| At present, marine pollution, especially heavy metal pollution aggravates in China, which result in deterioration of mudflat shellfish resources. In order to reveal the mechanism how heavy metals impact on bivalves, biochemical, cellular and immunological characteristics of M. veneriformis response to heavy metals exposure were studied. Heavy metal concentrations in M. veneriformis and in surface sediments along coastal area in Tianjin were investigated. Besides, it was studied that the bioaccumulation ability, assimilation and detoxification mechanism of cadmium and mercury by M. veneriformis, and the burden dose in different tissues. The toxic effect of heavy metals on structure and function of haemocytes in M. veneriformis was also studied. This study demonstrates the cellular response to heavy metal intimidation, which may reveal a correlation between heavy metal pollution and shellfish resources deterioration. The present work could help to establish fast and credible biomarker system, and provides scientific basis for ecological evaluation and restoration. In addition, the results may also provide biological reference for the implementation of marine water quality standards to protect these organisms. The results are as follows:1. According to the annual investigation on Tianjin coastal area, significant difference was found among the content of Cd, Pb, Cu, Zn, Mn, Cr and Ni both in M. veneriformis tissues and surface sediments. The heavy metal concentrations of Cd, Pb, Cu, Zn, Mn, Cr and Ni were 0.69-5.19,5.83-36.66,0.89-58.88,47.02-220.80, 121.89-1492.38,11.90-82.75 and 20.19-266.15μg/g in sediments,0.95-6.64, 0.37-14.15,2.91-23.35,23.55-111.81,13.17-50.45,1.98-24.56 and 1.70-56.17μg/g in M. veneriformis,1.02-6.01,0.23-13.53,12.09-42.28,36.93-133.05,15.27-73.80, 3.13-28.84 and 2.19-49.53μg/g in B. extra, respectively. The Cd in surface sediments was very high and had strong potential ecological risk. M. veneriformis has high bioaccumulation ability to Cd and Hg, thus it can be used as a valid bioindicator to heavy metal pollution.2. The acute toxicity of Cd and Hg to M. veneriformis was evaluated.The 24h LC50,48h LC50 and 96h LC50 for Cd was 15.96,5.15 and 2.38 mg/L, respectively, whereas for Hg was 3.71,0.61 and 0.21 mg/L, respectively. The safe concentration for M. veneriformis was 0.0238 mg/L for Cd and 0.0021 mg/L for Hg.3. The soft tissue of M. veneriformis showed different bioaccumulation ability to Cd and Hg. The increasing amount of Cd and Hg in tissues was 0.12-7.7μg/g and 0.002-0.024μg/g, the bioaccumulation rate of those were 0.3-6.2% and 0.11-0.68%, and the absorption rate constant of those were 0.07-1.10 and 0.001-0.005, respectively.4. Both Cd and Hg had significant effects on the metallothionein (MT) contents in mantles, gills and hepatopancreas during the whole experimental time for all concentration treatments (p<0.05) in comparison with controls. The tissue MT concentrations were hepatopancreas> gills> mantles. We suggested hepatopancrea as a valid tissue in monitoring marine heavy metal pollutions. The MT concentrations were significant correlated with Cd content in tissues, while the correlation of MT and Hg was not significant.5. Cadmium had a significant effect on activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidases (GPx), malondialdehyde (MDA), acid phosphatase (ACP) and alkaline phosphatase (ALP) in gills and hepatopancreas of M. veneriformis during the whole experimental time for all Cd treatment (P<0.05) in comparison with controls, and the activities changed like a peak. Hepatopancrea CAT and GPx activities were higher than gill, while SOD and MDA showed the opposite incline.6. The ultrastructural changes, lysosome membrane stability-neutral red retention (NRR), micronuleus frequency (MN) and total abnormal nucleus frequency (TAN) of hemocyte of M. veneriformis were measured exposed to cadmium and mercury. The significant effect of these biomarkers was found under diverse concentrations except 2μg/L Hg (P<0.05). The NRR, MN and TAN exposed to mercury were higher than to cadmium under the same concentration and exposed time. There was significant linear correlation among NRR, MN and TAN, which may be used as valid biomarkers to heavy metal contamination. |
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