Accumulation and disposition of hexabromocyclododecane (HBCD) in juvenile rainbow trout (Oncorhynchus mykiss)

Co-metabolic degradation and metabolite detection of hexabromocyclododecane by Shewanella oneidensis MR-1

Hexabromocyclododecane (HBCD) is a widely used brominated flame retardant; however, it is a persistent organic pollutant as well as affects the human thyroid hormones and causes cancer. However, the degradation of HBCD has received little attention from researchers. Due to its bioaccumulative and hazardous properties, an appropriate strategy for its remediation is required. In this study, we investigated the biodegradation of HBCD using Shewanella oneidensis MR-1 under optimized conditions. The Box-Behnken design (BBD) was implemented for the optimization of the physical degradation parameters of HBCD. S. oneidensis MR-1 showed the best degradation performance at a temperature of 30 °C, pH 7, and agitation speed of 115 rpm, with an HBCD concentration of 1125 μg/L in mineral salt medium (MSM). The strain tolerated up to 2000 μg/L HBCD. Gas chromatography-mass spectrometry analysis identified three intermediates, including 2-bromo dodecane, 2,7,10-trimethyldodecane, and 4-methyl-1-decene. The results provide an insightful understanding of the biodegradation of HBCD by S. oneidensis MR-1 under optimized conditions and could pave the way for further eco-friendly applications. KEY POINTS: • HBCD biodegradation by Shewanella oneidensis • Optimization of HBCD biodegradation by the Box-Behnken analysis • Identification of useful metabolites from HBCD degradation.

Inventorization and ecological risk assessment of tetrabromobisphenol A and hexabromocyclododecane in sediments from Guangdong coastal area of South China Sea

Brominated flame retardants (BFRs) exhibit excellent flame retardant properties and are widely used in various industries. Among the common BFRs, tetrabromobisphenol A (TBBPA) and hexabromocyclododecane (HBCDs) pose substantial ecological and human health risks due to their extensive application and long-range transport. This study established 131 sample collection sites along the coast of the South China Sea (SCS) in Guangdong Province to assess the concentration, distribution, inventory, and ecological risk of TBBPA and HBCDs in surface sediments. The concentrations of TBBPA in SCS sediments ranged from < limit of detection (LOD) to 80 μg/kg dry weight (dw), and those of HBCDs from < LOD to 18 μg/kg dw. The diastereoisomers of HBCDs (α-, β-, and γ-HBCD) in the sediment samples accounted for 36 %, 13 %, and 51 %, respectively. Human activities, particularly those associated with nearby electronic waste disassembly and textile and garment industries, considerably influenced the dispersion of TBBPA and HBCDs. The inventories of TBBPA and HBCDs in Guangdong Province's SCS were estimated to be 3.2 × 105 kg and 7.2 × 104 kg, respectively. The average risk quotient values ranged from <0.01 to 0.016, indicating a low to negligible environmental risk. This study provides deeper insights into the distribution and scientific significance of HBCDs and TBBPA in SCS sediment samples, elucidates the current state of BFR contamination, and offers recommendations for future research on environmental safety and human health in the region.

Maternal Transfer and Apical and Physiological Effects of Dietary Hexabromocyclododecane Exposure in Parental Fathead Minnows (Pimephales promelas)

Hexabromocyclododecane (HBCD) is a persistent organic pollutant that has been characterized as an endocrine disruptor, undergoes maternal transfer, and hinders development and growth in oviparous organisms. The present study examined the apical effects of dietary HBCD (11.5, 36.4, 106 mg/kg, wet wt) on adult fathead minnow exposed for 49 days and the subsequent accumulation and maternal transfer kinetics in adult tissue and eggs, respectively. Exposed adults displayed a significant increase in egg production in the medium treatment group, but no other significant effects were noted. Maternal transfer of dietary HBCD had a similar egg-to-muscle ratios (EMR) in the low and medium treatment groups (1.65 and 1.27 [wet wt], respectively). However, the high treatment group deviated from other treatments with an EMR of 4.2 (wet wt), potentially due to differences in total lipid content in food and/or reaching diffusion/lipid saturation limits in adult tissue, resulting in lower accumulation in the adult muscle tissue. A positive correlation was observed between egg HBCD concentration and time of exposure, which indicates that maternal transfer of HBCD is of concern in fish, and further studies should be conducted to fully elucidate the potential adverse effects that may be observed in the early life stage of oviparous organisms. Environ Toxicol Chem 2023;42:143-153. © 2022 SETAC.

Concentrations and Long-Term Temporal Trends of Hexabromocyclododecanes (HBCDD) in Lake Trout and Walleye from the Great Lakes

Hexabromocyclododecane (HBCDD) is a hazardous, persistent, bioaccumlative brominated flame retardant. To investigate how its use has affected the Great Lakes, total HBCDD (∑HBCDD) concentrations and temporal trends in homogenized whole fish samples from the Great Lakes region (1978 to 2016) were determined. ∑HBCDD concentrations (ng/g ww) for each lake are Erie (0.49-2.60), Ontario (3.12-8.90), Michigan (3.91-9.01), Superior (5.69-13.1), and Huron (5.57-13.7). Early years (1978 to 1992) showed no significant trend. However, recent trends (2004 to 2016) suggest concentrations are increasing in Lakes Erie and Ontario, decreasing in Lakes Superior and Michigan, and not changing in Lake Huron. Decreasing trends for Lakes Superior and Michigan are likely the result of decreased usage of the compound globally, regionally, and locally. For the other lakes, increasing or zero trends are consistent with food web changes due to invasive species and climate change, which has caused more intense storms and less ice cover leading to increased sediment resuspension.

Multi-generational effects and variations of stress response by hexabromocyclododecane (HBCD) exposure in the nematode Caenorhabditis elegans

In order to understand multi-generational effects and changes of stress response by hexabromocyclododecane (HBCD) exposure, the animal model Caenorhabditis elegans was chosen for toxicity study. Multiple endpoints, including the physiological levels (growth, reproduction, and locomotion behaviors), stress-related gene expressions, reactive oxygen species (ROS) production and degree of cell apoptosis, were evaluated on exposed nematodes and their progeny. Prolonged exposure to HBCD at concentrations of 2 nM-200 nM caused adverse physiological effects in the parental generation (F0), and these effects were also observed in the offspring under HBCD-free conditions (F1). HBCD-induced toxicities could be transferred from parent to offspring. The integrated gene expressions profiles showed that exposure to HBCD at concentrations of 20-200 nM resulted in obvious changes in stress-related gene expressions, which were more increased in F0 generation than in F1 generation. The increased expressions were pronounced in several genes related to oxidative stress and cell apoptosis, e.g., hsp-16.2, hsp-16.48, sod-1, sod-3 and cep-1 genes. Exposure to 200 nM of HBCD could significantly increase ROS production and degree of cell apoptosis in the F0 and F1 generations. Therefore, it was speculated that HBCD exposure induced oxidative stress and cell apoptosis, which resulted in the adverse physiological effects. This finding is helpful for understanding the multi-generational effects and evaluating the potential risk of HBCD.

Responses of antioxidant and biotransformation enzymes in Carassius carassius exposed to hexabromocyclododecane

The ubiquitous existence of hexabromocyclododecane (HBCD) in environmental matrices has made it attractive to both field investigators as well as laboratory researchers. However, literature on the biological effects caused by HBCD on aquatic vertebrates seldom exist. This has inevitably increased the difficulty of toxicological assessment in the aquatic environment. Juvenile crucian carp (Carassius carassius) were exposed (flow-through) to different concentrations of technical HBCD (nominal 2, 20, 200 μg L^-1) for 7 days to determine the responses of antioxidant and biotransformation enzymes. HBCD was found to be increasingly bioconcentrated in the fish livers as time proceeds. Also, the contribution of α-HBCD exhibited an enhancement from 13% in the exposure solutions to 24% in crucian carp, still much lower than in wild fishes (ca. 80%). HBCD induced activities of antioxidant enzymes in most cases, as well as increased level of lipid peroxidation. In contrast to the weak response of 7-ethoxyresorufin-O-deethylase (EROD), 7-pentoxyresorufin-O-depentylase (PROD) activity was generally induced in a time-dependent manner with peaks at day 2. Phase II enzyme Glutathione-S-transferase (GST) showed a dose-dependent induction with maximums in the 20 μg L^-1 treatment at all the four timepoints of 1, 2, 4 and 7 days. Some enzymatic responses showed good associations, indicating coordinated functions. To sum up, tHBCD exposure in the present circumstance had produced an ecological stress to crucian carp. The low levels of biotransformation and slow rates of bioisomerization suggest a possible long-term toxic effect, especially around HBCD point sources.

Bioconcentration and effects of hexabromocyclododecane exposure in crucian carp (Carassius auratus)

As a cycloaliphatic brominated flame retardant, hexabromocyclododecane (HBCD) has been widely used in building thermal insulation and fireproof materials. However, there is little information on the bioconcentration as well as effects with respect to HBCD exposure in the aquatic environment. To investigate the bioconcentration of HBCD in tissues (muscle and liver) and its biochemical and behavioural effects, juvenile crucian carp (Carassius auratus) were exposed to different concentrations of technical HBCD (nominal concentrations, 2, 20, 200 μg/L) for 7 days, using a flow-through exposure system. HBCD was found to concentrate in the liver and muscle with a terminal concentration of 0.60 ± 0.22 μg/g lw (lipid weight) and 0.18 ± 0.02 μg/g lw, respectively, at an environmentally-relevant concentration (2 μg/L). The total thyroxine and total triiodothyronine in the fish plasma were lowered as a result of exposure to the HBCD. Acetylcholinesterase activity in the brain was increased, while swimming activity was inhibited and shoaling inclination was enhanced after exposure to 200 μg/L HBCD. Feeding rate was suppressed in the 20 and 200 μg/L treatment groups. In summary, HBCD concentrations 10-100× higher than the current environmentally-relevant exposures induced adverse effects in the fish species tested in this study. These results suggest that increasing environmental concentrations and/or species with higher sensitivity than carp might be adversely affected by HBCD.

Application of liquid chromatography-tandem mass spectrometry in quantitative bioanalyses of organic molecules in aquatic environment and organisms

Analytical methods using liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) for the simultaneous determination of metabolites or contaminants (or both) in various tissues of aquatic organisms and in the aquatic environment have received increasing attention in the last few years. This review discusses the findings relevant to such procedures published between 2005 and 2015. The aim is to evaluate the advantages, restrictions, and performances of the procedures from sample preparation to mass spectrometry measurement. To support these discussions, a general knowledge on LC-MS/MS is also provided.

Accumulation and phytotoxicity of technical hexabromocyclododecane in maize

To investigate the accumulation and phytotoxicity of technical hexabromocyclododecane (HBCD) in maize, young seedlings were exposed to solutions of technical HBCD at different concentrations. The uptake kinetics showed that the HBCD concentration reached an apparent equilibrium within 96hr, and the accumulation was much higher in roots than in shoots. HBCD accumulation in maize had a positive linear correlation with the exposure concentration. The accumulation of different diastereoisomers followed the order γ-HBCD>β-HBCD>α-HBCD. Compared with their proportions in the technical HBCD exposure solution, the diastereoisomer contribution increased for β-HBCD and decreased for γ-HBCD in both maize roots and shoots with exposure time, whereas the contribution of α-HBCD increased in roots and decreased in shoots throughout the experimental period. These results suggest the diastereomer-specific accumulation and translocation of HBCD in maize. Inhibitory effects of HBCD on the early development of maize followed the order of germination rate>root biomass≥root elongation>shoot biomass≥shoot elongation. Hydroxyl radical (OH) and histone H2AX phosphorylation (γ-H2AX) were induced in maize by HBCD exposure, indicative of the generation of oxidative stress and DNA double-strand breaks in maize. An OH scavenger inhibited the expression of γ-H2AX foci in both maize roots and shoots, which suggests the involvement of OH generation in the HBCD-induced DNA damage. The results of this study will offer useful information for a more comprehensive assessment of the environmental behavior and toxicity of technical HBCD.

Enantioselective biotransformation of hexabromocyclododecane by in vitro rat and trout hepatic sub-cellular fractions

α-, β-, and γ-Hexabromocyclododecanes (HBCDs) were subjected to in vitro biotransformation experiments with rat and trout liver S9 fractions for different incubation times (10, 30, and 60 min) at 2 concentration levels (1 and 10 μM). The metabolic degradation of target HBCDs followed first order kinetics. Whereas β-HBCD undergoes rapid biotransformation (t0.5 = 6.4 and 38.1 min in rat and trout, respectively), α-HBCD appears the most resistant to metabolic degradation (t0.5 = 17.1 and 134.9 min). The biotransformation rate in trout was slower than in rat. Investigation of HBCD degradation profiles revealed the presence of at least 3 pentabromocyclododecene (PBCD) and 2 tetrabromocyclododecadiene (TBCD) isomers indicating reductive debromination as a metabolic pathway for HBCDs. Both mono- and di- hydroxyl metabolites were identified for parent HBCDs, while only mono hydroxyl metabolites were detected for PBCDs and TBCDs. Interestingly, δ-HBCD was detected only in trout S9 fraction assays indicating metabolic interconversion of test HBCD diastereomers during biotransformation in trout. Finally, enantioselective analysis showed significant enrichment of the (-)-α-HBCD enantiomer (EF = 0.321 and 0.419 after 60 min incubation in rat and trout, respectively). The greater enrichment of (-)-α-HBCD in rat than in trout underlines the species-specific differences in HBCD metabolism and the need for caution when extending similar results from animal studies to humans.

Trophic dynamics of hexabromocyclododecane diastereomers and enantiomers in fish in a laboratory feeding study

The laboratory trophic transfer of hexabromocyclododecanes (HBCDs) was studied using predatory (oscar) fish and a prey species (tiger barb) exposed to a technical HBCD. Gut absorption, dynamic changes of diastereomer pattern and enantiomer fractions, and potential metabolism of HBCDs were examined. Compared with β- or γ-HBCD, α-HBCD showed lower absorption efficiency in the gut of oscar fish. A predominance of γ-HBCD was observed in the tiger barb after 5 d HBCD-exposed and oscar feeding on the tiger barb for 16 d. After 20 d of depuration, 41.1% γ-HBCD and 42.7% β-HBCD disappeared, and α-HBCD exceeded the initial amount. The transformation from γ-HBCD predominance in the food to α-HBCD predominance in the oscar was attributed mainly to the isomerization of γ-HBCD (at least 3% and up to 22.7%) to α-HBCD. Selective enrichment of the (+) α- and (-) β-enantiomers and no enantioselective enrichment of γ-HBCD were observed in the tiger barbs. No enantioselective uptake of the 3 diasteromers was found in the oscar gut. The enantiomer fractions of α- and γ-diastereomers were significantly higher, but that of β-diastereomer were significantly lower in the oscars than in the tiger barbs, indicating enantioselective metabolism of the 3 diastereomers. Two HBCD monohydroxylated metabolites were detected in the 2 fish species, but their composition patterns differed, indicating a species-specific metabolism of HBCD in the studied fish species.

Enantiomer-specific bioaccumulation and depuration of hexabromocyclododecanes in zebrafish (Danio Rerio)

The bioaccumulation and depuration of six hexabromocyclododecane enantiomers in zebrafish were investigated through dietary exposure. The bioaccumulation of (±)α-HBCD, (±)β-HBCD and (±)γ-HBCD was concentration dependent. The results from their concentration trends and assimilation efficiencies suggested that zebrafish selectively bioaccumulate the (+) α-enantiomer and (+) γ-enantiomer compared to their corresponding (-)-enantiomers. However, (±)β-HBCD did not show enantioselectivity in the process of bioaccumulation. The depuration of all six enantiomers in zebrafish followed first-order kinetics except (-)γ-HBCD in low dose. These results showed that the bioaccumulation and depuration of HBCDs in zebrafish were enantiomer-selective. Each enantiomeric pair of HBCDs could display different biological process in organisms.

Diastereoisomer- and enantiomer-specific accumulation, depuration, and bioisomerization of hexabromocyclododecanes in zebrafish (Danio rerio)

In this study, zebrafish (Danio rerio) were exposed to two dietary concentrations of individual HBCD diastereoisomers (α-, β-, and γ-HBCD) for 42 days, followed by clean food for 21 days, to examine bioaccumulation, depuration, and enantiomer fractions (EFs) of HBCD diastereoisomers and to test the bioisomerization of HBCDs in fish. The depuration of α-, β-, and γ-HBCD in zebrafish followed the first-order process. Bioaccumulation parameters of the three diastereoisomers differed between low and high dose, suggesting that the bioaccumulation of them is concentration dependent. Calculated assimilation efficiencies (AEs), biomagnification factors (BMFs), and half-lives (t(1/2)) of α-HBCD were the highest among the three diastereoisomers. Furthermore, the study showed that zebrafish could biotransform γ-HBCD to α-HBCD. The highest AE, BMF, and t(1/2) of α-HBCD and bioisomerization of γ-HBCD to α-HBCD could explain why α-HBCD appears to be dominant in biota samples. The EFs for α- and γ-HBCD in zebrafish estimated at different times of bioaccumulation and depuration were all significantly greater than those in corresponding food (P < 0.05), indicating selective enrichment of (+) α-enantiomer and (+) γ-enantiomer relative to (-) α-enantiomer and (-) γ-enantiomer, respectively.

Diastereomer-specific uptake, translocation, and toxicity of hexabromocyclododecane diastereoisomers to maize

Hexabromocyclododecane (HBCD), a brominated flame retardant, has become a ubiquitous contaminant due to its wide application, persistence, and toxicity. HBCD diastereoisomers have different physical and chemical properties and may differ in their bioaccumulation and toxicity in plants. Accumulation and toxicity of α-, β-, and γ-HBCDs in maize were investigated in the present study. The accumulation was in the order β-HBCD > α-HBCD > γ-HBCD in roots and β-HBCD > γ-HBCD > α-HBCD in shoots. Both the inhibitory effect of the diastereoisomers on the early development of maize and the intensities of hydroxyl radical and histone H2AX phosphorylation in maize exposed to 2 μg L(-1) HBCD followed the order α-HBCD > β-HBCD > γ-HBCD, indicating the diastereomer-specific oxidative stress and DNA damage in maize. It was further confirmed that the generation of reactive oxygen species was one, but not the only, mechanism for DNA damage in maize exposed to HBCDs.

Kinetic study of γ-hexabromocyclododecane orally given to laying hens (Gallus domesticus). "Transfer of HBCD in laying hens"

INTRODUCTION

High concentrations of hexabromocyclododecane (HBCD) sometimes recorded in free-range hens' eggs are thought to be due to soil ingestion. Of the three stereoisomers of HBCD (α-, β-, and γ-HBCD), γ-HBCD is the main component in the commercial mixture, as well as in environmental matrices, whereas the isomer profile is α-dominated in biota. In fish and in mammals, this shift is thought to be due to a rapid elimination of γ-HBCD and to its bioisomerization to the more persistent α-HBCD. The aim of the current controlled study was to better understand the fate of ingested HBCD in laying hens. The isomer profile in soil being γ-dominated, excretion kinetics of γ-HBCD into egg yolk, and accumulation in liver and in abdominal fat were investigated.

MATERIALS AND METHODS

Forty-eight laying hens were individually housed and fed with a spiked diet containing 1.1-ng γ-HBCD per gram for 21 days and with a clean diet for the following 18 days. Hens were sequentially slaughtered throughout the 39-day experiment. α-, β-, and γ-HBCD were analyzed in egg yolk, in abdominal fat, and in liver by LC-MS/MS. α- and γ-HBCD were quantified in the three tissues, while β-HBCD was never quantified.

RESULTS AND CONCLUSION

Kinetics of the two isomers suggests that γ-HBCD is rapidly biotransformed and eliminated, and partly isomerized into the more persistent α-HBCD. Carry-over rate of ingested γ-HBCD to egg yolk was estimated at 1.2%. Estimated half-lives of γ-HBCD in egg yolk, in abdominal fat, and in liver were 2.9, 13, and 0.41 days, respectively.

Hexabromocyclododecanes (HBCDs) in marine fishes along the Chinese coastline

This study reports concentrations of hexabromocyclododecanes (HBCDs) in two species of marine fish, large yellow croaker (Pseudosciaenacrocea) and silver pomfret (Pampusargenteus) (n = 46), from nine Chinese coastal cities (Dalian, Tianjin, Qingdao, Shanghai, Zhoushan, Wenzhou, Fuzhou, Quanzhou and Xiamen). HBCDs were detectable in all samples analyzed, indicating ubiquitous contamination of these compounds in the Chinese coastal environment. The average total HBCD concentration was 3.7 ng g⁻¹ lipid weight (range: 0.57-10.1 ng g⁻¹ lipid weight), which is relatively lower than other regions of the world, especially Europe, where HBCDs are intensively used. Among the three individual HBCD isomers (α-, β- and γ-HBCD) in all fish samples, the α-isomer showed a remarkable predominance (from 87.5% to 100% of total contribution), indicating its higher bioaccumulative potential. Geographically, the highest HBCD level present in fish was found in Dalian in northern China, and the lowest occurred in Wenzhou. Estimated daily intakes of HBCDs via fish consumption for the Chinese population were 0.004-1.00 ng kg body weight⁻¹ d⁻¹. These exposure levels were much lower than the effect levels.

Diastereomer-specific bioaccumulation of hexabromocyclododecane (HBCD) in a coastal food web, Western Norway

The present study reports diastereomer-specific accumulation of HBCD from a point source in five marine species representing a typical food web in a Norwegian coastal area. Samples of mussels, polychaetes, crabs and seabird eggs were analyzed for the diastereomers α-, β- and γ-HBCD, as well as lipid content and stable isotopes of nitrogen ((15)N/(14)N) to estimate trophic level. Accumulated HBCD did not correlate well with lipid content for most of the species, thus wet-weight based concentrations were included in an assessment of biomagnification. In contrast to β- and γ-HBCD, the α-diastereomer increased significantly with trophic level, resulting in magnification factors >1 in this coastal marine ecosystem. Data for poikilotherms did not show the same positive correlation between the α-diastereomer and trophic position as homeotherms. The apparent biomagnification of the α-HBCD could be due to bioisomerization or diastereomer-specific elimination that differed between poikilotherms and homeotherms.

Bioavailability of hexabromocyclododecane to the polychaete Hediste diversicolor: exposure through sediment and food from a contaminated fjord

Knowledge of the bioavailability of brominated flame retardants for sediment-dwelling organisms is limited. The present study investigated bioaccumulation of hexabromocyclododecane (HBCD) in the ragworm Hediste diversicolor exposed to field-contaminated sediments and food. Sediments and blue mussels (Mytilus edulis) were sampled from a fjord on the Norwegian west coast known to be highly contaminated with the brominated flame retardant HBCD. During a four-week experiment, ragworms were either exposed to sediment containing moderate or high concentrations of HBCD, or they were fed contaminated soft tissue from mussel. Although mean sediment concentrations were as high as 40 microg/g dry weight in the contaminated sediments, no HBCD were detected in the exposed ragworms (detection limit up to 2.0 ng/g wet wt). In contrast, the exposure to contaminated food (mussel) resulted in elevated body concentrations of 9.1 ng HBCD/g wet weight. The partitioning of HBCD in the test-sediments appeared to be influenced by the quantity and quality of the sediment organic matter (up to 15% total organic carbon). The presence of small plastic beads (<2 mm), which were shown to contain a mean of 50 microg/g dry weight of HBCD, could explain the low bioavailability of the flame retardant in the sediment. Hexabromocyclododecane in food appeared to be readily assimilated in the ragworm, however, and the diastereomer pattern indicated a relative increase of alpha-HBCD from mussel to ragworm. Hence, in the sampled areas of the fjord, trophic transfer appeared to be a more important mechanism for the entry of HBCD into the local benthic food web than sediment-to-biota accumulation.