| Cadmium pollution is a serious concern in coastal areas of China,and the salinity of seawater in this area is lower than the normal value due to tidal cycles,climate change and freshwater inflow.At present,limited studies on the effect of low salinity on the toxic effects of Cd have been published.Previous studies mainly focused on the use of acute toxicity experiments to determine the median lethal concentration,or by chronic toxic experiments exploring the absorption rate and the Cd bioaccumulation under low salinity condition.However,there is still a lack of research exploring the effect of low salinity on Cd toxicity from multiple biological processes,such as ion balance,oxidative stress and energy metabolism.Therefore,it is difficult to reveal the mechanism of the effect of low salinity on Cd toxicity.In this study,adult oysters Crassostrea gigas were exposed to Cd at four concentrations(5,20,80 and 240μg·L-1)under normal（31.4 psu）or low（15.7 psu）salinity condition for 7 and 28 days.The potential impact of low salinity on the toxicological effects of Cd was investigated by measuring the changes of Cd and essential elements and biomarkers.Combining with the integrated biomarker response（IBR）method,the toxicity differences of single and combined exposure to Cd and low salinity were evaluated.Details of the thesis were listed below.（1）The bioaccumulation of Cd in oyster gills increased with the increase of Cd exposure concentrations,and the contents of some essential metal elements and the activities of some enzymes also changed significantly.Cd interfered with Mn,Fe and Mg ion balance by activating divalent metal ion transporters on the cell membrane,and could also affect Na and K contents by inhibiting the activity of NKA.Cd exposure inhibited the activity of antioxidant enzymes in oyster gills,and induced oxidative stress and lipid peroxidation.Exposure to high concentrations of Cd might increase the Fe contents by up-regulating the expression of Fe2+transport-related genes,which would increase the burden on the antioxidant system through the Fenton reaction.The response results of endpoints related to energy metabolism showed that Cd exposure interfered with key metabolic pathways,such as TCA cycle and glycolysis in oyster gills,suggesting that the body consumed energy to repair oxidative damage and maintain ion balance,and might adopt a strategy of anaerobic metabolism to meet the increased energy demand caused by Cd exposure.Compared with the exposure for 7 days,the contents of some essential metal elements and endpoints related to antioxidant and energy metabolism in oyster gills also changed significantly after exposure for 28 days,suggesting that long-term exposure to Cd could increase disturbance of ion balance,oxidative stress and energy metabolism disorder.（2）Low salinity had limited effects on the bioaccumulation of Cd in oyster gills,but aggravated the toxicological effects of Cd,mainly because Cd and low salinity had synergistic effects in inducing disturbance of ion balance,oxidative stress and energy metabolism disorder.Further analysis of the mechanism by which low salinity aggravated the toxic effect of Cd found that low salinity could significantly inhibit the activity of antioxidant enzymes,and caused the contents of GSH and MDA to increase significantly,indicating that low salinity induced oxidative stress and lipid peroxidation;low salinity could also increase the Fe contents by up-regulating the expression of Fe2+transport-related genes in the oyster gills,thereby increasing the burden of the antioxidant system through the Fenton reaction.Especially under the combined stress of Cd and low salinity,the change of redox balance caused by low salinity could further aggravate the oxidative stress induced by Cd.Further IBR analysis showed that the combined effect of Cd and low salinity was much greater than that of single Cd exposure,indicating that low salinity aggravated the toxicological effects of Cd on oysters.Therefore,the impact of low salinity on the toxicity of environmental contaminants should be fully considered to comprehensively evaluate their environmental risks in coastal zones.（3）The effects of low salinity on the toxic effects of Cd changed with time.After 7 d and 28 d of exposure,there were significant differences in the effects of Cd and low salinity combined stress on the variations of essential elements and biomarkers.Especially in the disturbance of ion balance:After 7 days of exposure,the combined stress of Cd and low salinity could induce hormesis indicated by Mn contents;the combined stress of high concentration of Cd and low salinity might enhance the NSCCs of the cytoplasmic membrane to increase K contents,and the decrease of Zn contents might be caused by regulating the gene expression of biological pathways such as Zn2+absorption or efflux.After 28 days of exposure,the combined stress of low concentration of Cd and low salinity might lead to the loss of some unknown osmotic substances in the gills,and then stimulated the body to increase the contents of Na and Mg to maintain the osmotic balance.In addition,the trend of ALT activities in the gills cultured in low salinity condition changed significantly after 7 and 28 days of exposure:After 7 days of exposure,the combined stress of Cd and low salinity induced an increase in ALT activities to compensate for the shortage of pyruvate supply caused by the decreased PK activities;The activities of ALT decreased after 28 days of exposure indicating that the combined stress might cause damage to the gills,suggesting that the tolerance of oysters subjected to long-term low salinity stress to Cd was reduced. |
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