Stereoisomer-Specific Trophodynamics of the Chiral Brominated

Subscriber access provided by University of Sussex Library

Characterization of Natural and Affected Environments

Stereoisomer-specific Trophodynamics of the Chiral Brominated Flame Retardants HBCD and TBECH in a Marine Food Web, with Implications for Human Exposure Yuefei Ruan, Xiaohua Zhang, Jian-Wen Qiu, Kenneth M.Y. Leung, James C.W. Lam, and Paul K.S. Lam Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02206 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 36

Environmental Science & Technology

1

Stereoisomer-specific Trophodynamics of the Chiral Brominated

2

Flame Retardants HBCD and TBECH in a Marine Food Web, with

3

Implications for Human Exposure

4

Yuefei Ruan†, Xiaohua Zhang‡, Jian-Wen Qiu§, Kenneth M.Y. Leung , James

5

C.W. Lam†,‡,*, Paul K.S. Lam†, ,*

6

‖

⊥

†

State Key Laboratory in Marine Pollution (SKLMP), Research Centre for the Oceans

7

and Human Health, Shenzhen Key Laboratory for the Sustainable Use of Marine

8

Biodiversity, City University of Hong Kong, Hong Kong SAR, China

9

‡

Department of Science and Environmental Studies, The Education University of Hong of Kong, Hong Kong SAR, China

10 11

§

12

‖ The Swire Institute of Marine Science and School of Biological Sciences, The

Department of Biology, Hong Kong Baptist University, Hong Kong SAR, China

University of Hong Kong, Hong Kong SAR, China

13 14

⊥

Department of Chemistry, City University of Hong Kong, Hong Kong SAR, China

James C. W. Lam*

Paul K. S. Lam*

Department of Science and

State Key Laboratory in Marine

Environmental Studies, The Education

Pollution, Department of Chemistry,

University of Hong Kong, Hong Kong

City University of Hong Kong, Hong

SAR, China

Kong SAR, China

Tel: +852-2948-8537

Tel: +852-3442-6828

Fax: +852-2948-7676

Fax: +852-3442-0303

E-mail: [email protected]

E-mail: [email protected]

[email protected] 15

1

ACS Paragon Plus Environment


16

ABSTRACT

17

Stereoisomers of 1,2,5,6,9,10-hexabromocyclododecane (HBCD) and 1,2-dibromo-4-

18

(1,2-dibromoethyl)-cyclohexane (TBECH) were determined in sediments and 30

19

marine species in a marine food web to investigate their trophic transfer. Lipid content

20

was found to affect the bioaccumulation of ΣHBCD and ΣTBECH in these species.

21

Elevated biomagnification of each diastereomer from prey species to marine mammals

22

was observed. For HBCD, biota samples showed a shift from γ- to α-HBCD when

23

compared with sediments and technical mixtures; trophic magnification potential of

24

(‒)-α- and (+)-α-HBCD were observed in the food web, with trophic magnification

25

factors (TMFs) of 11.8 and 8.7, respectively. For TBECH, the relative abundance of γ-

26

and δ-TBECH exhibited an increasing trend from abiotic matrices to biota samples;

27

trophic magnification was observed for each diastereomer, with TMFs ranging from 1.9

28

to 3.5; the enantioselective bioaccumulation of the first eluting enantiomer of δ-TBECH

29

in organisms at higher TLs was consistently observed across samples. This is the first

30

report on the trophic transfer of TBECH in the food web. The estimated daily intake of

31

HBCD for Hong Kong residents was approximately 16-times higher than that for the

32

general population in China, and the health risk to local children was high based on the

33

relevant available reference dose.

34

KEYWORDS

35

HBCD; TBECH; Enantiomer; Food web; Trophic Transfer; Biomagnification

36

2


Page 2 of 36

Page 3 of 36


37

INTRODUCTION

38

Concerns over brominated flame retardants (BFRs) have been growing since the

39

1990s, as many exhibit environmental persistence, bioaccumulation, and toxicity, in

40

wildlife and humans.1,2 Numerous studies have reported that two kinds of extensively

41

used

42

hexabromocyclododecane (HBCD), are globally distributed in the environment and

43

have been found in different environmental matrices, including biota, indicating the

44

liver and thyroid hormone homeostasis as the main targets for toxicity.3–5 For most other

45

BFRs, such as 1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (TBECH), however,

46

only limited data are available on their production and use, exposure status, and

47

toxicological information.6–8 Considering that all formulations of commercial PBDE

48

mixtures and HBCD technical products were recently included under the Stockholm

49

Convention on Persistent Organic Pollutants (POPs) in Annex A for global elimination,

50

research is shifting toward the use of the other BFRs as alternatives, as these alternatives

51

have not been ascertained to be safer for environmental and public health.9,10

BFRs,

polybrominated

diphenyl

ethers

(PBDEs)

and

1,2,5,6,9,10-

52

Several kinds of BFRs, including HBCD and TBECH, can exhibit optical activities

53

as chiral compounds. For instance, HBCD theoretically includes 6 diastereomeric pairs

54

of enantiomers and 4 meso forms, and commercial mixtures mainly consist of α-

55

(~12%), β- (~9%), and γ-diastereomers (~79%);11–13 TBECH exhibits stereochemical

56

diversity as 4 diastereomeric pairs of enantiomers, and the technical formulation

57

contains equimolar amounts of α- and β-diastereomers.14 Many studies have revealed

58

that HBCD exhibits high diastereo- and enantio-selectivity in organisms.3,15,16 A

59

diastereomeric shift from γ- to α-HBCD was frequently observed in biota at higher

60

trophic levels (TLs), whereas the preferential bioaccumulation of (−)-α-HBCD was

61

revealed more often in certain animal species, independent of biological factors, such 3



62

as body composition, age/sex variation, nutritive condition, and tissue distribution.15–17

63

Although TBECH is increasingly detected in the environment and biota globally,

64

TBECH concentrations are primarily quantified as the sum of all stereoisomers

65

(ΣTBECH), due to difficulties in the chromatographic separation of every

66

diastereomer.18–20 No study has investigated TBECH enantiomeric patterns in any

67

environmental matrix, except our recent work on cetacean blubber.21 The trophic

68

transfer of POPs and POP-like pollutants in food webs is a vital component for

69

assessing their ecological and health risks. Biomagnification of HBCD has been

70

observed through correlation between the TL and the concentration in organisms.22–30

71

However, most studies do not fully consider the precise predator-prey feeding

72

relationships, and even fewer focus on the trophodynamics of different HBCD

73

enantiomers.31,32 Up to now, the biomagnification potential of TBECH is still

74

uncharacterized.

75

Our recent study reported on the occurrence of HBCD enantiomers and meso isomers

76

as well as TBECH enantiomers in two resident species of marine mammals ― finless

77

porpoises (Neophocaena phocaenoides) and Indo-Pacific humpback dolphins (Sousa

78

chinensis) ― in the South China Sea.21 Elevated accumulation of these chiral BFRs

79

with the preferential enrichment of specific stereoisomers was observed in these marine

80

top predators. To build on this earlier work, the present study aims to (1) determine the

81

occurrence and trophodynamics of specific HBCD and TBECH stereoisomers in the

82

marine food web with the marine mammals as apex predators and (2) assess the food

83

web magnification potentials of these stereoisomers and the potential role of diet on

84

their levels in the marine mammals. Particular emphasis is placed on understanding the

85

behavior and fate of chiral BFRs in marine ecosystems.

86

MATERIALS AND METHODS 4


Page 4 of 36

Page 5 of 36


87

Sample Collection. A total of 34 surface sediments were collected from Hong

88

Kong waters in June, 2012, from sampling sites corresponding to the Environmental

89

Protection Department’s regular sediment monitoring stations.33 Marine organisms,

90

including 5 mollusk species, 6 crustacean species, and 19 fish species, were collected

91

via bottom trawling in Hong Kong waters from July to November, 2012 (Table 1).

92

These marine species, together with the two previously studied cetacean species from

93

this region, constitute a marine food web pertaining to low-trophic-level marine lives

94

to the marine top predators.34 The sampling area covered the western, southern, and

95

eastern Hong Kong waters. Further details for the samples and sampling sites are

96

presented in Table S1 and Figure S1 in the Supporting Information (SI).

97

Sample Preparation and Instrumental Analysis. Details of the standards,

98

reagents and solvents can be found in the SI. Sample extraction was accomplished using

99

previously established methods with modest modifications, as described in the SI.

100

HBCD stereoisomers were analyzed by liquid chromatography‒tandem mass

101

spectrometry using an Eclipse Plus C18 column (4.6 mm i.d. × 100 mm, 3.5 μm)

102

coupled to a NUCLEODEX β-PM column (4 mm i.d. × 200 mm, 5 μm) for separation,

103

and TBECH enantiomers were determined using gas chromatography–mass

104

spectrometry using a CHIRALDEX B-TA capillary column (30 m × 0.25 mm i.d., 0.12

105

µm film thickness) for separation. More information on the instrumental analysis is

106

provided in the SI.

107

Quality Assurance and Quality Control (QA/QC). The limit of detection

108

(LOD) was defined as an instrument signal-to-noise ratio (SNR) of ≥ 3:1. Detailed

109

QA/QC measures for calibration curves, procedure blanks, and matrix recovery tests

110

are given in the SI. The LODs of the samples ranged from 0.05 to 0.2 ng/g dry weight

111

(dw) in sediments and 0.3 to 0.8 ng/g lipid weight (lw) in biota for HBCD stereoisomers, 5



112

and from 0.1 to 0.5 ng/g dw in sediments and 0.8 to 1.8 ng/g lw in biota for TBECH

113

stereoisomers. Matrix recoveries ranged from 83% to 110% for sediments and 72% to

114

106% for bio-samples. Surrogate recoveries ranged from 78% to 95%, and the

115

concentration data were corrected by the surrogates.

116

Stable Isotope Analysis and TL Determination. Information on the stable

117

isotope analysis is given in the SI. TLs of individual organisms were determined based

118

on the assumption that zooplankton occupies TL 2.0 using the following equation:

119

TL consumer = 2.0 + (δ15N consumer – δ15N primary consumer) / 3.8

120

where δ15N primary consumer is the stable nitrogen isotope value of zooplankton (9.7) and

121

3.8 is the isotopic trophic enrichment factor.35,36

122

Biota–Sediment Accumulation Factor (BSAF), Bioaccumulation Factor

123

(BMF), and Trophic Magnification Factor (TMF) Calculations. The BSAF

124

calculation was based on the assumption that sediment is the dominant exposure

125

pathway for benthic organisms using the following equation:

126

BSAF = C biota (ng/g, lw) / C sediment [ng/g, total organic carbon (TOC)]

127

where C biota is the average lipid-normalized concentration in benthos and C sediment is the

128

TOC-normalized concentration in sediments.37 BMF was defined as the ratio of the

129

average lipid-normalized concentration between predator and prey.37 TMF was used to

130

describe the biomagnification, and the calculation was based on the correlations

131

between average lipid-normalized concentrations and TLs using the following equation:

132

ln C biota (ng/g, lw) = a + b × TL

133

where a and b represent the constant and the slope of the linear regression, respectively.

134

Slope b was used to calculate TMF using the equation TMF = e b.38

135

Estimated Daily Intake (EDI) Assessment. The consumption rates (CRs) of

136

marine crustaceans, mollusks, and fishes in the Hong Kong population were obtained 6


Page 6 of 36

Page 7 of 36


137

from a dietary survey of the general population conducted in Hong Kong in 2005-2007

138

and were 27.51 g wet weight (ww)/day, 23.17 g ww/day, and 65.98 g ww/day,

139

respectively.39 The average body weight (BW) was 66.2 kg for adults and 21.1 kg for

140

children in China.29 All the marine organisms investigated in this study are popular

141

seafood items in Hong Kong. To evaluate human exposure to HBCD and TBECH via

142

seafood consumption, the EDI for local residents was calculated using the following

143

equation:

144

EDI = CR × C biota (ng/g, ww) / BW

145

Statistical Analysis. Statistical analysis was performed using Microsoft Excel

146

2016 to generate descriptive statistics, and all other statistical procedures were

147

conducted using IBM SPSS Statistics 20.0 and OriginLab OriginPro 2015. Different

148

data treatments for values below the LOD, i.e., excluding values below the LOD, using

149

1/2 the LOD instead, or setting those values to zero, was confirmed to have no effect

150

on the conclusion and interpretation of the results. Thus, values below the LOD were

151

replaced with 1/2 LOD, as was done in our previous studies.21,40 Enantiomer fractions

152

(EFs) were used to describe enantiomeric patterns. Methods used for statistical analysis

153

and detail information on the EF calculation are provided in the SI.

154

RESULTS AND DISCUSSION

155

Levels of ΣHBCD and ΣTBECH in Sediments and Biota. ΣHBCD and

156

ΣTBECH were detected in most of the sediment samples. ΣHBCD was detected in all

157

the sampled biota species, while ΣTBECH was detected in 26 of the 30 investigated

158

species. These results indicate the widespread occurrence of HBCD and TBECH in the

159

marine environment of Hong Kong. No significant spatial differences (p > 0.05) were

160

found in the concentrations of ΣHBCD or ΣTBECH in biota species with similar TLs

7



161

among different sampling areas (SI Table S1). Thus, concentrations from the same

162

marine species were pooled for data analysis. ΣHBCD and ΣTBECH concentrations in

163

sediments (dw) from different sampling areas and in each marine species (dw and lw)

164

are summarized in Table 1 and illustrated in SI Figure S2.

165

In Hong Kong waters, surface sediment concentrations were between < 0.1 (below

166

the LOD) and 239 ng/g dw for ΣHBCD, and were between < 0.5 and 5.10 ng/g dw for

167

ΣTBECH. ΣHBCD levels were approximately 1 order of magnitude higher than those

168

of ΣTBECH. Considering the median concentrations, surface sediments from central

169

waters accumulated more ΣHBCD and ΣTBECH than the other areas. The correlation

170

between TOC content and sediment concentration was not significant for both ΣHBCD

171

and ΣTBECH (p > 0.05), implying the minor role of TOC in shaping the sediment

172

partitioning of HBCD and TBECH. These findings suggest a domination of local

173

sources of HBCD and TBECH from human activities accounting for their occurrence

174

in sediments from the study region. A strong positive correlation was found between

175

the ΣHBCD and ΣTBECH concentrations in sediments (p < 0.0001), indicating their

176

probably similar emission source and/or pathway. Following the method of Liu et al.

177

(2014),41 for the Hong Kong’s 12 coastal council districts relevant to our sampling sites,

178

a significant positive correlation was found in this study between the population density

179

and the average BFR (ΣHBCD + ΣTBECH) concentration (p < 0.05). Since the 1980s,

180

the dominant industry in Hong Kong has shifted from manufacturing to services.

181

Therefore, BFRs in sediments from Hong Kong waters may have arisen from

182

urbanization (e.g., using BFR-containing products). Overall, ΣHBCD concentrations in

183

sediments from Hong Kong waters in this study were relatively high compared with a

184

previous study on their occurrence in sediments from the Pearl River Delta (PRD)

185

region collected in 2013, whereas ΣTBECH concentrations in our study were slightly 8


Page 8 of 36

Page 9 of 36

186


lower compared with that study.42

187

In biota, the lipid content varied largely between the sampled marine species, and the

188

values ranged from 4.9% to 29%. The concentrations of ΣHBCD and ΣTBECH in these

189

biota on dry weight basis were significantly positively correlated with the lipid contents

190

(Pearson’s r = 0.695 and 0.498, respectively, p < 0.01) (SI Figure S3), indicating that

191

lipid content plays a vital role in the bioaccumulation of HBCD and TBECH.

192

Accordingly, the concentrations below are expressed in ng/g lw. No significant

193

difference in ΣHBCD concentrations was found among fishes, crustaceans, and

194

mollusks (p > 0.05), while ΣTBECH levels were significantly higher in mollusks than

195

the other groups (p < 0.05). Tao et al. reported higher levels of ΣTBECH than ΣHBCD

196

found in wild fish species including mackerel and tuna.10 In the present study, ΣHBCD

197

concentrations in all three biota groups were all significantly higher than those of

198

ΣTBECH (p < 0.05, SI Figure S2), in conformity with a previous finding of the lower

199

bioaccumulation potential of TBECH than HBCD in zebrafish (Danio rerio) after

200

dietary exposure.43 This may also reflect the higher production/use of HBCD than

201

TBECH in the PRD in recent years.

202

Data for HBCD levels in marine organisms have been documented for a number of

203

areas and species. In marine fishes and invertebrates, the reported HBCD levels ranged

204

from low ng/g lw up to 5,200 ng/g lw, with high levels typically found in recent years

205

in Asian countries, especially China.3,29,30,44 The ΣHBCD concentrations in bio-samples

206

in our study (3.01–93.2 ng/g lw) were approximately 1 order of magnitude higher than

207

those in marine fish samples collected along the Chinese coastline in 2008 (0.57–10.1

208

ng/g lw),45 but were 1–2 orders of magnitude lower than those reported in marine

209

species collected from several estuaries in Bohai Sea, East China.29,30,46 Overall, the

210

ΣHBCD levels in marine organisms in our study are among the moderate levels reported 9



211

worldwide. Very limited information is available for TBECH in wild aquatic species.

212

The ΣTBECH concentrations in bio-samples in our study (n.d.–13.9 ng/g lw) were

213

slightly higher than those reported in Nordic mussels and fishes (up to 1.6 ng/g ww)

214

and in Canadian Arctic beluga blubber (0.9–11.3 ng/g lw).47,48 The current results

215

suggest that marine organisms in the marine environment of Hong Kong are susceptible

216

to considerable TBECH exposure.

217

BSAFs and BMFs in Marine Organisms. In this study, BSAFs were determined

218

for the estimation of contaminant uptake by biota from sediment, and BMFs between

219

prey species and marine mammals were calculated to further investigate the

220

accumulation of contaminants from diet to top predators. The dataset of the marine

221

mammals was published previously.21 The assessed BSAFs for crustaceans and

222

mollusks are given in SI Table S2, while the identified prey list and calculated BMFs

223

for the two cetacean species are given in SI Table S3.

224

The BSAFs of ΣHBCD and ΣTBECH ranged from 0.02 to 0.28 and from 0.02 to

225

0.08, respectively, i.e., none of these crustacean and mollusk species exhibited a BSAF

226

greater than 1. The BSAF ranges of ΣHBCD for invertebrates in our study overlap with

227

those reported in marine bivalves from South Korea (0.01–0.15),49 but are much lower

228

than those determined from a laboratory study on bivalves assembled from brackish

229

coastal bays (log BSAF > 1.25).50 This discrepancy is likely ascribed to the large

230

variations between field and laboratory exposure. At the present time, there are no

231

available BSAFs of TBECH for comparison.

232

Considering the BSAFs of HBCD diastereomers (α, β, and γ), α-HBCD contributed

233

the most to the bioaccumulation of ΣHBCD in each invertebrate species, whereas γ-

234

HBCD always accounted for the least. This provides a pathway for α-HBCD to be the

235

most readily transferred HBCD diastereomer in the marine food web as shown in this 10


Page 10 of 36

Page 11 of 36


236

study. For TBECH, no significant differences in BSAFs were found among the four

237

diastereomers. However, this does not necessarily indicate an identical bioaccumulation

238

potential for different TBECH diastereomers in these organisms, as BSAF represents

239

an integrated consequence of various processes, including bioavailability, uptake and

240

depuration, as well as bioisomerization.15

241

The feeding habits of finless porpoises (N. phocaenoides) and Indo-Pacific

242

humpback dolphins (S. chinensis) in Hong Kong waters were identified in two previous

243

studies via the examination of stomach contents collected from stranded cetaceans.51,52

244

These marine mammals are the top predators in this investigated food web, and there is

245

some dietary overlap between the two cetacean species, sharing some important prey

246

items (e.g., anchovies and croakers). Notwithstanding, porpoises and dolphins have

247

dissimilar prey preferences, which can be reflected in their distribution around Hong

248

Kong: porpoises prefer cephalopods and usually appear in deeper and clearer saline

249

waters (mainly in the southern and eastern waters of Hong Kong), whereas dolphins

250

prefer demersal and shoaling fishes of the murky and brackish waters of estuaries, i.e.,

251

mostly in northwestern waters of Hong Kong close to the Pearl River Estuary (PRE).53

252

Among the collected marine species in our study, 11 pairs of predator-prey relationships

253

were identified for each cetacean species (SI Table S3).

254

For the predator-prey pairs of porpoises, the BMFs of ΣHBCD and ΣTBECH were

255

all above 1 and ranged from 15.0 to 192 and from 2.23 to 19.8, respectively, suggesting

256

the biomagnification of HBCD and TBECH from the studied prey to porpoises. Much

257

higher BMFs were observed for the predator-prey pairs of dolphins (SI Table S3), with

258

values ranging from 325 to 4,160 for ΣHBCD and from 9.80 to 111 for ΣTBECH. This

259

signifies that dolphins have greater potential to accumulate HBCD and TBECH via

260

food chains as compared to porpoises. For the 8 common preys between porpoises and 11



261

dolphins, much higher BMFs were observed in ΣHBCD than in ΣTBECH. Therefore,

262

similar to the BSAF results, ΣHBCD exhibited higher biomagnification potential than

263

ΣTBECH in cetacean species. Future studies should examine the influence of other

264

biological factors, such as age/sex variations and nutritive conditions.

265

Considering BMFs for HBCD diastereomers (α, β, and γ), the BMFs of α-HBCD

266

were the largest, in accordance with this diastereomer predominating cetacean species,

267

as reported in our previous study.21 Although the BMFs of β-HBCD also suggest its

268

high biomagnification potential, it was detected in marine mammals at relatively low

269

levels, possibly because this diastereomer is present in relatively small quantities in

270

technical products and is susceptible to biotransformation.54,55 For TBECH, the BMFs

271

of α- and β-TBECH were significantly lower than those of γ- and δ-TBECH (p < 0.01).

272

The large BMFs of γ- and δ-TBECH are in agreement with the rapidly increasing

273

proportions of these diastereomers in cetacean species, as described in our previous

274

study.21 In comparison, although the BMFs of α- and β-TBECH suggest their relatively

275

“low” biomagnification potential, these diastereomers still prevailed in cetacean

276

samples, partially because α- and β-TBECH are generally the exclusive components in

277

commercial mixtures.

278

Trophic Transfer in the Marine Food Web. To assess the trophic transfer of

279

HBCD and TBECH in the investigated food web, regression analysis was conducted

280

between the lipid-normalized concentrations (ln-transformed) and the TLs, and TMF

281

was derived from the slope of the regression line. Only species with detectable

282

contaminants were selected for this analysis. Thus, 32 species and 28 species (each

283

including two cetacean species) were included in the TMF calculation for HBCD and

284

TBECH, respectively. The results of the regression analysis are shown in Table 2. Note

285

that TMF > 1 indicates that the contaminant is biomagnified. 12


Page 12 of 36

Page 13 of 36


286

The increase in the ΣHBCD concentration with the TL was statistically significant,

287

with a TMF of 7.9 (p < 0.0001). In addition, the concentrations of (±)-α-, (+)-α-, and

288

(–)-α-HBCD were significantly positively correlated with the TL (p < 0.0001), with

289

TMFs of 10.3, 8.7 and 11.8, respectively, indicating the strong preferential

290

biomagnification of α-HBCD in this food web. There were no significant correlations

291

in the concentrations of (±)-β- and (±)-γ-HBCD with the TL. The trophodynamics of

292

HBCD enantiomers in this food web are consistent with the observation in the eastern

293

Canadian Arctic marine food web and in a freshwater food web from an electronic

294

waste recycling site in South China.31,32 Compared with the reported TMFs of α-HBCD

295

(mostly 2~3) in other studies,23,25,26,28–32 our TMFs were approximately 4-fold higher

296

and comparable to the most biomagnified organic contaminants [e.g., 2,2',3,3',4,4'-

297

hexachlorobiphenyl (PCB-128) and oxychlordane, with reported average TMFs of 8.72

298

and 10.31, respectively].56 Our results indicate that α-HBCD can be substantially

299

magnified in the present food web and transferred to top predators. Although β- and γ-

300

HBCD were not trophic-magnified, the BMFs observed in this study indicate their

301

biomagnification potential. It is important to note that this is the first investigation on

302

the trophodynamics of HBCD in a subtropical marine food web.

303

ΣTBECH was also found to be trophic-magnified in the investigated food web, with

304

a TMF of 2.2 (p < 0.05). Additionally, the concentrations of α-, β-, γ- and δ-TBECH

305

were significantly positively correlated with the TL (p < 0.05), with TMFs of 2.2, 1.9,

306

3.4 and 3.5, respectively, suggesting the possible preferential biomagnification of γ- and

307

δ-TBECH in this food web. However, the obtained TMF can be influenced by

308

interspecies ecological (e.g., variations in food intake) and organismal parameters such

309

as metabolism, reproductive status, migration, and age.57 This is the first study that

310

demonstrates the trophic transfer of TBECH in a food web. There are no available field 13



311

data on TMF of TBECH for further comparison. The observed TMF for ΣTBECH in

312

the present study was much lower than that of ΣHBCD, revealing the lower

313

bioaccumulation potential of TBECH in organisms at higher TL. However, the TMFs

314

of TBECH diastereomers were comparable to the abovementioned TMFs of α-HBCD

315

in other studies, which is likely due to the difference in food web configuration. It

316

should be noted that the range of TLs in our study (with apex predators included) was

317

broader than that in most of other studies, possibly resulting in a higher TMF obtained.

318

Reports on TMFs for non-PBDE and non-HBCD BFRs are scarce. Trophic

319

magnification was observed for hexabromobenzene (HBB) in a freshwater food web

320

from an electronic waste recycling site in South China, with a reported TMF of 1.76,

321

while trophic dilution (TMF < 1) through this food web was found for 1,2-bis(2,4,6-

322

tribromophenoxy)ethane (BTBPE) and pentabromotoluene (PBT).32,58 Trophic dilution

323

for BTBPE was also observed in the Lake Maggiore (Italy) food web.27 In the Lake

324

Winnipeg (Canada) food web, BTBPE concentrations were not significantly correlated

325

with the TL, while decabromodiphenyl ethane (DBDPE) was biomagnified with a

326

reported TMF of 8.6.23 The above findings indicate that several unregulated BFRs, such

327

as TBECH, HBB, and DBDPE, can have high trophic magnification potentials. The

328

magnification of TBECH diastereomers provides clear evidence of an expanding class

329

of bioaccumulative substances that urgently require assessment for their potential

330

ecological risks.

331

HBCD and TBECH Diastereomeric Profiles and Enantiomeric Patterns.

332

Figure 1 shows the diastereomeric compositions in the collected samples for all

333

examined HBCD and TBECH diastereomers. The samples were sorted into six groups:

334

technical products, sediments, mollusks, crustaceans, fishes, and cetaceans. For HBCD

335

[Figure 1 (A)], the γ-diastereomer was the predominant component in technical 14


Page 14 of 36

Page 15 of 36


336

products, accounting for 85.3% of ΣHBCD. In sediments and crustaceans, γ-HBCD still

337

exhibited the principal abundance, but the contribution of α-HBCD in sediments

338

(19.2%) and crustaceans (32.8%) was much higher than that in technical products

339

(8.5%). In mollusks and fishes, a diastereomeric shift in the relative abundance from γ-

340

to α-HBCD was observed, where α- and γ-HBCD accounted for approximately 75%

341

and 20% of ΣHBCD, respectively. In cetaceans (apex predators), α-HBCD showed a

342

superlative preponderance (97.1%), while γ-HBCD existed only in a trace proportion

343

(1.1%), similar to the compositions of β-, δ-, and ε-HBCD. The stereoselective

344

biomagnification changed the diastereomeric compositions in these marine organisms.

345

A significant positive correlation was observed between the α-HBCD proportion and

346

the TL (Pearson’s r = 0.416, p < 0.05), while significant negative correlations were

347

found for the proportions of β- and γ-HBCD (Pearson’s r = –0.406 and –0.403,

348

respectively, p < 0.05). The δ-HBCD meso form has been found in samples of fish and

349

birds, previously reported by other studies.59–61 However, apart from our previous study

350

on marine mammals,21 to the best of our knowledge, this is the first report on the

351

presence of ε-HBCD in biota. Although δ- and ε-HBCD were widespread in our

352

collected samples, they existed at trace levels in general, and no significant correlations

353

were observed between the proportions of these two meso isomers and the TL (p > 0.05).

354

The percentage of α-HBCD to ΣHBCD increased with the TL in these marine organisms.

355

Among α-, β-, and γ-HBCD, the α-diastereomer had the highest uptake efficiency, the

356

lowest depuration rate, and the longest half-life, which could explain the prevalence of

357

α-HBCD in higher TL organisms.62–64 Other explanations may include the rapid

358

biotransformation of β- and γ- to α-HBCD and/or the preferential metabolism of β- and

359

γ- over α-HBCD.54,55,65

360

For TBECH [Figure 1 (B)], α- and β-TBECH were almost the exclusive components 15



361

in technical products, accounting for 57.3% and 42.5%, respectively, of ΣTBECH. In

362

sediments and marine organisms, except cetaceans, the composition of β-TBECH

363

remained almost unchanged (roughly 40~45%), while the proportion of α-TBECH

364

decreased by nearly 10%, and the contribution of γ- and δ-TBECH (approximately 5%

365

and 7%, respectively) was much higher than that in technical TBECH products (each

366

0.1%). In cetaceans, the composition of α-TBECH remained almost unaltered

367

compared with that in sediments and organisms at lower TLs, while a sharp decrease

368

by over 10% in the proportion of β-TBECH was observed, along with roughly doubled

369

proportions of γ- and δ-TBECH. These findings were at variance with those reported in

370

Canadian arctic beluga blubber,48 in herring gull egg pools,66 and in UK supermarket

371

fish,10 where the proportions of β-TBECH were higher than those of α-TBECH. The

372

changes observed in the diastereomeric compositions of TBECH in our marine

373

organisms might result from stereoselective biomagnification. A significant negative

374

correlation was found between the β-TBECH proportion and the TL (Pearson’s r = –

375

0.583, p < 0.01), while significant positive correlations were observed for γ- and δ-

376

TBECH (Pearson’s r = 0.571, p < 0.01 and Pearson’s r = 0.633, p < 0.001, respectively).

377

No significant correlation was found for α-TBECH (p > 0.05). Overall, the relative

378

abundance of γ- and δ-TBECH exhibited an increasing trend from technical products to

379

abiotic matrices and finally to biota samples, and the higher TMFs of γ- and δ-TBECH

380

may allow their facile transfer to top predators. A previous study suggested that β-

381

TBECH was metabolized much faster than technical TBECH mixtures by human liver

382

microsomes.67 This could partially explain the sharp decrease in the proportion of β-

383

TBECH rather than α-TBECH in the studied marine mammals.

384

A pair of enantiomers for a compound has identical physicochemical properties, and

385

the integrity of racemates is preserved (i.e., EF = 0.5) when the enantiomeric pair is 16


Page 16 of 36

Page 17 of 36


386

subjected to achiral interactions, such as abiotic environmental processes.59 Deviation

387

from an EF of 0.5 indicates an enantiomeric enrichment from biologically-mediated

388

processes. SI Figure S4 shows the EFs in the collected samples, which were sorted into

389

the same six groups as those for the diastereomeric profiles.

390

Although the preferential enrichment of specific enantiomers of HBCD occurred in

391

some samples of technical products, sediments, mollusks, and crustaceans [SI Figure

392

S4 (A)], the average EFs for each HBCD diastereomer did not significantly deviate

393

from the EF ranges in standard solutions (p > 0.05) and were considered racemic

394

residues of HBCD enantiomers. Most of the samples of fishes and cetaceans had

395

significantly lower EFs than the range in standard solutions (p < 0.05), indicating that

396

the statistically selective enrichment of (–)-enantiomers relative to (+)-enantiomers

397

occurred for α-, β-, and γ-HBCD in these species.

398

A previous study suggested that cytochrome P450 enzymes (CYP450) played a

399

prominent role in the enrichment of (−)- over (+)-α-HBCD via in vitro metabolism

400

rather than stereoisomerization by human liver microsomes.65 If this also occurs in vivo

401

for other mammals and fishes which share certain similarities in xenobiotic-

402

metabolizing CYP450 with humans,68 the result could partially explain the preferential

403

bioaccumulation of (−)- over (+)-α-HBCD in the marine fishes and mammals examined.

404

The selective enrichment of (−)-α-HBCD could also arise from the higher

405

bioaccumulation potential of (−)- relative to (+)-α-HBCD, and the comparatively higher

406

TMF reported in this study further lends support to this idea. Additionally, a plot of the

407

EF of α-HBCD versus the TL (Figure 2) reveals a small but significant (p < 0.05)

408

decrease in the EF with the TL and suggests the enantioselective enrichment of (−)-α-

409

HBCD. Possible mechanisms for the preferential enrichment of (−)-β- and (−)-γ-HBCD

410

are still unknown, and more studies are needed. 17



411

Samples of technical products, sediments, mollusks, and crustaceans exhibited

412

racemic residues of TBECH enantiomers (p > 0.05) [SI Figure S4 (B)]. In most of the

413

fish samples, only the EFs of δ-TBECH were significantly different from (higher than)

414

the EF ranges in standard solutions (p < 0.05), indicating that the preferential

415

enrichment of the first eluting enantiomer of δ-TBECH (E1-δ-TBECH) relative to the

416

second eluting enantiomer (E2-δ-TBECH) occurred in our fish species. In cetaceans,

417

the EFs of α- and δ-TBECH were significantly higher than the EF ranges in standard

418

solutions, while the EFs of γ-TBECH were significantly lower (p < 0.05), indicating the

419

preferential enrichment of E1-α-TBECH, E2-γ-TBECH, and E1-δ-TBECH in these

420

species, whereas no significant deviations were observed for β-TBECH (p > 0.05). Two

421

trends consistent across samples are the enantioselective bioaccumulation of E1-δ-

422

TBECH in organisms at higher TLs and the virtually unchanged β-TBECH

423

enantiomeric composition. The enantioselectivity of TBECH could be potentially

424

attributed to the differences in uptake, metabolism, and excretion in biota, which

425

necessitates further investigation. For the first time, the enantioselectivity of TBECH

426

in a food web was reported.

427

Human Exposure to HBCD and TBECH Diastereomers via Seafood

428

Consumption. Dietary intake is one of the main routes for human exposure to

429

chemical pollutants, including several BFRs.4 In Hong Kong, seafood is one of the main

430

food sources for local residents. Accordingly, a preliminary estimation of the dietary

431

exposure of the Hong Kong population to HBCD and TBECH diastereomers through

432

seafood consumption was conducted based on the dataset of mollusks, crustaceans, and

433

fishes acquired from the present study. It should be noted that the EDI for children

434

might be overestimated because the consumption rate is the average for the local

435

population, and the local population may intake HBCD and TBECH through 18


Page 18 of 36

Page 19 of 36


436

consumption of other foods or other pathways such as inhalation, dust ingestion, and

437

dermal absorption (occupational exposure is not expected to be significant in Hong

438

Kong).69

439

As shown in Table 3, the total EDIs of ΣHBCD via seafood consumption for children

440

and adults were 4.07 and 1.30 ng/kg/day, respectively. This estimated exposure for local

441

adults in our study was approximately 16-times higher than the value for the general

442

Chinese population via aquatic animal-origin food consumption (0.08 ng/kg/day),70 but

443

was slightly lower than that for the Scottish population via shellfish consumption (5.9–

444

7.9 ng/kg/day),71 while was comparable to that for the Japanese population via fish

445

consumption (1.3–3.7 ng/kg/day),72 and was much higher than that for the Dutch

446

population via seafood consumption (0.06–0.17 ng/kg/day).73

447

The human health risk from dietary exposure of HBCD can be assessed using the

448

hazard quotient (HQ) approach, which is derived by comparing the EDI with a

449

reference dose (RfD) (HQ = EDI/RfD).40 A HQ below 1 indicates little or no health

450

concern. Based on a no-observed-adverse-effect-level (NOAEL) of 450 mg/kg/day in

451

terms of increased liver weights and abnormal fatty accumulation in rats with an

452

uncertainty factor of 3000, the US National Research Council suggested an oral RfD

453

for ΣHBCD of 150,000 ng/kg/day.74 Following this method, a NOAEL of 35

454

μg/kg/week in terms of impaired lipid and glucose homeostasis in mice fed a high-fat

455

diet was used to derive the more sensitive RfD (1.67 ng/kg/day) used in this study.75

456

Consequently, the HQs for the local children and adults in this study were 2.44 and 0.78,

457

respectively, indicating the higher risk of ΣHBCD exposure to children through seafood

458

consumption. However, this preliminary assessment has certain limitations, and the

459

RfD could be refined when more toxicity data become available. The major

460

diastereomeric fraction of human intake from the present food web was α-HBCD 19



461

through the consumption of crustaceans and fishes, while mollusk consumption mainly

462

contributed to the intake of γ-HBCD. However, most toxicity data were established

463

from technical mixtures, which contain much more γ-HBCD, and very few studies have

464

reported the differentiation in toxicity between individual HBCD stereoisomers.16

465

Given that α-HBCD seemed to cause the least cytotoxicity in liver cells across

466

studies,76,77 the actual human health risk posed by ΣHBCD may be overestimated.

467

The total EDIs of ΣTBECH for children and adults via seafood consumption were

468

0.78 and 0.25 ng/kg/day, respectively, where the major diastereomeric fraction of

469

human intake was α- and β-TBECH (nearly 90%). Our results of α- and β-TBECH via

470

fish consumption for the Hong Kong residents (4.78 and 4.12 ng/day, respectively,

471

where BW was excluded for comparison) were higher than those reported for the

472

Swedish mothers (3.5 and 0.60 ng/day, respectively).78 Based on a NOAEL of 20.35

473

mg/kg/day in terms of mild renal lesions and inflammation in rats with an uncertainty

474

factor of 3,000,74,79 an oral RfD for ΣTBECH of 6,800 ng/kg/day was proposed in this

475

study. The calculated HQ of ΣTBECH was

Stereoisomer-Specific Trophodynamics of the Chiral Brominated

Recommend Documents