| On the basis of the investigation of the content and distribution of heavy metals in sediments and benthic organisms from the sewage stream in Guangzhou city, the antagonistic effect of selenium on the harm of mercury (Hg2+) to the tissues in swordtail fish (Xiphophorus helleri), the 22nd offspring purely cultivated by Pearl River Fishery Research Institute, were measured by physiological and biochemical indexes and observation of electron microscopy. The main study in this paper included as follows: the content and distribution of heavy metals in sediments and benthic organisms from the sewage stream in Guangzhou city; the acute toxicity and joint toxicity of mercury and selenium to swordtail fish (Xiphophorus helleri); the damage of mercury to the indexes of antioxidant system in the gills and livers in swordtail (including the measurement of the activities of total antioxidative capacity[T~AOC], Superoxide dismutase [SOD], Glutathione peroxidase[GSH-Px] and the concentration of malondiald-ehyde [MDA])and the relief effects of selenium on it, as well as the physiological damage of mercury on the tissues, namely: the antagonistic effect of Na+-K+~ATPase activity on the tissues between selenite and mercury, and the ultrastructural damage under the exposure of mercury.Employing the flame atomic absorption spectrophotometer, we measured and evaluated the concentrations and distribution of heavy metals in sediments and benthic organisms collected from the sewage stream in Guangzhou. The average concentrations of Cd, Cu, Zn, Pb, Cr, Ni in sediments were 5. 61, 302.12, 727.03, 163.37, 67.70, 84. 58 g/g dry wt ; 0.78, 29.20, 268.38, 10.75, 3.89, 3. 44 Mg/g dry wt in benthic organisms, respectively. The results indicated that all streams in Guangzhou have been polluted by heavy metals in different degree. Chigang stream, Haizhu stream and Shangxia stream are the most serious. Industry activities are responsible for the pollution mainly. The concentrations of heavy metals in benthos(aquatic oligochaetes) is remarkablely correlated with that in sediments (P<0. 01, P<0. 05), so the former can be used as bioindicator of Ni, Cu and Cr.The acute and joint toxicity to swordtail fish (Xiphophorus helleri) were studied in the present test. The LC50 of divalent mercury to swordtail fish were 0.84, 1.35, 1.71 mg/L in 24, 48 and 96 h respectively. The LC50 of tetravalent selenium to swordtail fish were 6.64, 12.59, 17.91 mg/1, respectively. The joint toxicity of the two toxicants to swordtail fish was synergistic in 24 h and antagonistic in 48h and 96h, both when treated as the ratio 1:1 of toxicity and concentration. The results also showed that since the high sensitivity to mercury and selenium, the swordtail fish could be a relatively ideal environmental biomarker.The swordtails were exposed to Hg2+ and Se4+ for 7 d and by examining the change of the antioxidant T-AOC, SOD, GSH-Px and MDA gills and liver tissues, we researched the protective effect of microelement Se4+ on the heavy metal mercury (Hg2+), the results showed that obvious damages to the antioxidant system and membrane system by all the selected con- centrations of Hg2+. The antioxidant abilities of protective enzymes were enhanced in restricted range by Hg2+ stress in the early period and the lower concentrations, which were weaken because of the further exposure,especially in liver. Compared to the single Hg2+ treated, se4++Hg2+ the mixed exposure could restrain the fall of the activity of T?AOC, SOD and GSH ?Px, reducing the level of lipid peroxidation, and alleviating membrane damage, represented with the heighten Na+-K+-ATPase activity and reduced MDA concentration, resulting in the increased resistance to Hg2+damage. However, Se4+ were incapable of preventing from the damage of Hg2+ ultimately, along with the exposure period, regardless of the fact that the obvious enhancement of T- AOC and ATPase activity.Applying electric microscopic technique, we oberserved theultrastructure of gill and liver in the swordtails treated with...
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