enter aquatic ecosystems through the transport of tire wear particles in runoff during a precipitation event. The mass mortality of coho salmon following precipitation events in urban areas lead to the discovery that 6PPDQ is the likely cause due to coho salmon's relatively high sensitivity to 6PPDQ. The assessment of 6PPDQ toxicity to other aquatic species has expanded, but it has focused on fish. This study investigated the toxicity of 6PPDQ to four freshwater invertebrate species, larval burrowing mayfly (Hexagenia spp.), juvenile cladoceran (Daphnia magna), file ramshorn snail embryo (Planorbella pilsbryi), and adult washboard mussel (Megalonaias nervosa). For all four species, the highest concentration of 6PPDQ tested did not result in significant mortality. This translated into the determination of the highest concentration that did not cause significant mortality (NOEC) for Hexagenia spp., D. magna, P. pilsbryi, and M. nervosa of 232.0, 42.0, 11.7, and 17.9 μg/L, respectively. The data from this study indicate that freshwater invertebrates are not as sensitive to 6PPDQ as some salmonid species (e.g., coho salmon Oncorhynchus kisutch). This study also analyzed 6PPDQ in road runoff from around the city of Guelph in Ontario, Canada. 6PPQ was detected in all samples but the concentration was two orders of magnitude lower than the NOECs for the four tested species of freshwater invertebrate.

Uptake and Biotransformation of the Tire Rubber-derived Contaminants 6-PPD and 6-PPD Quinone in the Zebrafish Embryo (Danio rerio)

N-(1,3-Dimethylbutyl)-N'-phenyl-p-phenylenediamine (6-PPD) is a widely used antioxidant in tire rubber known to enter the aquatic environment via road runoff. The associated transformation product (TP) 6-PPD quinone (6-PPDQ) causes extreme acute toxicity in some fish species (e.g., coho salmon). To interpret the species-specific toxicity, information about biotransformation products of 6-PPDQ would be relevant. This study investigated toxicokinetics of 6-PPD and 6-PPDQ in the zebrafish embryo (ZFE) model. Over 96 h of exposure, 6-PPD and 6-PPDQ accumulated in the ZFE with concentration factors ranging from 140 to 2500 for 6-PPD and 70 to 220 for 6-PPDQ. A total of 22 TPs of 6-PPD and 12 TPs of 6-PPDQ were tentatively identified using liquid chromatography coupled to high-resolution mass spectrometry. After 96 h of exposure to 6-PPD, the TPs of 6-PPD comprised 47% of the total peak area (TPA), with 4-hydroxydiphenylamine being the most prominent in the ZFE. Upon 6-PPDQ exposure, >95% of 6-PPDQ taken up in the ZFE was biotransformed, with 6-PPDQ + O + glucuronide dominating (>80% of the TPA). Among other TPs of 6-PPD, a reactive N-phenyl-p-benzoquinone imine was found. The knowledge of TPs of 6-PPD and 6-PPDQ from this study may support biotransformation studies in other organisms.

Automated, High-Throughput Analysis of Tire-Derived p-Phenylenediamine Quinones (PPDQs) in Water by Online Membrane Sampling Coupled to MS/MS

The tire-derived contaminant N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine quinone (6-PPDQ) was recently identified as a potent toxin to coho salmon (Oncorhynchus kisutch). Studies investigating 6-PPDQ have employed solid-phase extraction (SPE) or liquid-liquid extraction (LLE) with liquid chromatography-mass spectrometry (LC-MS), providing excellent sensitivity and selectivity. However, cleanup and pre-enrichment steps (SPE/LLE) followed by chromatographic separation can be time- and cost-intensive, limiting sample throughput. The ubiquitous distribution of 6-PPDQ necessitates numerous measurements to identify hotspots for targeted mitigation. We recently developed condensed phase membrane introduction mass spectrometry (CP-MIMS) for rapid 6-PPDQ analysis (2.5 min/sample), with a simple workflow and low limit of detection (8 ng/L). Here, we describe improved quantitation using isotopically labeled internal standards and inclusion of a suite of PPDQ analogues. A low-cost autosampler and data processing software were developed from a three-dimensional (3D) printer and Matlab to fully realize the high-throughput capabilities of CP-MIMS. Cross-validation with a commercial LC-MS method for 10 surface waters provides excellent agreement (slope: 1.01; R2 = 0.992). We employ this analytical approach to probe fundamental questions regarding sample stability and sorption of 6-PPDQ under lab-controlled conditions. Further, the results for 192 surface water samples provide the first spatiotemporal characterization of PPDQs on Vancouver Island and the lower mainland of British Columbia.

Long-term exposure to 6-PPD quinone reduces reproductive capacity by enhancing germline apoptosis associated with activation of both DNA damage and cell corpse engulfment in Caenorhabditis elegans

Recently, 6-PPD quinone (6-PPDQ), a derivative of tire antioxidant 6-PPD, was reported to have acute toxicity for organisms. However, the possible reproductive toxicity of 6-PPDQ is still largely unclear. In this study, the reproductive toxicity of 6-PPDQ after long-term exposure was further investigated in Caenorhabditis elegans. Exposure to 1 and 10 μg/L 6-PPDQ reduced the reproductive capacity. Meanwhile, exposure to 1 and 10 μg/L 6-PPDQ enhanced the germline apoptosis, which was accompanied by upregulation of ced-3, ced-4, and egl-1 expressions and downregulation of ced-9 expression. The observed increase in germline apoptosis in 1 and 10 μg/L 6-PPDQ exposed nematodes was associated with the enhancement in DNA damage and increase in expressions of related genes of cep-1, clk-2, hus-1, and mrt-2. The detected enhancement in germline apoptosis in 1 and 10 μg/L 6-PPDQ exposed nematodes was further associated with the increase in expressions of ced-1 and ced-6 governing the cell corpse engulfment process. Molecular docking analysis indicated the binding potentials of 6-PPDQ with three DNA damage checkpoints (CLK-2, HUS-1, and MRT-2) and corpse-recognizing phagocytic receptor CED-1. Therefore, our data suggested the toxicity on reproductive capacity by 6-PPDQ at environmentally relevant concentrations by enhancing DNA damage- and cell corpse engulfment-induced germline apoptosis in organisms.

Bioretention Cells Provide a 10-Fold Reduction in 6PPD-Quinone Mass Loadings to Receiving Waters: Evidence from a Field Experiment and Modeling

Road runoff to streams and rivers exposes aquatic organisms to complex mixtures of chemical contaminants. In particular, the tire-derived chemical 6PPD-quinone (N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine-quinone) is acutely toxic to several species of salmonids, which are critical to fisheries, ecosystems, and Indigenous cultures. We therefore urgently require interventions that can reduce loadings of 6PPD-quinone to salmonid habitats. Herein, we conducted a spike and recovery experiment on a full-scale, mature bioretention cell to assess the efficacy of stormwater green infrastructure technologies in reducing 6PPD-quinone loadings to receiving waters. We then interpreted and extended the results of our experiment using an improved version of the "Bioretention Blues" contaminant transport and fate model. Overall, our results showed that stormwater bioretention systems can effectively mitigate >∼90% of 6PPD-quinone loadings to streams under most "typical" storm conditions (i.e., < 2-year return period). We therefore recommend that stormwater managers and other environmental stewards redirect stormwater away from receiving waters and into engineered green infrastructure systems such as bioretention cells.