Phytotoxicity of Brominated Diphenyl Ether-47 (BDE-47) and Its Hydroxylated and Methoxylated Analogues (6-OH-BDE-47 and 6-MeO-BDE-47) to Maize (Zea mays L.)

Polybrominated diphenyl ethers (PBDEs) decreased the protein quality of rice grains by disturbing amino acid metabolism

Polybrominated diphenyl ethers (PBDEs) in soils posed potential risks to crop growth and food safety due to their prevalence and persistence. PBDEs were capable of being absorbed and accumulated into crops, impacting their growth, whereas the interference on metabolic components and nutritional composition deserves further elucidation. This study integrated a combined non-targeted and targeted metabolomics method to explore the influences of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), 2,2',4,4',5-pentabromodiphenyl ether (BDE-99) and decabromodiphenyl ether (BDE-209) on the metabolic responses of rice (Oryza sativa). Metabolic pathways, which were associated with sugars, organic acids, and amino acids, were significantly disturbed under PBDE stresses. Particularly, 75% of the marked altered pathways belonged to amino acid metabolism, with alanine/aspartate/glutamate metabolism being commonly enhanced. The degradation of aspartic acid promoted the formation of downstream amino acids, among which the levels of lysine, methionine, isoleucine, and asparagine were increased by 1.31-3.15 folds compared to the control. Thus, the antioxidant capacity in rice plants was enhanced, particularly through the significant promotion of ascorbic acid-glutathione (AsA-GSH) cycle in rice leaves. The amino acids were promoted to resist reactive oxygen species (ROS) efficiently, thus were deficient for nutrient storage. When exposed to 4 μmol/kg PBDEs, the contents of amino acids and proteins in grains decreased by 9.1-32.1% and 8.6-34.8%, respectively. In particular, glutelin level was decreased by 5.6-41.2%, resulting in a decline in nutritional quality. This study demonstrated that PBDEs deteriorated the protein nutrition in rice grains by affecting amino acid metabolism, providing a new perspective for evaluating the ecological risks of PBDEs and securing agricultural products.

Bioaccumulation of polystyrene nanoplastics and BDE-209 induced oxidative stress, photosynthesis and growth impairments in floating fern Salvinia natans

Aquatic ecosystems are facing increasing exposure to pollutants, posing potential threats to the stability and wellness of aquatic species. This study focused on evaluating the impacts of single and combined exposure to 80 nm polystyrene nanoplastics (PS-NPs, 0.1, 1, 10, 20 mg/L) and decabromodiphenyl ether (BDE-209, 300 ng/L) for 14 days on the bioaccumulation, growth, photosynthesis and oxidative stress in the free-floating fern Salvinia natans. PS-NPs primarily accumulated in the epidermis and trichomes of S. natans. Meanwhile, the levels of superoxide dismutase (SOD) and malondialdehyde (MDA) were significantly increased, while those for peroxidase (POD), catalase (CAT), total antioxidant capacity (T-AOC), and relative growth rate (RGR) decreased. Furthermore, the chlorophyll contents in submerged leaves were decreased, while those in floating leaves were increased at PS-NPs concentrations of 0.1 and 1 mg/L. However, the chlorophyll contents in both submerged and floating leaves displayed a decreasing trend with increasing concentrations of PS-NPs. Under the co-exposure of PS-NPs and BDE-209, the contents of MDA were significantly elevated, whereas CAT, POD, SOD, T-AOC and RGR were significantly decreased (p < 0.05). Our results revealed that, compared to single exposure, more pronounced ecotoxic effects are observed in S. natans under co-exposure to PS-NPs and BDE-209. These findings offer valuable perspectives into the possible environmental risks of BDE-209 and PS-NPs in freshwater ecosystems, contributing to the development of effective management strategies for protecting aquatic organisms and ecosystems. This research highlights the urgent need to understand the toxic effects of emerging contaminants on different aquatic organisms, emphasizing the importance of protecting and preserving aquatic ecosystems.

Cellular and physiological mechanisms of halogenated and organophosphorus flame retardant toxicity

Flame retardants (FRs) are chemical substances used to inhibit the spread of fire in numerous industrial applications, and their abundance in modern manufactured products in the indoor and outdoor environment leads to extensive direct and food chain exposure of humans. Although once considered relatively non-toxic, FRs are demonstrated by recent literature to have disruptive effects on many biological processes, including signaling pathways, genome stability, reproduction, and immune system function. This review provides a summary of research investigating the impact of major groups of FRs, including halogenated and organophosphorus FRs, on animals and humans in vitro and/or in vivo. We put in focus those studies that explained or referenced the modes of FR action at the level of cells, tissues and organs. Since FRs are highly hydrophobic chemicals, their biophysical and biochemical modes of action usually involve lipophilic interactions, e.g. with biological membranes or elements of signaling pathways. We present selected toxicological information about these molecular actions to show how they can lead to damaging membrane integrity, damaging DNA and compromising its repair, changing gene expression, and cell cycle as well as accelerating cell death. Moreover, we indicate how this translates to deleterious bioactivity of FRs at the physiological level, with disruption of hormonal action, dysregulation of metabolism, adverse effects on male and female reproduction as well as alteration of normal pattern of immunity. Concentrating on these subjects, we make clear both the advances in knowledge in recent years and the remaining gaps in our understanding, especially at the mechanistic level.

Biotransformation kinetics and pathways of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) and its hydroxylated and methoxylated derivatives (6-OH-BDE-47 and 6-MeO-BDE-47) in earthworms (Eisenia fetida)

As a class of persistent organic pollutant, polybrominated diphenyl ethers (PBDEs) and their hydroxylated and methoxylated derivatives (OH-PBDEs and MeO-PBDEs) have been widely detected in soil environments. However, studies on the bioavailability and transformation of PBDEs and their derivatives in soil organisms remain scarce. In this study, a detailed kinetic investigation on the accumulation and biotransformations of BDE-47, 6-MeO-BDE-47 and 6-OH-BDE-47 in earthworms (Eisenia fetida) exposed to artificially contaminated soils was conducted. The uptake and elimination kinetics of BDE-47, 6-MeO-BDE-47 and 6-OH-BDE-47 by earthworms were in accordance with a one-compartment first-order kinetic model. The bioaccumulation factors (BAFs) followed the order 6-MeO-BDE-47 > 6-OH-BDE-47 > BDE-47. All three compounds could undergo step-by-step debromination to produce lower brominated analogs in earthworms. Both BDE-47 and 6-OH-BDE-47 could be transformed to MeO-PBDEs, whereas no transformation from 6-OH-BDE-47 or 6-MeO-BDE-47 to PBDEs or from BDE-47 and 6-MeO-BDE-47 to OH-PBDEs took place in the earthworms. Methoxylation was proposed as a potential metabolic pathway to form MeO-PBDEs in earthworms, with the metabolic rates for the methoxylation of BDE-47 and 6-OH-BDE-47 being 27.7 and 5.1 times greater, respectively, than that of the debromination metabolism. The isomers of 6-MeO-BDE-47 and 6-OH-BDE-47 were formed via the addition of methoxy/hydroxy groups or via bromine shifts on benzene ring in the earthworms. This study provides comprehensive information for a better understanding of the accumulation and biotransformation of PBDEs and their derivatives in earthworms.

Polybrominated diphenyl ethers interact with the key protein involved in carbohydrate metabolism in rice

Rice exposed to organic pollutants such as polybrominated diphenyl ethers (PBDEs) usually experiences reduced biomass and increased soluble sugar content. This study showed that 2, 2', 4, 4'-tetrabromodiphenyl ether (BDE-47) led to increased glucose, fructose, and sucrose in rice leaves, accompanied by decreased photosynthetic rate and biomass. In order to identify the key enzyme that BDE-47 interacted with, a diazirine-alkynyl photoaffinity probe was designed, and photoaffinity labeling based chemoproteomics was conducted. Among all differentially expressed proteins, fructose-1, 6-bisphosphate aldolase (FBA) involved in carbohydrate metabolism was most likely the target protein of BDE-47. Spectral techniques and molecular docking analysis further revealed that the pollutant-protein interaction was driven by hydrophobic force. BDE-47 inhibited FBA catalytic efficiency by competing with its substrate, fructose-1, 6-diphosphate (F-1, 6-P), leading to soluble sugar accumulation, photosynthetic rate decline and biomass reduction. This study unraveled the influencing mechanism of PBDEs on rice by combining the novel photoaffinity labeling-based chemoproteomics with conventional proteomics. The improved knowledge on direct interaction between organic pollutants and proteins will help alleviate the harmful effects of soil pollution on plants.

Bromophenol Induced Multiple Stress Responses in Rice Plants: Impact of Doses and Congener Structures

Bromophenols (BPs) have both natural and artificial sources in the environment and are frequently detected in plants. Herein, the ubiquitous 2,4,6-TriBP was hydroponically exposed to rice seedlings at two concentrations (0.2 and 2.0 mg/L) to characterize the dose-dependent abiotic stress responses of rice plants to BPs. The 2,4,6-TriBP induced oxidative damage to rice roots and subsequently inhibited plant transpiration and growth at the end of exposure in both concentrations. Moreover, the gene expression of OsUGT72B1 and the activity of glycosyltransferases of exposed rice roots were 2.36-to-4.41-fold and 1.23-to-1.72-fold higher than that of the blank controls after 24 h, following the formation of glycoconjugates in response to 2,4,6-TriBP exposure. It was notable that the glycosylation rates also showed a dose-effect relationship in rice roots. One and six glycoconjugates of 2,4,6-TriBP were detected in 0.2 and 2.0 mg/L exposure groups, respectively. Considering the detected species of glycoconjugates for four other types of BPs, the numbers of bromine atoms were found to dramatically affect their glycosylation process in rice plants. These results improve our fundamental understanding of the impact of congener structures and exposure concentrations of organic contaminants on the glycosylation process in response to phytotoxicity.

Biomarkers responses and polybrominated diphenyl ethers and their methoxylated analogs measured in Sparus aurata from the Lagoon of Bizerte, Tunisia

This study aimed to the examination of the levels and effects of organobromine compounds (polybrominated diphenyl ethers: PBDEs and methoxylated brominated diphenyl ethers: MeO-PBDEs), in Sparus aurata native to the Lagoon of Bizerte. For that, different biomarkers of exposure (somatic indices, superoxide dismutase, and catalase activities) and effect (malondialdehyde level, histopathologic alterations, and DNA damage) as well as pollutant levels were measured in specimens collected from this impacted ecosystem and the Mediterranean Sea as a reference site. Bizerte Lagoon PBDE fish levels were higher than the Mediterranean Sea, whereas MeO-PBDEs were higher in the reference site. Fish from Bizerte Lagoon presented a higher hepatosomatic index, lower catalase and superoxide dismutase activity, higher level of malondialdehyde, and higher percentage of DNA tail in comparison to fish from the reference area. The histological study of the liver indicated substantial lesions in fish from the polluted site. The results showed strong positive correlations between the concentrations of the PBDE or MeO-PBDE and the MDA and DNA tail % levels and negative correlations for the activities of enzymes of SOD and CAT. Consequently, these findings could suggest a potential link between exposure to these pollutants and the observed biomarker responses in the Bizerte Lagoon seabream. Taken together, these results highlight the importance of biomarker selection and the selected sentinel fish species as useful tools for biomonitoring of aquatic pollution.

The aryl hydrocarbon receptor: A predominant mediator for the toxicity of emerging dioxin-like compounds

The aryl hydrocarbon receptor (AHR) is a member of the basic helix-loop-helix/Per-ARNT-Sim (bHLH-PAS) family of transcription factors and has broad biological functions. Early after the identification of the AHR, most studies focused on its roles in regulating the expression of drug-metabolizing enzymes and mediating the toxicity of dioxins and dioxin-like compounds (DLCs). Currently, more diverse functions of AHR have been identified, indicating that AHR is not just a dioxin receptor. Dioxins and DLCs occur ubiquitously and have diverse health/ecological risks. Additional research is required to identify both shared and compound-specific mechanisms, especially for emerging DLCs such as polyhalogenated carbazoles (PHCZs), polychlorinated diphenyl sulfides (PCDPSs), and others, of which only a few investigations have been performed at present. Many of the toxic effects of emerging DLCs were observed to be predominantly mediated by the AHR because of their structural similarity as dioxins, and the in vitro TCDD-relative potencies of certain emerging DLC congeners are comparable to or even greater than the WHO-TEFs of OctaCDD, OctaCDF, and most coplanar PCBs. Due to the close relationship between AHR biology and environmental science, this review begins by providing novel insights into AHR signaling (canonical and non-canonical), AHR's biochemical properties (AHR structure, AHR-ligand interaction, AHR-DNA binding), and the variations during AHR transactivation. Then, AHR ligand classification and the corresponding mechanisms are discussed, especially the shared and compound-specific, AHR-mediated effects and mechanisms of emerging DLCs. Accordingly, a series of in vivo and in vitro toxicity evaluation methods based on the AHR signaling pathway are reviewed. In light of current advances, future research on traditional and emerging DLCs will enhance our understanding of their mechanisms, toxicity, potency, and ecological impacts.

The phytotoxicity of exposure to two polybrominated diphenyl ethers (BDE47 and BDE209) on photosynthesis and the response of the hormone signaling and ROS scavenging system in tobacco leaves

To reveal the response and adaptative mechanism of plants to the organic pollutants PBDEs, physiological and transcriptomic techniques were used to study the effects of exposure to BDE47 and BDE209 on tobacco (Nicotiana tabacum L.) plant growth, physiological function and response of key genes. Exposure to both BDE47 and BDE209 inhibited the growth of tobacco plants. The number of down-regulated DEGs following exposure to BDE47 was significantly higher than that following exposure to BDE209. Enrichment analysis using the KEGG showed that BDE47 and BDE209 primarily affected tobacco leaf photosynthesis-antenna proteins, photosynthesis, plant hormone signal transduction and α-linolenic acid metabolism. BDE47 primarily inhibits the synthesis of Chl a, and BDE209 has a more significant impact on Chl b. Most photosynthesis-related DEGs were concentrated in PSII and PSI; the number of down-regulated DEGs in PSI was significantly higher than that in PSII, and the range in which the PSI activity was reduced was also higher than that of PSII, i.e., PSII and PSI (particularly PSI) were sensitive to the effects of exposure to BDE47 and BDE209 on photosynthesis. The increase of the ratio of regulatory energy dissipation played an important protective role in alleviating the photoinhibition of PSII. Exposure to BDE47 and BDE209 can lead to the accumulation of ROS in tobacco leaves, but correspondingly, the activities of antioxidant enzymes SOD, POD, CAT, APX and GPX and the up-regulated expression of their coding genes play an important role in preventing excessive oxidative damage. Exposure to BDE47 and BDE209 promoted the up-regulation of gene expression related to Pro synthesis. In particular, the Pro synthetic process of the Orn pathway was promoted. Exposure to BDE47 and BDE209 induced the up-regulated expression of genes related to the synthesis of ABA and JA, promoted the synthesis of ABA and JA, and activated ABA and JA signal transduction pathways. In conclusion, both BDE47 and BDE209 inhibit the synthesis of chlorophyll and hinder the process of light energy capture and electron transfer in tobacco leaves. BDE47 was more toxic than BDE209. However, tobacco leaves can also adapt to BDE47 and BDE209 by regulating the antioxidant system, accumulating Pro and initiating the hormone signal transduction process. The results of this study provide a theoretical basis for the phytotoxicity mechanism of PBDEs.

Bioaccumulation and biotransformation of tetrabromoethylcyclohexane (TBECH) in maize (Zea mays L.): Stereoselective driving roles of plant biomacromolecules

The bioaccumulation and biotransformation of tetrabromoethylcyclohexane (TBECH) in maize were investigated. Furthermore, the roles of plant biomacromolecules such as lipid transfer proteins (LTPs), CYP and GST enzymes in driving the biological processes of TBECH stereoisomers were explored. The uptake and translocation of TBECH in maize were diastereo- and enantio-selective. Isomerization from α- to δ-TBECH and β- to γ-TBECH, and metabolites of debromination, hydroxylation and TBECH-GSH adducts were identified in maize roots. The gene expressions of LTPs, CYPs and GSTs were extensively changed in maize after exposure to technical TBECH. CYP and GST enzyme activities as well as GST31 and CYP71C3v2 gene expressions were selectively induced or inhibited by TBECH diastereomers over time. TBECH was able to dock into the active sites and bind with specific residues of the typical biomacromolecules ZmLTP1.6, GST31 and CYP71C3v2, indicating their roles in the bioaccumulation and metabolization of TBECH. Binding modes and affinities to biomacromolecules were significantly different between α- and β-TBECH, which contributed to their stereo-selectivity. This study provided a deep understanding of the biological fate of TBECH, and revealed the driving molecular mechanisms of the selectivity of TBECH stereoisomers in plants.

Reproductive stimulation and energy allocation variation of BDE-47 and its derivatives on Daphnia magna

As endocrine disrupting chemical, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) is widely distributed in water environment with a high detection rate. 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) and 6-methoxy-2,2',4,4'-tetrabromodiphenyl ether (6-MeO-BDE-47) are two main derivatives of BDE-47. To explore the aquatic risk of BDE-47 and its derivatives, the effects of them and their ternary mixture on the reproduction, growth, energy allocation, and neurological and antioxidant responses of Daphnia magna were monitoring during different exposure periods, i.e., daphnids exposed to compounds for 21 days or pre-exposed to compounds for 14 days and then recovered 7 days in clean water. In general, in 21-day test, reproductive parameters of exposed daphnids were significantly stimulated, and the growth and enzymatic activities of super oxidase dimutase (SOD), glutathione peroxidase (GPx) and acetylcholinesterase (AChE) were significantly depressed by the single- or mixture compounds. In (14 + 7)-day test, the levels of body length, number of living offspring per female and the enzyme activities recovered to some degree. However, after 7 days of recovery in pollution free medium, the reproductive parameters and enzymatic activities of D. magna were unable to restore control values. These results showed that D. magna has a tendency that the energy allocated to reproduction was greater than that to grow after exposure. The energy distribution of D. magna occurred autonomously after being exposed, which can make it better adapt to environmental changes. Moreover, based on the behavioral and enzymology indicators of D. magna, the spider chart's application in the characteristic analysis of function indicators of D. magna implied that SOD, GPx and AChE could become sensitive biomarkers for different exposure periods. Those findings enable us to better understand BDE-47 and metabolites, and are conducive to better take measures to solve the pressure it brings.

Maize plant (Zea mays) uptake of organophosphorus and novel brominated flame retardants from hydroponic cultures

The root uptake and root-shoot translocation of seven organophosphorus flame retardants (OPFRs) and four novel brominated flame retardants (NBFRs) were assessed in this investigation using hydroponic grown maize plants (Zea mays). Three initial liquid concentrations for each considered compound were examined (i.e., 0.3 μg L^-1, 3 μg L^-1, 30 μg L^-1). The results indicated that the 30 μg L^-1 treatments were phytotoxic, as they resulted in a significant decrease in shoot dry weight. Plant-driven removal of the tested FRs decreased with the increasing initial spiking level and were reportedly higher for the NBFRs (range 42%-10%) than OPFRs (range 19%-7%). All the considered FRs were measured in the roots (range 0.020-6.123 μg g^-1 dry weight -DW-) and shoots (range 0.012-1.364 μg g^-1 DW) of the tested plants, confirming that there was uptake. Linear relationships were identified between the chemical concentrations in the plant parts and the tested hydroponic concentrations. Root concentration factors were positively correlated with the specific lipophilicity (i.e., logK_ow) of the tested FRs and were determined to be higher for the NBFRs than the OPFRs. The NBFRs had a higher root uptake rate than the OPFRs, and this trend was more significant with the increasing treatment concentrations. Shoot/root concentration factors were found to be lower than the unity value for 10 of the 11 tested compounds. These results can be related to the specific molecular configurations and the occurrence of different functional groups in the tested compounds. The results will help to improve risk assessment procedures and fine tune our understanding of human receptor responses to the ingestion of maize crops grown on agricultural sites irrigated with water contaminated by FRs.

Plant accumulation and transformation of brominated and organophosphate flame retardants: A review

Plants can take up and transform brominated flame retardants (BFRs) and organophosphate flame retardants (OPFRs) from soil, water and the atmosphere, which is of considerable significance to the geochemical cycle of BFRs and OPFRs and their human exposure. However, the current understanding of the plant uptake, translocation, accumulation, and metabolism of BFRs and OPFRs in the environment remains very limited. In this review, recent studies on the accumulation and transformation of BFRs and OPFRs in plants are summarized, the main factors affecting plant accumulation from the aspects of root uptake, foliar uptake, and plant translocation are presented, and the metabolites and metabolic pathways of BFRs and OPFRs in plants are analyzed. It was found that BFRs and OPFRs can be taken up by plants through partitioning to root lipids, as well as through gaseous and particle-bound deposition to the leaves. Their microscopic distribution in roots and leaves is important for understanding their accumulation behaviors. BFRs and OPFRs can be translocated in the xylem and phloem, but the specific transport pathways and mechanisms need to be further studied. BFRs and OPFRs can undergo phase I and phase II metabolism in plants. The identification, quantification and environmental fate of their metabolites will affect the assessment of their ecological and human exposure risks. Based on the issues mentioned above, some key directions worth studying in the future are proposed.

Comparison of 6:2 chlorinated polyfluorinated ether sulfonate (6:2 Cl-PFESA) and perfluorooctane sulfonate (PFOS) accumulation and toxicity in mung bean

With the regulation of perfluorooctanesulfonate (PFOS), 6:2 chlorinated polyfluoroalkyl ether sulfonate (6:2 Cl-PFESA) has been used as a potential PFOS alternative in electroplating. In this study, the uptake, translocation and phytotoxicity of PFOS and 6:2 Cl-PFESA in mung bean (Vigna radiata (Linn.) Wilczek.) were investigated. The uptake kinetics of PFOS and 6:2 Cl-PFESA fit the Michaelis-Menten equation well, suggesting that the uptake is a carrier-mediated process. The root concentration factor (RCF) of 6:2 Cl-PFESA (34.55 mL g^-1 dw) was 1.27 times that of PFOS (27.11 mL g^-1 dw), and the translocation factor (TF) of 6:2 Cl-PFESA (0.177) was 1.07 times that of PFOS (0.165). Exposure to 6:2 Cl-PFESA and PFOS both resulted in the inhibition of mung bean seedling development. Treatment with 6:2 Cl-PFESA and PFOS led to the concentration-dependent elevation of malondialdehyde (MDA), carbonyl groups, and phosphorylated histone H2AX (γ-H2AX) levels in mung bean roots. The MDA and carbonyl group contents induced by 6:2 Cl-PFESA were 1.10-1.35 and 1.03-1.14 times, respectively, those of PFOS. The hydroxyl free radical (·OH) levels in mung bean roots after exposure to PFOS and 6:2 Cl-PFESA were elevated significantly, and the ·OH levels induced by 6:2 Cl-PFESA were higher than those induced by PFOS. Hydroxyl free radical levels were positively correlated with the MDA and carbonyl group contents in mung bean roots (p < 0.05). The dynamic changes in some antioxidative enzyme activities in mung bean seedlings were determined, including peroxidase (POD), superoxide dismutase (SOD), and catalase (CAT). The results demonstrated the phytotoxicities of 6:2 Cl-PFESA and PFOS to mung bean in the early developmental stage. 6:2 Cl-PFESA is more harmful to mung beans than PFOS. The production of hydroxyl radical is the mechanism that causes the toxicity of PFOS and 6:2 Cl-PFESA toward plants.

The phytotoxicities of agricultural soil samples from a coal gangue stacking area to several maize cultivars (Zea mays L.)

In Shanxi, a major energy province in China, environmental pollution caused by coal gangue accumulation is becoming an increasingly serious problem. In addition, crops are the first trophic level in the human food chain, and the security and production of crops are closely related to human well-being. The objective of this study was to estimate the phytotoxicities of agricultural soil samples contaminated by coal gangue accumulation using maize (Zea mays L.) as a model organism. Finally, a tolerant maize cultivar was screened for coal gangue stacking areas. Seven cultivars of maize seeds were treated with agricultural soil leachate around the coal gangue stacking area at various concentrations of 0, 1:27, 1:9, 1:3, and 1:1. The results revealed that the agricultural soil leachate treatment could inhibit seed germination and the growth of roots and shoots and that the soil leachate-induced phytotoxicities were cultivar-dependent. At the same exposure concentration, tolerant maize cultivar displayed lower toxicity symptoms than sensitive maize cultivar in terms of growth inhibition, oxidative damage, and DNA damage. Stronger activities of antioxidant enzymes were observed in the tolerant maize cultivar than in the sensitive maize cultivar, indicating that the difference between cultivars in antioxidant capacity is one reason for the difference in plant tolerance. Our study provides experimental evidence for the ecological risk assessment of soil and the selection of maize cultivars with high environmental pollutant tolerance for use in coal gangue stacking areas.

Toxic effects of 2,4,4'- trichlorobiphenyl (PCB-28) on growth, photosynthesis characteristics and antioxidant defense system of Lemna minor L

Polychlorinated biphenyls (PCBs) are a common category of persistent man-made organic pollutants that are widespread in the ambient environment. Although Lemna minor L. is an extensively applied plant for aquatic remediation in ecotoxicology research worldwide, little is known regarding its responses to the potentially toxic effects of PCBs. For this study, a 14-day dissolved exposure was conducted to explore the effects of 2,4,4'- trichlorobiphenyl (PCB-28) on the growth, photosynthesis characteristics and antioxidant defense system of L. minor plants. We found that 100 and 200 μg/L of PCB-28 decreased the fresh weight, chlorophyll and protein content, and activities of superoxide dismutase, peroxidase, glutathione S-transferase, and nitroreductase, whereas plasma membrane permeability, and the malondialdehyde and reactive oxygen species concentrations were increased. However, it was observed that 5 and 20 μg/L of PCB-28 had no significant effects on these physiological indices. The ultra-structure of chloroplast demonstrated that 100 and 200 μg/L PCB-28 severely damaged the chloroplast structures. Moreover, correlation analysis revealed that the content of reactive oxygen species had negative correlations with the fresh weight, chlorophyll and protein content, as well as the activities of superoxide dismutase, peroxidase, glutathione S-transferase, and nitroreductase, but had positive correlations with the malondialdehyde content and plasma membrane permeability. This work provides valuable data toward elucidating the physiology and biochemistry of PCBs induced phytotoxicity.

Emerging environmental pollutants hydroxylated polybrominated diphenyl ethers: From analytical methods to toxicology research

Hydroxylated polybrominated diphenyl ethers (OH-PBDEs) are of particular concern due to their ubiquitous distribution and adverse health effects. Significant progress has been made in the characterization of OH-PBDEs by using mass spectrometry (MS). In this review, we summarize applications of MS-based techniques in detection, environmental and biota distribution, and potential health risk effects, hoping to unfold an overall picture on account of current knowledge of OH-PBDEs. The analytical methodologies are discussed from sample pretreatment to MS analysis. The methods including gas chromatography-MS (GC-MS), liquid chromatography-MS (LC-MS), and ion mobility spectrometry-MS (IMS-MS) are discussed. GC-MS is the most frequently adopted method in the analysis of OH-PBDEs due to its excellent chromatographic resolution, high sensitivity, and strong ability for unknown identification. LC-MS has been widely used for its high sensitivity and capability of direct analysis. As a newly developed technique, IMS-MS provides high specificity, which greatly facilitates the identification of isomers. OH-PBDEs pervasively existed in both abiotic and biotic samples, including humans, animals, and environmental matrices. Multiple adverse health effects have been reported, such as thyroid hormone disruption, estrogen effects, and neurotoxicity. The reported potential pathological mechanisms are also reviewed. Additionally, MS-based metabolomics, lipidomics, and proteomics have been shown as promising tools to unveil the molecular mechanisms of the toxicity of OH-PBDEs. © 2020 John Wiley & Sons Ltd. Mass Spec Rev.

Physiological Responses and Phytotoxicities of Lythrum salicaria to Decabromodiphenyl Ether (BDE-209)

Decabromodiphenyl ether (BDE-209), a member of a major group of brominated flame retardants, is detected in aquatic environments at considerable levels and induces physiological and toxic effects on aquatic plants. In this study, the physiological responses induced by and the toxic effects of BDE-209 at different concentrations (0, 0.2, 0.5 and 1.0 mg L^-1) in Lythrum salicaria were examined. OJIP transient curves indicated that BDE-209 treatment negatively affected photosystem II (PSII) grouping. Additionally, the results showed that BDE-209 inhibited seedling development and elevated reactive oxygen species (ROS), phosphorylated histone H2AX (γ-H2AX), malondialdehyde (MDA) levels and antioxidative enzyme activities in the roots and shoots of L. salicaria. The results revealed that BDE-209 exposure contributed to ROS accumulation, which was considered as the probable toxicity mechanism. The current results provided an insight into the development of L. salicaria with high BDE-209 tolerance.

Biochemical and molecular responses of maize (Zea mays L.) to 1,2-dibromo-4-(1,2 dibromoethyl) cyclohexane (TBECH) diastereomers: Oxidative stress, DNA damage, antioxidant enzyme gene expression and diversity of root exudates

The phytotoxicities of TBECH diastereomers to plants at the biochemical and molecular levels were investigated in a hydroponic study by using maize as a model plant. The results showed that TBECH could induce the production of two species of reactive oxygen species (ROS), O₂^•- and H₂O₂, in maize tissues. The accumulation of ROS was the highest when maize was exposed to β-TBECH. TBECH enhanced the phosphorylation of plant histone, and the contents of γ-H2AX in maize followed the order β-TBECH > αβ-TBECH > γδ-TBECH > γ-TBECH. Transcriptome profiling revealed that antioxidant enzyme genes (AEGs) were over-expressed in maize when stressed by technical grade TBECH. The RT-PCR detection further validated that three typical AEGs, including CAT, SOD, and POD genes, were time-dependent and selectively expressed under the influence of TBECH diastereomers. Molecular compositions of maize root exudates characterized by FT-ICR-MS were significantly different among the four groups of TBECH diastereomer treatments. TBECH diastereomers specifically affected the chemical diversity and abundance of root exudates. New insights into the biochemical effects of TBECH on plants are provided in this work, which is helpful to deepening the understanding of their stereo-selectivity.

Fate of 4-bromodiphenyl ether (BDE3) in soil and the effects of co-existed copper

The quantitative fate of polybrominated diphenyl ethers (PBDEs) in soil is unknown. Furthermore, the effects of co-contamination by toxic copper on the behavior of PBDEs have not been investigated. Using a ¹⁴C-tracer, we studied mineralization, metabolism, and formation of non-extractable residues (NERs) of one PBDE congener, i.e., the 4-bromodiphenyl ether (BDE3) in oxic soil for 50 days, without and with amendment of Cu (400 mg kg^-1 soil dw). BDE3 rapidly dissipated with a half-life of 5.5 days and large amounts of CO₂ (38.8 ± 0.3% of initial applied amount at the end of incubation) and NERs (42.5 ± 0.4%) were rapidly produced. One hydroxylated metabolite (4'-HO-BDE3) was formed (8.1 ± 0.6%) at the beginning of the incubation, but then decreased to 2.2 ± 0.4%. Only BDE3 occurred in physico-chemically entrapped NERs, amounting to 9.2 ± 0.7%, while only 4'-HO-BDE3 in ester-linked NERs (10.9 ± 0.7%). The addition of Cu strongly reduced the kinetics constants of the transformations (including dissipation, mineralization, and NER-formation), the predicted maximal amounts of mineralization, as well as covalent binding of 4'-HO-BDE3 to soil. The results provide first quantitative insights into the fate of low-brominated congeners of PBDEs in soil and indicate that co-contamination by Cu may increase the environmental risks of biodegradable PBDEs in soil by increasing their persistence.

The bioaccumulation, elimination, and trophic transfer of BDE-47 in the aquatic food chain of Chlorella pyrenoidosa-Daphnia magna

As a persistent organic pollutant, 2,2',4,4'-tetrabromodiphenyl ether (BDE-47) has been widely detected in aquatic environments. However, studies on the fate and transfer of BDE-47 in the aquatic food chain remain scarce. In this study, we investigated the bioaccumulation and elimination of BDE-47 in Chlorella pyrenoidosa, as well as the trophic transfer and biomagnification of BDE-47 in the "C. pyrenoidosa-Daphnia magna" food chain, using C-14 radioactive tracer technology. After 96 h of BDE-47 exposure, the algae accumulated 88.98% ± 0.59% of the initial radioactivity from the medium, and 36.09% ± 9.22% of the accumulated residues in the algae occurred in the form of bound residues. During 96 h of elimination, only 13% ± 0.50% of accumulated radioactivity in the algae was released into the medium. After 24 h of exposure, D. magna accumulated 35.99% ± 2.55% of the initial radioactivity via water filtration from the medium, and 31.35% ± 1.92% of the accumulated radioactivity in D. magna occurred as bound residues. However, D. magna accumulated 66.89% ± 2.37% of the accumulated radioactivity in the algae via food uptake from the contaminated algae, with a high portion of radioactivity observed as bound residues (83.40% ± 0.97% of accumulated radioactivity in D. magna). This indicated a reduction in the environmental risk of BDE-47. There was obvious biomagnification in the food chain between C. pyrenoidosa and D. magna (biomagnification factors, BMFs>1), resulting in environmental hazard transfer in the aquatic food chain. However, no metabolite was found during the exposure experiment, and further studies should be carried out to investigate the intrinsic mechanisms of the trophic transfer of BDE-47, especially in multilevel food chains. Therefore, this study elucidated the effect of dietary uptake on the bioaccumulation of BDE-47 in D. magna and provided new insight for future analysis regarding the bioaccumulation and biomagnification of organic pollutants in the food chain.

Responses of plants to polybrominated diphenyl ethers (PBDEs) induced phytotoxicity: A hierarchical meta-analysis

Biologists have extensively investigated the toxicity of polybrominated diphenyl ethers (PBDEs) on plants in ecosystems, where experiments revealed that PBDEs can promote, inhibit, or have no significant effects on the physiological and biochemical functionality of plants. These studies have stimulated many theoretical works that aimed to elucidate the differences in the toxicity of PBDEs on various plants. However, there has been no quantitative attempt to reconcile theory with the results of empirical experiments. To close this gap between theory and experiments, we conducted a hierarchical meta-analysis to examine the toxicity of PBDEs on plants and confirmed potential sources of variation across numerous studies. Through the analysis of 1299 observations garnered from 41 studies, we revealed the significant toxicity of PBDEs on plants. This result was verified to be robust and showed no signs of bias. Our study affirmed that functional indexes can contribute to variations that lead to the toxicity of PBDEs on various plants. Furthermore, we found that lower congeners PBDEs were more toxic to plants than higher congeners PBDEs, and higher plants were more resistant to PBDEs induced phytotoxicity than lower plants. For interactive effects, only specific PBDEs concentrations had significant effects on glutathione S-transferase activities, and experimental durations had no significant impacts on any functional indexes. These results reconciled empirical studies and assisted us with elucidating the ecotoxicology of PBDEs induced phytotoxicity.

Interactions between decabromodiphenyl ether and lead in soil-plant system

Pot experiments were conducted under abiotic conditions to investigate the interactive influence of decabromodiphenyl ether (BDE-209) and lead (Pb) on the seed germination, germ length, root exudation and physiological characteristics of tall fescue (Festuca arundinaceae), and the uptake, accumulation of Pb and BDE-209 in the plant tissues. Results show that seed germination and germ length were impacted by Pb but less influenced by BDE-209. BDE-209 spiking (10 and 50 mg/L) could alleviate the toxicity of high Pb concentration on seed germination and growth. The chlorophyll content was significantly increased at 500 mg/kg Pb but declined at 2000 mg/kg Pb. Low-level Pb contamination (500 mg/kg) activated antioxidase activity; however, 2000 mg/kg Pb significantly reduced the antioxidase activity. Plant biomass slightly decreased at 500 mg/kg Pb but significantly declined at 2000 mg/kg Pb. The addition of a moderate dosage of BDE-209 (10-50 mg/kg) lessened Pb phytotoxicity, leading to improved plant growth relative to the case of Pb spiking alone. The exudate secretion was significantly enhanced by Pb addition, but BDE-209 spiking only caused slightly increased secretion. Pb could interfere with BDE-209 adsorption and translocation of tall fescue by affecting physiological behavior of the plant, but BDE-209 exhibited little influence on the Pb fate in the plant. Overall, BDE-209 had slight interference on the impact of Pb towards tall fescue. The results demonstrate the complex interactive effects of organic pollutants and heavy metals in the soil-plant system.

Could mangrove plants tolerate and remove BDE-209 in contaminated sediments upon long-term exposure?

Polybrominated diphenyl ethers (PBDEs) such as BDE-209, the commonest congener, are known to be toxic. A 24-months study using mangrove mesocosms with mixed mangrove species, namely Avicennia marina (Am), Aegiceras corniculatum (Ac) and Kandelia obovata (Ko), or without any plant was conducted to examine toxicity, removal, translocation and uptake of BDE-209. At month 24, BDE-209 stimulated the production of root superoxide radical (O₂^-*), and leaf and root malondialdehyde (MDA) of Ko, enhanced leaf O₂^-* of Ac, but did not affect the production of O₂^-* and MDA in Am. These findings indicated that the tolerance to BDE-209 was species-specific, with Am being the most tolerant and Ko the most sensitive species. In leaf and root, BDE-209 stimulated peroxidase (POD) activity in both Ac and Ko, and superoxide dismutase (SOD) in Am. After 24-months, more than 60% and 40% of BDE-209 in contaminated sediments were removed in planted and unplanted groups, respectively, with more PBDEs in upper than bottom sediment layers. This study demonstrates that planting tolerant species such as Avicennia marina with high uptake could remedy PBDEs in contaminated sediments.

Insight into the tolerance, biochemical and antioxidative response in three moss species on exposure to BDE-47 and BDE-209

Responses of Hypnum plumaeforme, Thuidium cymbifolium, and Plagiomnium cuspidatum to short-term (96 h) BDE-47 and BDE-209(0, 0.005, 0.05, 0.5, and 5 μM, respectively) stress were investigated. Both BDE-47 and BDE-209 increased the lipid peroxidation in the three moss species, malondialdehyde (MDA) content increased with the elevated concentration of contaminants, and followed the order: P. cuspidatum > H. plumaeforme > T. cymbifolium on exposure to different concentrations. BDE-47 and BDE-209 stimulated the superoxide dismutase (SOD) and peroxidase (POD) activity of the three moss species, indicating that they played an important role in preventing oxidative stress. Reactive oxygen species (ROS) accumulation was positively correlated with the level of contaminants. The response of anti-oxidative enzymes to BDE-47 and BDE-209 stress differed among the three species. At 5  μM BDE-47 and BDE-209 treatment, the chlorophyll content of T. cymbifolium was even a little higher than the control group. Proline played an important role for the scavenging of ROS in P. cuspidatum and T. cymbifolium. In summary, BDE-47 was more toxic to the three moss species than BDE-209. P. cuspidatum was the most sensitive and T. cymbifolium was the most tolerant species to BDE-47 and BDE-209 stress. The strong resistance and tolerance of T. cymbifolium, combined with sensitive/moderate anti-oxidative response could elucidate its potential use as bio-indicator in the ecological risk assessment of BDE-47 and BDE-209 contamination.

Epigenetic Effects of Polybrominated Diphenyl Ethers on Human Health

Disruption of epigenetic regulation by environmental toxins is an emerging area of focus for understanding the latter's impact on human health. Polybrominated diphenyl ethers (PBDEs), one such group of toxins, are an environmentally pervasive class of brominated flame retardants that have been extensively used as coatings on a wide range of consumer products. Their environmental stability, propensity for bioaccumulation, and known links to adverse health effects have evoked extensive research to characterize underlying biological mechanisms of toxicity. Of particular concern is the growing body of evidence correlating human exposure levels to behavioral deficits related to neurodevelopmental disorders. The developing nervous system is particularly sensitive to influence by environmental signals, including dysregulation by toxins. Several major modes of actions have been identified, but a clear understanding of how observed effects relate to negative impacts on human health has not been established. Here, we review the current body of evidence for PBDE-induced epigenetic disruptions, including DNA methylation, chromatin dynamics, and non-coding RNA expression while discussing the potential relationship between PBDEs and neurodevelopmental disorders.

Solubility, uptake, and translocation of BDE 47 as affected by DOM extracted from agricultural wastes

Dissolved organic matter (DOM) extracted from wheat straw (SDOM) and cow manure (MDOM) were used to investigate their effects on the solubilization, uptake, and translocation of 2,2',4,4'-tetrabromodiphenyl ether (BDE-47). Partition coefficients (K_DOC) of BDE 47 between water and the two types of DOM were measured by the solubility enhancement method. The uptake and translocation of BDE 47 by wheat plants were explored by hydroponic exposure experiments. In the range of 0 to 100 mg/L of DOM, the solubility of BDE 47 increased with increasing concentrations of DOM. The log [K_DOC] values of BDE 47 in SDOM and MDOM solutions were 5.77 and 5.31, respectively. The log [K_DOC] values of BDE 47 in SDOM solutions were higher than those in MDOM solutions, which might be ascribed to the higher content of aliphatic carbon and lower molecular weight of SDOM. The addition of DOM (50 mg/L) significantly increased the accumulation of BDE 47 in the shoots of wheat plants. Wheat straw DOM had greater effect than MDOM in enhancing the accumulation of BDE 47. This study demonstrated the potential risk of BDE 47 to plants resulting from DOM-facilitated transport or the changes in metabolic properties.

Growth, physiological function, and antioxidant defense system responses of Lemna minor L. to decabromodiphenyl ether (BDE-209) induced phytotoxicity

Polybrominated diphenyl ethers (PBDEs), represent one of the new types of persistent organic pollutants (POPs) that are currently found in ambient aquatic ecosystems. Lemna minor L. is a floating freshwater plant, which is widely employed for phytotoxicity studies of xenobiotic substances. For this study, we investigated the growth, physiological functions, and antioxidant capacities of L. minor, which were exposed to 0-20 mg L^-1 decabromodiphenyl ether (BDE-209) for 14 days. A logistic model was suitable for describing the growth of L. minor when the BDE-209 concentration was in the range of from 0 to 15 mg L^-1. When exposed to 5 and 10 mg L^-1 BDE-209, the growth of L. minor was significantly increased, where the intrinsic rate (r) and the maximum capacity of the environment (K) of L. minor were significantly higher than those of the control. In this case, the chlorophyll content and soluble proteins were also markedly increased. Moreover, the photosynthetic function (Fv/Fm, PI) was enhanced. However, for 15 mg L^-1 BDE-29 treated group, the growth of L. minor was significantly inhibited, with decreases in chlorophyll and the soluble protein content, until the L. minor yellowed and expired under a concentration of 20 mg L^-1. Photosynthetic functions were also negatively correlated with increasing increments of BDE-209 (15 and 20 mg L^-1). The malondialdehyde (MDA), superoxide anion radical (O₂^̄·) content, and permeability of the plasma membranes increased with higher BDE-209 concentrations (0-20 mg L^-1). The superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities of L. minor increased when the BDE-209 concentration ranged from 0 to 10 mg L^-1; however, the activities of SOD and POD were decreased. Only the CAT activity remained higher in contrast to the control group under 15-20 mg L^-1 BDE-209. These results demonstrated that 15 mg L^-1 BDE-209 imparted high toxicity to L. minor, which was a consequence of the overproduction of reactive oxygen species (ROS), which conveyed oxidative damage to plant cells. This study provided a theoretical understanding of BDE-209 induced toxicity as relates to the physiology and biochemistry of higher hydrophytes.

Growth and antioxidative response of two mangrove plants to interaction between aquaculture effluent and BDE-99

Mangroves are subject to contamination of polybrominated diphenyl ethers (PBDEs) due to waste and wastewater disposal, and aquaculture effluent (AE) from nearby aquaculture activities. However, the response of mangrove plants to these two stresses and their interaction has seldom been reported. A six-month microcosm study, planted with either Kandelia obovata (Ko) or Avicennia marina (Am), the two most dominant species in South China mangrove swamps, was conducted to investigate the effects of BDE-99, and the interactions of BDE-99 (one of the most abundant PBDE congeners) and AE on growth and physiological responses of these plants. In addition to mixed stressors, both stressors were also applied individually. Results showed that Avicennia was more tolerant to BDE-99 contamination than Kandelia, as reflected by the reduced biomass, but increased superoxide radical (O₂^-⁎) release and malondialdehyde (MDA) content in Kandelia. Addition of AE alleviated toxicity of BDE-99 in Kandelia by promoting biomass but lowering oxidative stress and MDA production. The hormesis model also demonstrated that the interaction between BDE-99 and AE on leaf and root MDA and O₂^-⁎ content in both Kandelia and Avicennia were mostly antagonistic. Activities of catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) in both leaf and root of Kandelia were reduced by BDE-99. On the contrary, BDE-99 significantly enhanced the three enzyme activities in Avicennia root at month 3. Addition of AE also significantly enhanced root CAT, POD and SOD activities, and leaf SOD in both plant species to remove excess ROS produced under BDE-99 exposure. These results indicated that the tolerance of mangrove plants to oxidative stresses depended on antioxidative enzymes that were inducible.

Effects of biochar on 2, 2', 4, 4', 5, 5'-hexabrominated diphenyl ether (BDE-153) fate in Amaranthus mangostanus L.: Accumulation, metabolite formation, and physiological response

The accumulation and metabolism of 2, 2', 4, 4', 5, 5'-hexabrominated diphenyl ether (BDE-153) in Amaranthus mangostanus L. (amaranth) as affected by different concentrations of biochar (1.3 to 26.6 g/L) under hydroponic conditions exposed to 10 μg/L BDE-153 after 10 days were investigated. Biochar significantly reduced BDE-153 shoot and root content by 27.5-61.6% and 73-95.3%, respectively. In general, BDE-153 migration from solution to amaranth decreased with increasing the doses of biochar. BDE-153 metabolites altered with doses of biochar. The ratio of de-BDEs to BDE-153 in root was polynomial correlated to biochar dose (R² = 0.9356**). Root and shoot Fe content was positively correlated with the BDE-153 amounts (R² = 0.948** and 0.822*, respectively). Though the higher biochar dose could obviously control BDE-153 uptake by the vegetable, the toxicity was caused more significantly. For instances, the high concentration of biochar at 26.6 g/L reduced pigment content, increased total ROS, and elevated antioxidant enzyme activity. At the same time, the O₂^- intensity was linearly positively correlated with de-BDEs in root (R² = 0.7324*) while photosynthetic parameter F_v/fm intensity was polynomial correlated to BDEs in shoot (R² = 0.9366*). Transmission electron microscopy (TEM) confirmed that exposure to BDE-153 and high concentration biochar at 26.6 g/L severely altered the chloroplasts in terms of the organelle shape and the presence of starch granules in the chloroplast. Taken together, biochar as a soil amendment could significantly control BDE-153 uptake and enhance BDE-153 metabolism in vegetables, but considering the dose of biochar to avoid its toxicity with higher dose.

Neurobehavioral effects of two metabolites of BDE-47 (6-OH-BDE-47 and 6-MeO-BDE-47) on zebrafish larvae

Two metabolites, OH-BDEs and MeO-BDEs, of polybrominated diphenyl ethers (PBDEs) were ubiquitously detected in animal tissues and environmental samples, drawing a widely public concern to their toxicity. The comparison of toxicity between PBDEs and their metabolites has been a focus in recent years, however, comparisons seldom involve neurobehavioral toxicity of PBDEs metabolites in published works. In this study, zebrafish larvae were exposed to 6-OH-BDE-47 and 6-MeO-BDE-47 and their neurobehavioral traits (including locomotion, path angle, and social activity) were recorded using the instrument Zebrabox; meanwhile, light illumination was used as stimuli in the test duration. The results showed larvae were more active in dark periods than light periods, and preferred turning right (+) to left (-). Effects of the two metabolites varied in different behavioral indicators. They induced different effects on path angle but did not reverse the left-right asymmetry. 6-OH-BDE-47 did not induce the effects on larval locomotion and social activity, but mainly decreased average and routine turn numbers; 6-MeO-BDE-47 promoted larvae responsive turns but inhibited social activity. This study offered new experimental means to the neurobehavioral toxicity of various PBDE metabolites. Further studies may focus on the toxic mechanisms of specific neurobehavioral traits.

Metabolomic analysis of two rice (Oryza sativa) varieties exposed to 2, 2', 4, 4'-tetrabromodiphenyl ether

Polybrominated diphenyl ethers (PBDEs) are toxic chemicals widely distributed in the environment, but few studies are available on their potential toxicity to rice at metabolic level. Therefore we exposed ten rice (Oryza sativa) varieties to 2,2',4,4'-tetrabromodiphenyl ether (BDE-47), a predominant congener of PBDEs, in hydroponic solutions with different concentrations. Two varieties that showed different biological effects to BDE-47, YY-9 and LJ-7, were screened as sensitive and tolerant varieties according to changes of morphological and physiological indicators. Metabolic research was then conducted using gas chromatography-mass spectrometry combined with diverse analyses. Results showed that LJ-7 was more active in metabolite profiles and adopted more effective antioxidant defense machinery to protect itself against oxidative damages induced by BDE-47 than YY-9. For LJ-7, the contents of 13 amino acids and 24 organic acids, especially l-glutamic acid, beta-alanine, glycolic acid and glyceric acid were up-regulated significantly which contributed to scavenging reactive oxygen species. In the treatment of 500 μg/L BDE-47, the contents of these four metabolites increased by 33.6-, 19.3-, 10.6- and 10.2-fold, respectively. The levels of most saccharides (such as d-glucose, lactulose, maltose, sucrose and d-cellobiose) also increased by 1.7-12.4 fold which promoted saccharide-related biosynthesis metabolism. Elevation of tricarboxylic acid cycle and glyoxylate and dicarboxylate metabolism enhanced energy-producing processes. Besides, the contents of secondary metabolites, chiefly polyols and glycosides increased significantly to act on defending oxidative stress induced by BDE-47. In contrast, the levels of most metabolites decreased significantly for YY-9, especially those of 13 amino acids (by 0.9%-67.1%) and 19 organic acids (by 7.8%-70.0%). The positive metabolic responses implied LJ-7 was tolerant to BDE-47, while the down-regulation of most metabolites indicated the susceptible nature of YY-9. Since metabolic change might affect the yield and quality of rice, this study can provide useful reference for rice cultivation in PBDEs-polluted areas.

The phytotoxicities of decabromodiphenyl ether (BDE-209) to different rice cultivars (Oryza sativa L.)

Decabromodiphenyl ether (BDE-209), as a major component of brominated flame retardants, has been detected in the agricultural soil in considerable amount. Given that BDE-209 is toxic, ubiquitous and persistent, BDE-209 might induce toxic effects on rice cultivars planted in contaminated soil. A comparative study was conducted on phytotoxicities and GC-MS based antioxidant-related metabolite levels to investigate the differences of phytotoxicities of BDE-209 to rice cultivars in Yangtze River Delta of China. Rice seedlings were treated with BDE-209 at 0, 10, 50, 100 and 500 μg/L in a hydroponic setup. Results showed that BDE-209-induced phytotoxicites were cultivar-dependent and that the antioxidant defense systems in the cultivars were disturbed differently. Among the three selected cultivars (Jiayou 5, Lianjing 7 and Yongyou 9), Jiayou 5 and Lianjing 7 displayed lower toxic effects than Yongyou 9 in terms of the growth inhibition, lipid peroxidation and DNA damage. The increases of antioxidant enzymes were significantly higher in Jiayou 5 and Lianjing 7 than those in Yongyou 9. Multivariate analysis of antioxidant-related metabolites in the three cultivars indicated that l-tryptophan and l-valine were the most important ones among 10 metabolites responsible for the separation of cultivars. The up-regulation of l-tryptophan and l-valine were likely plant strategies to increase their tolerance. The current results provided an insight into the development of rice cultivars with higher BDE-209 tolerance.

Single and mixture toxicities of BDE-47, 6-OH-BDE-47 and 6-MeO-BDE-47 on the feeding activity of Daphnia magna: From behavior assessment to neurotoxicity

Although 2,2',4,4'-tetrabrominated diphenyl ether (BDE-47), 6-hydroxy-2,2',4,4'-tetrabromodiphenyl ether (6-OH-BDE-47) and 6-methoxy-2,2',4,4'-tetrabromodiphenyl ether (6-MeO-BDE-47) clearly disrupt the endocrine system, current knowledge of their single and/or mixture toxicities on other behaviors of aquatic organisms remains limited. In the present study, Daphnia magna was used to investigate the single and mixture toxicities of BDE-47, 6-OH-BDE-47 and 6-MeO-BDE-47 as measured by inhibition of feeding during exposure and post-exposure periods. Additionally, the biochemical performance, i.e., the activities of super oxidase dismutase (SOD), glutathione peroxidase (GPx) and acetylcholinesterase (AChE) of the test organism was studied to investigate the potential mechanisms of the toxicity of the target compounds. The three target compounds produced an obvious depressive effect on feeding behavior during the exposure period, and the effect increased with increasing concentrations. D. magna was most sensitive to 6-OH-BDE-47. The toxicity of the ternary mixture showed an obvious concentration-dependent effect, whereas the binary mixture toxicity showed the characteristics of hormesis. During the post-exposure period, overcompensation occurred, indicating a short-term effect of the target compounds on D. magna. Additionally, significant changes occurred in neurological responses, indicating that these compounds might have neurobehavioral toxicity in D. magna. The decrease in oxidative stress enzymes (SOD and GPx) indicated that the antioxidant response of D. magna was destroyed.

Uptake, translocation, and metabolism of hydroxylated and methoxylated polychlorinated biphenyls in maize, wheat, and rice

Hydroxylated polychlorinated biphenyls (OH-PCBs) have been found in the environment with high toxicity. Recently, methoxylated polychlorinated biphenyls (MeO-PCBs) were identified as new pollutants and detected in sewage sludge. This study presents a detailed investigation on the uptake, translocation, and metabolism of OH-PCBs and MeO-PCBs in typical crops including maize, wheat, and rice. The interconversion between OH-PCBs and MeO-PCBs were observed. Demethylation of MeO-PCBs was favored over methylation of OH-PCBs. The metabolites were mainly generated in the roots and then translocated to the shoots. Analog-specific differences showed that the accumulation amounts of MeO-PCBs were higher than those of OH-PCBs in the crops. The translocation abilities followed this order: 3'-OH-CB-65 > 4'-OH-CB-101 > 3'-MeO-CB-65 > 4'-MeO-CB-101. The conversion rates were generally higher for 4'-OH-CB-101 than 3'-OH-CB-65 and higher for 4'-MeO-CB-101 than 3'-MeO-CB-65. Interspecies variability among the crops was also observed. The amounts of metabolites and acropetal translocation inside the plants were the greatest for maize. However, the concentration of compounds normalized by the mass of corresponding plant tissue was highest in wheat. These findings provide valuable information for a better understanding of the phytoaccumulation and phytotransformation of OH-PCBs and MeO-PCBs.

Uptake, Translocation, and Biotransformation of Organophosphorus Esters in Wheat (Triticum aestivum L.)

The uptake, translocation and biotransformation of organophosphate esters (OPEs) by wheat (Triticum aestivum L.) were investigated by a hydroponic experiment. The results demonstrated that OPEs with higher hydrophobicity were more easily taken up by roots, and OPEs with lower hydrophobicity were more liable to be translocated acropetally. A total of 43 metabolites including dealkylated, oxidatively dechlorinated, hydroxylated, methoxylated, and glutathione-, and glucuronide- conjugated products were detected derived from eight OPEs, with diesters formed by direct dealkylation from the parent triesters as the major products, followed with hydroxylated triesters. Molecular interactions of OPEs with plant biomacromolecules were further characterized by homology modeling combined with molecular docking. OPEs with higher hydrophobicity were more liable to bind with TaLTP1.1, the most important wheat nonspecific lipid transfer protein, consistent with the experimental observation that OPEs with higher hydrophobicity were more easily taken up by wheat roots. Characterization of molecular interactions between OPEs and wheat enzymes suggested that OPEs were selectively bound to TaGST4-4 and CYP71C6v1 with different binding affinities, which determined their abilities to be metabolized and form metabolite products in wheat. This study provides both experimental and theoretical evidence for the uptake, accumulation and biotransformation of OPEs in plants.

Antioxidative response of Kandelia obovata, a true mangrove species, to polybrominated diphenyl ethers (BDE-99 and BDE-209) during germination and early growth

A 3-months microcosm experiment with mangrove sediment spiked with PBDEs and planted with propagules of Kandelia obovata was conducted to investigate PBDE toxicity and antioxidative responses of the germinated seedlings. BDE-99 suppressed germination rate, leaves formation and growth of mangrove seedlings. The leaves and roots of BDE-99 treated seedlings had significantly higher superoxide (O₂^-) release, malondialdehyde (MDA) and total polyphenol (TP) content, and peroxidase (POD) activity than the control. BDE-209 increased activities of all three antioxidative enzymes, catalase (CAT), POD and superoxide dismutase (SOD) in roots, but in leaves, only CAT activity was stimulated. The MDA content of BDE-209 treated seedlings was less than the control. PBDEs were found in plant tissues of the treated seedlings. These results indicated that even though PBDEs were taken up in tissues, K. obovata, due to its antioxidative defense enzymes, could tolerate PBDEs and could be used for the bioremediation of PBDE-contaminated environments.

Linking toxicity profiles to pollutants in sludge and sediments

Obtaining a complex picture of how pollutants synergistically influence toxicity of a system requires statistical correlation of chemical and ecotoxicological data. In this study, we determined concentrations of eight potentially toxic metals (PTMs) and four groups of organic pollutants in 15 sewage sludge and 12 river sediment samples, then linked measured contaminant concentrations to the toxicity of each matrix through constrained correspondence analysis (CCA). In sludge samples, Hg, As, hexachlorohexane (HCH), polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD) influenced the toxicity profiles, with the first four having significant effects and HBCD being marginally significant. In sediment samples, Hg, As, PBDEs, hexachlorobenzene (HCB), dichlorodiphenyltrichloroethane (DDT), HBCD, HCH and polycyclic aromatic hydrocarbons (PAHs) were found to explain toxicity profiles with Hg, As, PBDEs, HCB, DDT, HBCD, and HCH having significant effects and PAHs being marginally significant. Interestingly, HCH was present in small amounts yet proved to have a significant impact on toxicity. To the contrary, PAHs were often present in high amounts, yet proved to be only marginally significant for sediment toxicity. These results indicate that statistical correlation of chemical and ecotoxicological data can provide more detailed understanding of the role played by specific pollutants in shaping toxicity of sludge and sediments.

Transformation of hydroxylated and methoxylated 2,2',4,4',5-brominated diphenyl ether (BDE-99) in plants

The occurrence and fate of hydroxylated polybrominated diphenyl ethers (OH-PBDEs) and methoxylated polybrominated diphenyl ethers (MeO-PBDEs) have received significant attention. However, there is limited information on the metabolism relationship between OH-pentaBDEs and MeO-pentaBDEs that were frequently detected with relatively high concentrations in the environment. In this study, the biotransformation between OH-BDE-99 and MeO-BDE-99 was investigated in rice, wheat, and soybean plants. All the three plants can metabolize OH-BDE-99 to corresponding homologous methoxylated metabolites, while the transformation from MeO-BDE-99 to OH-BDE-99 could only be found in soybean. The conversion of parent compounds was the highest in soybean, followed by wheat and rice. Transformation products were found mainly in the roots, with few metabolites being translocated to the shoots and solution after exposure. The results of this study provide valuable information for a better understanding of the accumulation and transformation of OH-PBDEs and MeO-PBDEs in different plants.

Direct and dissolved oxygen involved photodegradation of MeO-PBDEs in water

Photodegradation has been proved to be a crucial way of elimination for polybrominated diphenyl ethers (PBDEs) and hydroxylated PBDEs (HO-PBDEs). However, it is still unknown whether methoxylated PBDEs (MeO-PBDEs) can also undergo photodegradation. In this study, 4'-MeO-BDE-17, 5-MeO-BDE-47, 5'-MeO-BDE-99, 6-MeO-BDE-47 and 6-MeO-BDE-85 were selected as targets to investigate their photodegradation in water. Meanwhile, the effects of dissolved oxygen on the photoreactions of MeO-PBDEs were also unveiled. Simulated sunlight experiments indicate that 6-MeO-BDE-47 resisted photodegradation for 20h, while other MeO-PBDEs underwent relatively fast photodegradation, which was greatly susceptible to the substitution patterns of methoxyl and bromine. Photo-excited MeO-PBDEs (except 6-MeO-BDE-47) can sensitize dissolved oxygen to generate singlet oxygen ((1)O2) and superoxide anion radical (O2(-)). The generated (1)O2 cannot degrade the MeO-PBDEs, whereas O2(-) was reactive with MeO-PBDEs. The contribution of dissolved oxygen to the photodegradation of 4'-MeO-BDE-17 and 6-MeO-BDE-85 was negligible; while the negative contribution was observed for 5-MeO-BDE-47 and 5'-MeO-BDE-99. Hydrodebromination was a crucial photodegradation pathway for MeO-PBDEs (excluding 4'-MeO-BDE-17 and 6-MeO-BDE-47). Eventually, direct photolysis half-lives of MeO-PBDEs except 6-MeO-BDE-47 in the surface waters at 40 N latitude were calculated to be 1.35-3.46d in midsummer and 6.39-17.47d in midwinter.

Uptake, translocation and biotransformation kinetics of BDE-47, 6-OH-BDE-47 and 6-MeO-BDE-47 in maize (Zea mays L.)

This study presents a detailed kinetic investigation on the uptake, acropetal translocation and transformation of BDE-47, 6-OH-BDE-47 and 6-MeO-BDE-47 in maize (Zea mays L.) by hydroponic exposure. Root uptake followed the order: BDE-47 > 6-MeO-BDE-47 > 6-OH-BDE-47, while 6-OH-BDE-47 was the most prone to acropetal translocation. Debromination rates of BDE-47 were 1.31 and 1.46 times greater than the hydroxylation and methoxylation rates, respectively. Transformation from BDE-47 to lower brominated OH/MeO-PBDEs occurred mainly through debromination first followed by hydroxylation or methoxylation. There was no transformation from 6-OH-BDE-47 or 6-MeO-BDE-47 to PBDEs. Methylation rate of 6-OH-BDE-47 was twice as high as that of 6-MeO-BDE-47 hydroxylation, indicating methylation of 6-OH-BDE-47 was easier and more rapid than hydroxylation of 6-MeO-BDE-47. Debromination and isomerization were potential metabolic pathways for 6-OH-BDE-47 and 6-MeO-BDE-47 in maize. This study provides important information for better understanding the mechanism on plant uptake and transformation of PBDEs.