Screening Of Biomarkers In Bivalve Meretrix Meretrix And Ecotoxicological Effects Assessment Based On Available Heavy Metals In Gulf Sediments

Different chemical speciations of heavy metals in marine sediments have greatlydifferent chemical stability, migration, transformation, bioavailability and potentialtoxicity. It is difficult to evaluate ecotoxicological effects of heavy metals effectivelyonly by analyzing the total contents of heavy metals. Most ecotoxicological effectsstudies about heavy metals’ chemical speciation in sediment focused on thebioaccumulation and lethal effect. Molecular biomarkers can efficiently reflectinteractions between pollutants and the target molecules of cells, and they have aseries of advantages such as high sensitivity and low cost. Nevertheless, assessmentsof heavy metals’ ecotoxicological effects in sediments used to be based on the totalcontents of heavy metals. No studies with respect to relations of biomarkers andheavy metals chemical speciation in sediments on the under sub-lethal condition hadbeen reported.In this study,11stations were set up in the Beibu Gulf coast area by differentfunction, such as breeding areas, sewage zones, and the surface sediments andadvantages benthic bivalve (Meretrix meretrix) collected. Chemical speciation (acidextractable, reducible, oxidizable and residual fraction) of seven typical heavy metals(Hg, As, Cu, Pb, Zn, Cd, Cr), aluminum, TOC, particle size, physical and chemicalproperties in sediments were determined. And we determined12kinds of biochemicalparameters in gills and viscera of M. meretrix: superoxide dismutase (SOD), catalase(CAT), glutathione peroxidase (GPx), glutathione peptide sulfur transferase (GST),thibabituric acid reactive substance (TBARS), reduced glutathione(GSH), oxidizedglutathione (GSSG), metallothioneins (MTs), acetylcholinesteras (AChE), oxidizedand reduced glutathione ratio (GSH/GSSG), total glutathione (tGSH), oxidative stressindex (OSI). The distribution of each chemical speciation and total contents of heavy metals were analyzed. Correlation analysis was used to find out relationships betweenbiochemical indicators in gills, viscera and available and total contents of heavymetals in sediments. The response pattern and sensitivity to available and totalcontents of heavy metals were compared. Then two sets of biomarker combinationswere screened. The second generation integrated biomarker response (IBRv2) indexeswere calculated based on the two combinations, which can reflect the biologicalresponse mechanism of environmental damage. The two sets of biomarkers werecompared by assessing the responses’ sensitivity and reliability, and an effectiveevaluation method was established for the determination of heavy metalcomprehensive toxicity in sediment based on true biological effects. Thus theapplication effect of biomarkers screening and toxicity assessment the based onavailable heavy metals was evaluated. Following are the main conclusions:(1) All of the measured heavy metals’ available contents in the Beibu Gulfintertidal sediments, except Cr, were consistent with the total contents. Cr, Hg and Asmainly occurred in the residual fraction, while Cd dominantly presented in thenon-residual fractions in the sediments. There were six stations for Pb and threestations for Zn and Cu in an available form-based, and the remaining stations areresidual form-based. The distribution of available Hg, Cd, Cr were significantlyaffected by the physicochemical properties of sediments. The organic matter and fineparticles contents in sediments could affect the distribution of available Hg, Cd and Cr,and there were no significant relationship between the physical and chemicalproperties of sediment and total contents of heavy metals was. The traditionalchemical evaluation results based on chemical speciation of heavy metals insediments are not consistent and not adequately consider biological effects and thecombined effect of contaminants.(2) Biochemical parameters in gills and viscera of M. meretrix varied patternsand sensitivity of responses to different types and forms of heavy metals, and thereare obvious differences in the organization and spatial distribution. The biochemicalparameters of M. meretrix were more sensitive to available heavy metals than toresidual and total. Available heavy metal content reflected the biological toxic effects of heavy metals in sediments better than the total. The glutathione antioxidant systemof M. meretrix had a sensitive response to a variety of different chemical speciation ofheavy metals, and they could be used as biomarkers of heavy metal pollution insediments.(3) The screened biomarkers combination screened according to the responses oftotal contents of heavy metals are: GSHg/GSSGg, OSIg, TBARSg, GSTv, GSHv,GSHv/GSSGv(subscript g and v represent gill and viscera respectively), and thescreened biomarkers combination according to the responses of available heavymetals are: GSHg/GSSGg、OSIg、GSTg、GSSGg、CATg、GSTv. Compared withbiomarkers combination screened according to the total contents of heavy metals, thecombination based on available contents had higher sensitivity and credibility, and theintegrated biomarker index (IBRv2A), calculated according to it had a more effectiveindication of metal pollution than IBRv2T, which was calculated based on the former.The order of comprehensive toxicity of heavy metals in different site’s sedimentsobtained from IBRv2Aanalysis is: S5(Yingpan shipside)> S1(Liusha Bay)>S6(Xichang)> S8(Luerhuan river)> S10(western of Fangchenggang)> S9(eastern ofFangchenggang)> S7(Xiniujiao)> S3(Lemin)> S4(Tieshangang)> S11(Yueliang Bay)>S2(Jianghong).（4）Compared with traditional risk assessment method, the integrated biomarkerindex method based on available heavy metals, is better to reflect the ecotoxicologicaleffects of heavy metals in sediments, and had has a series of advantages such assensitive, reliable, fast and convenient, low cost and show good prospects.