Bioaccumulation,Biomagnificatoin And Biotransformation Of Halogenated Persistent Organic Pollutants(HOPs) And Organophosphorus Flame Retardants(PFRs) In Fish

Halogenated persistent organic pollutants?HOPs?,such as polychlorinated biphenyls?PCBs?,polybrominateddiphenylethers?PBDEs?and dichlorodiphenyltrichloroethane?DDT?,and organophosphorus flame retardants?PFRs?are both toxic organic compounds that have attracted wide attention.The bioaccumulation and biotransformation of these contaminants have always been one of the important research topics in the field of environmental sciences.Numerous previous studies have reported the biotransformation process of HOPs,which mainly focused on the determination of metabolites and on the changes of the composition of the parent compounds.However,these conventional methods are not able to comprehensive evaluate the biotransformation process,due to its complexity in vivo.Moreover,although biotransformation has been shown to have greatly influence on the bioaccumulation and biomagnification potential of organic pollutants in organism,no efficient method exists for characterizing its effect to date.For chiral compounds,such as o,p'-DDT and some PCB congeners,the examination of changes in their enantiomeric fractions?EFs?can provide direct evidence of the stereoselective biotransformation process,whilst not available to give insights into its mechanisms.The extent of stable isotope fractionation during biotransformation process allows for the qualitative and quantitative assessment of pollutant biodegradation,as well as the elucidation of the reaction mechanisms using compound-specific and enantiomer-specific carbon isotope analysis?CSIA and ESIA?.Therefore,in the present study,the bioaccumulation of several PCB and PBDE congeners and o,p'-DDT?HOPs?in common carp?Cyprinus carpio?and tropic transfer in two artificial food chains?spiked food?tiger barb?redtail catfish,spiked food?tiger barb?oscar fish?was investigated.CSIA and ESIA were used to trace the biotransformation process of HOPs?o,p'-DDT,PCBs and PBDEs?in fish,and to investigate the stereoselective biotransformation of chiral chemicals and its internal mechanism.We also attempted to quantify the influence of chemical biotransformation on the biomagnification of organic pollutants using CSIA and ESIA.Besides,researches regarding the bioaccumulation,tissue distribution,and biotransformation of PFRs in aquatic organisms are still in the early stage;only limited studies have reported the in vivo distribution of diesters?DAPs,hydrolysis metabolites of PFRs?in fish,while data on hydroxylated PFRs?HO-PFRs?remains scarce.As thus,a further study was conducted by exposing common carp via water to seven PFRs[tris?2-chloroethyl?phosphate?TCEP?,tris?2-chloroisopropyl?phosphate?TCIPP?,tris?1,3-dichloro-2-propyl?phosphate?TDCIPP?,tri-n-butyl phosphate?TNBP?,tris?2-butoxyethyl?phosphate?TBOEP?,triphenyl phosphate?TPHP?,and 2-ethylhexyl diphenyl phosphate?EHDPHP?],which are frequently detected in aquatic ecosystems.The bioaccumulation and tissue distribution of PFRs and their major metabolites?DAPs and HO-PFRs?were investigated in different tissues?i.e.muscle,liver,gonad,gill,intestine,brain,kidney,and serum?.The results of this study are of great significance for a comprehensive understanding of the bioaccumulation and biotransformation of these organic compounds in aquatic organisms,as well for evaluating the influence of biotransformation process on biomagnification for these compounds.In the experiment of exposing PCB and PBDE congeners to common carp,significantly debromination was found for BDE 85,BDE 99,and BDE 153;five metabolic debromination congeners?BDEs 28,47,49,66,and 101?were also frequently detected in fish tissues,while no debromination was observed for BDE 100and BDE 154.Several hydroxylated PBDEs?HO-PBDEs?were detected in sera of common carp,however,neither hydroxylated PCBs?HO-PCBs?nor methylsulfone PCBs?MeSO₂-PCBs?were detected in the sera or fish tissues of the carp throughout the experiment.The results of the carbon isotope composition(?¹³C value)demonstrated that significantly isotopic fractionation has occurred for PCB 8,PCB 18and PCB 45,and BDE 153,with an isotopic enrichment factor??_C?-1.99‰,-1.84‰,-1.70‰,and-0.86‰,respectively.The absence of?¹³C values for BDE 85 and BDE99 in fish was caused by their rapid metabolism,and no obvious isotope fractionation was observed for most PCB congeners,and BDE 100 and BDE 154.Stereoselective biotransformation was observed for PCB 45,PCB 91 and PCB 95.The ESIA results showed that significantly isotopic fractionation was occurred for both atropisomers of PCB 45,and the?_C of E₁-PCB 45?-1.63‰?was close to that of E₂-PCB 45?-1.74‰?,indicating both atropisomers were metabolized by similar reaction mechanisms.However,because E2 was biotransformed faster than E1,steps,such as uptake of the substrate into the cell and binding of the substrate to the enzyme,other than isotope sensitive carbon bond cleavage might be rate-limiting for the metabolism of PCB 45,and those steps were stereoselective.As for PCB 91 and PCB 95,only one atropisomer of each congener was involved in the biotransformation process.In the experiment of two artificial food chains,significantly debromination was also found for BDE 85,BDE 99,and BDE 153 in tiger barb?TB?,and it can biotransfom o,p'-DDT to form o,p'-DDD and o,p'-DDE as well.Although there are differences in assimilation efficiencies?AE?and depuration rate?k_d?,the calculated biomagnification factors?BMFs?for all PCBs in redtail catfish?RF?were similar to those in the oscar fish?OF?.Debromination metabolism of PBDEs occurred in the OF,but was absent in the RF,which resulted in an increase in the BMF for debromination products and a decrease in the BMF for reactants of debromination.Of the eight chiral chemicals,only o,p'-DDD in the OF exhibited difference in AE between two enantiomers.On the contrary,all chiral chemicals with the exception of o,p'-DDD in the RF showed more or less differences in AE values between the two enantiomers.This result indicates that the enantioselective absorption occurring in the RF is completely different from that in the OF.Differences in the k_d values were observed between the two enantiomers for the six chiral PCBs,but not for o,p'-DDD and o,p'-DDT in the RF.As for the OF,enantiomer-specific k_d was found in all chiral chemicals except for PCB 95 and PCB149.K_d plays a more crucial role than?in determining the enantioselective accumulation in the predator fish.MeSO₂-CBs were detected in the livers of both RF and OF,and HO-PBDEs were frequently detected in the serum,which provided important but limited information on the biotransformation of certain PCB and PBDE congeners in fish species.The results of CSIA demonstrated that no biotransformation for PCB 28,PCB 52,PCB 101 and PCB 138 occurred in both food chains.However,a heavy isotope enrichment was observed for all the other PCB congeners in both RF and OF,except for 149 in the OF.Of all the BDE congeners,only BDE153 in the OF showed a heavy carbon isotope enrichment due to its debromination(The concentrations of BDE 99 and BDE 85 in the OF were too low to measure?¹³C due to their rapid debromination.).The?_C values for o,p'-DDT,and PCB 8 and PCB 18 are comparable between the RF and the OF,suggesting similar biotransformation mechanism for these compounds;whereas the?_C values for other PCB congeners in the OF are approximately two times those in the RF,implying different biotransformation mechanisms exist for these chemicals in these two fish species.A significant positive correlation between the k_d and the??¹³C/?¹³C_initial,and a significant negative correlation between the BMF and the??¹³C/?¹³C_initialnitial were observed for chemicals/enantiomers that had significant alterations in isotopic composition in both the RF and OF.Meanwhile,significant linear positive correlations were also observed between the?BMF/?k_d and??¹³C/?¹³C_initial for PCB congeners and atropisomers in the RF and the OF.These results suggest that the biotransformation rate and the influence of biotransformation of chemicals on the BMF in the two food chains can be predicted by the stable carbon isotope fractionation of chemicals in the predator fish.In the experiment of aqueous exposure of PFRs to common carp,the log K_OW of PFRs was significantly negatively correlated to the percentages of individual PFRs to the total PFRs in serum,whereas significantly positive correlations were observed in all other tissues.All PFRs exhibited relatively low bioaccumulation potential in fish tissues.Significant correlations betweenthe log K_OW of PFRs and their log bioconcentration factor(BCF_ww)were also found in all investigated tissues except for serum.This suggests that the hydrophobicity of PFRs played a significant role in the distribution and body compartment accumulation of PFRs in common carp.The bioaccumulation potential of PFRs in serum was different from the other tissues,probably due to its specific properties.DAP and HO-PFRs were quantified as the major metabolites.Their levels in liver and intestine were significantly higher than in other tissues.Biotransformation processes also played a crucial role in the accumulation of PFRs in fish.Our results provide critical information for further understanding the bioconcentration,tissue distribution and metabolism of PFRs in fish.