| The effects of heavy metal pollution-a traditional pollution-on local environments and organisms can be substantial and long lasting. More importantly, toxic metals can be taken up by marine organisms, entering the food chain and be potentially transferred to the upper trophic levels, which can eventually lead to adverse effects on humans due to the consumption of contaminated seafood. Mercury, one of typical traditional heavy metal pollutants, has been demonstrated its harmful effects to aquatic organisms by numerous studies. With the rapid development of nanotechnology, heavy use of nanomaterials and make them become one of emerging contaminants. Nano-titanium oxide (nano-TiO2), with mixed crystal lattice and photocatalytic activity, is the most potential application as a nanomaterial. In the environment, it is rare that there is only one toxicant present. Nanomaterials can interact with other toxicants (such as heavy metals) to give different responses on target organisms. The combined effects are not the simple sum of the single effect. There is now substantial research showing the interaction mechanism between heavy metals. While the interaction mechanism between heavy metals and nanomaterials will obviously require further investigation. Marine bivalves are the most frequently used biological monitoring indicators of marine pollution. In this study, Ruditapes philippinarum was chosen as the biological monitoring indicator. A 96-h acute toxicity test was conducted to investigate the median lethal concentration (LC50) of Hg(Ⅱ) under different nano-TiO2 concentrations (0,0.50,5.00 mg L-1). Then a 72-h acute toxicity test was conducted to investigate the enzymes activity, including catalase (CAT), superoxide dismutase (SOD), glutathione S-transferase (GST), and acetylcholine esterase (AChE), induced by Hg(Ⅱ) (0,1.00,50.00μg L-1) in the presence of low concentrations of nano-TiO2 (0,0.50 mg L-1). Accumulated mercury in R.philippinarum after 3-d exposure to Hg(Ⅱ) at different concentrations in the absence and presence of 0.50 mg L-1 nano-TiO2 was quantified to evaluate the combined effects of Hg(II) and nano-TiO2 on R.Philippinarum. The aim of the study was to provide basis for interaction mechanism between toxicants and combined effects on aquatic organisms.Results showed:1. The effects on acute toxicity of Hg(II):In the presence of nano-TiO2 (0.50, 5.00 mg L-1),96 h LC50 (0.862,0.820 mg L-1) were lower than absence one (0.950 mg L-1). The nano-TiO2 enhanced toxicity of Hg(Ⅱ) to R.Philippinarum. And, under the condition of high concentration of the nano-TiO2,96 h LC50 was much lower than that of low concentration and the Hg(Ⅱ) alone group, which may be related to the interaction mechanism between Hg(Ⅱ) and nano-TiO2.2. The effects on enzyme activity:The SOD activity was significantly increased by 50.00μg L-1 Hg(Ⅱ) exposure treatment compared to the control group (62.591± 11.84 U mg’prot). The SOD activity was highest in clams treated with 50.00μg L-1 Hg(II) in presence of 0.50 mg L-1 nano-TiO2 (89.63±6.08 U mg-1prot), and it was significantly higher than that in the group in absence of nano-TiO2 (19.03%); The CAT activity was significantly increased by each Hg(Ⅱ) exposure treatment compared to the no Hg(Ⅱ) ones. Exposure to 50μg L-1 Hg(Ⅱ) in presence of 0.50 mg L-1 nano-TiO2 resulted in the highest CAT activity (24.772.29 U mg-1prot), and it was significantly higher than that in the group in absence of nano-TiO2 (25.10%); The GST activity was significantly higher upon exposure to high concentration of Hg(Ⅱ) (50μg L-1) than in the no Hg(Ⅱ) treatment. The presence of nano-TiO2 further increased the effect of high concentration of Hg(Ⅱ) on GST activity (65.75±3.95 U mg-1prot), and the GST activity was 25.21% higher than that in the group in absence of nano-Ti02.3. The effects on the nervous system:There was no significant differences in AChE activity between the exposure groups and the control.4. The effects on mercury bioaccumulation:The mercury accumulation amount of each exposure group was increased with the increase of Hg(Ⅱ) concentration. For the 2 high Hg(Ⅱ) concentration groups, the accumulation of with nano-TiO2 group is higher than that of the without nano-TiO2 one.The results of this study show that the nano-TiO2 can enhance toxicity effect of Hg(Ⅱ) on R.Philippinarum. In the presence of nano-TiO296 h LC50 were reduced. And the higher nano-TiO2 concentration made it reduced more. The activity of various enzyme (CAT, SOD, GST, AChE), as biomarkers, presented different change. The presence of nano-TiO2 also made the mercury accumulation increased in the tissue of R.Philippinarum. Nano-TiO2, as the carriers of Hg(Ⅱ), may enhanced the toxicity of Hg(Ⅱ) to R.Philippinarum by increasing the mercury bioaccumulation. |
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