Toxic Effects Of Heavy Metal Pollutants Exposure On Phascolosoma Esculenta

In recent years, rapid economic growth and development in the region has led to excessive release of heavy metal pollutants into the wetland. The accumulation of contaminants in wetland sediments is likely to pose serious environmental problems in mangrove ecosystem, which seriously endangered the growth, survival, idioplasm and farming of macrofauna. This paper studied the effects of heavy metal (Cd, Pb, Cu and Zn) on acute toxicity, oxygen consumption rate (OCR) and ammonia excretion rate (AER), the survival and growth, metal contents and bioconcentration factors (BCF) in the muscle, digestive enzymes activities including amylase, cellulose and lipase, antioxide enzymes activities including superoxide dismutase (SOD) and catalase (CAT), metallothionein (MT) content of Phascolosoma esculenta (Chen et Yeh), meanwhile discussed the toxic and detoxicfied courses of heavy metal in P. esculenta. The results were showed as follows and hoped to provide scientific data for ecotoxicology research on mangrove macrofauna and can guide the reserve and restoration of mangrove ecosystem. And this paper was helpful to the establishment of the evaluation system of sipuncula safe aquaculture, processing and consumption, which was a guideline for the development of ecological fishery.1. Single and joint acute toxicity test of Cd²⁺, Pb²⁺, Cu²⁺ and Zn²⁺ to Phascolosoma esculentaThe 48-h LC₅₀ values (95% confidence interval estimation) of Cd²⁺, Pb²⁺, Cu²⁺, Zn²⁺, (Zn²⁺+Cd²⁺), (Zn²⁺+Pb²⁺), (Zn²⁺+Pb²⁺+Cu²⁺) and (Zn²⁺+Pb²⁺+Cu²⁺+Cd²⁺) were 10.69 (7.40～15.44), 29.43 (22.69～38.17), 52.77 (30.09⁹2.55), 59.67 (40.48～87.94), 109.74 (32.41～371.63), 72.82 (42.84～123.79), 17.05 (11.18～26.01) and 11.94 (9.01～15.81) mg/L, respectively. The 96-h LC₅₀ values (95% confidence interval estimation) of those were 4.48 (3.34～5.99), 10.59 (9.24～12.80), 6.77 (4.27¹0.72), 10.91 (8.36～14.17), 13.21 (5.97～29.24), 11.73 (9.08～15.15), 0.46 (0.12～1.69) and 3.05 (2.22～4.19) mg/L, respectively. The safe concentrations of Zn²⁺, Cu²⁺, Pb²⁺and Cd²⁺ for P. esculenta were 0.55, 0.34, 0.53 and 0.22 mg/L, respectively. For (Zn²⁺+Cd²⁺), (Zn²⁺+Pb²⁺), (Zn²⁺+Pb²⁺+Cu²⁺) and (Zn²⁺+Pb²⁺+Cu²⁺+Cd²⁺), the 48-h AI values were -16.914 (-25.699～-10.464), -2.695 (-16.169～-6.999), -1.783 (-4.1472～-1.391) and -0.949 (-2.862～-0.970) respectively, while the corresponding 96-h AI values were -6.358 (-9.692～-3.918), -1.183 (-7.303～-4.257), 2.928 (-3.101～1.353), -0.699 (-2.331～-0.75), respectively. Under laboratory condition, the toxicity sequence of the four heavy metals was Cd>Cu>Pb>Zn. Except the 96-h AI value of (Zn²⁺+Pb²⁺+Cu²⁺), the other 48-h AI and 96-h AI values were negative, indicating that the other mixture of these metals had little additive (potentially antagonism) effect on mortality of P. esculenta. The joint toxicity of (Zn²⁺+Pb²⁺+Cu²⁺) to P. esculenta in 96 h was greater than those of any single species of these heavy metals, showing a more toxic (potentially synergistic) effect. And this revealed that the joint toxicity of heavy metals was complex.2. Effects of Cd²⁺, Pb²⁺, Cu²⁺, Zn²⁺ on OCR and AER of Phascolosoma esculentaThe effect of heavy metals on these physiological variables of P. esculenta can be discerned. The respiration and ammonia excretion were inhibited by nonessential Cd²⁺ or Pb²⁺, the oxygen consumption rate (OCR) and the ammonia excretory rate (AER) were reduced, and low values of the O/N ratio were obtained, indicating a protein catabolism. On the other hand, essential Cu²⁺ or Zn²⁺ had a marked inhibition effect on ammonia excretion, and induction at low, moderate concentrations but inhibition at a high concentration on respiration. High O:N values were obtained in individuals which could indicate a lipid catabolism. The degree of inhibition by (Cd²⁺+Zn²⁺) was lower than that of Cd²⁺ individually, which might indicate that mixture of these two metals reduced Cd toxic effect.3. Effects of Cd²⁺, Zn²⁺ and (Cd²⁺+Zn²⁺) on the survival and growth of Phascolosoma esculentaIn this part, the survival rate and the weight gain of P. esculenta were conducted. Results suggested that the survival rate of Cd²⁺ groups were higher than 45 %, while higher than 50 % in Zn²⁺ or (Cd²⁺+Zn²⁺) and furthermore it decreased in accordance with the increase of metal ions concentrations. The similar decrease tendency of weight and the weight gain was also observed. The degree of inhibition by (Cd²⁺+Zn²⁺) to the survival rate and the weight gain was lower than that of Cd²⁺ or Zn²⁺ individually, which might indicate that mixture of these two metals had little additive (potentially antagonism) effect compared with single metal toxicity. 4. Changes of metal contents and bioconcentration factors (BCF) of the muscle in Phascolosoma esculentaThe results showed that the contents of Cd or Zn of the body muscle in P. esculenta were elevated with the increasing concentration of Cd²⁺ or Zn²⁺. The BCF of Cd or Zn was reduced in the increasing concentration group. The BCF was as large as 7042 of Cd, while 1432 of Zn in the single toxicity, respectively. That indicated Cd or Zn was accumulated strongly by P. esculenta, and the toxic effects of metals were pricked up consequently. The few contents of Cd or Zn and lower BCF in joint toxicity of (Cd²⁺+Zn²⁺) might indicate that mixture of these two metals had little additive (potentially antagonism) effect compared with single metal toxicity.5. Effects of Cd²⁺, Zn²⁺ and (Cd²⁺+Zn²⁺) on the digestive enzymes activity in Phascolosoma esculentaIn this part, the effects of cadmium, zinc and mixture of both metals on the digestive enzymes activity in P. esculenta were investigated. The inhibition effect of heavy metals on activities of some important digestive enzymes, e.g. the amylase and cellulase, could be noted in the intestines in varied extents when increasing the level of Cd²⁺, Zn²⁺ or (Cd²⁺+Zn²⁺), respectively. The degree of inhibition on the cellulase was higher than that of amylase. In contrast, the activity of the lipase was found to increase in all treated groups and represented a dose-effect and time-effect relationship. The phenomena above suggests that the variations on the activity of amylase, cellulase and lipase enzymes can indicate sensitively the inhibition effect and toxicity of Cd²⁺, Zn²⁺ in P. esculenta. The degree of inhibition or induction by (Cd²⁺+Zn²⁺) was lower than that of Cd²⁺ or Zn²⁺ individually, which might indicate that mixture of these two metals had little additive (potentially antagonism) effect compared with single metal toxicity.6. Effects of Cd²⁺, Zn²⁺ and (Cd²⁺ + Zn²⁺) on the antioxidant enzymes activity in Phascolosoma esculentaFor the changes of antioxidant defense system after metals exposure, the results showed that with the prolonged time and increased concentrations of metal ions, activities of SOD and CAT were induced significantly (p<0.05) and the variation of the activity induced by different concentrations of metal ions showed a parabola trend in the intestines. The SOD activity increased greatly with the exposure time at a low concentration of Cd²⁺ (0.005 mg/L) , while the activity was induced significantly during the initial period but decreased rapidly later at moderate (0.05 mg/L, 0.1 mg/L) and high (0.5 mg/L) concentrations. The SOD activity was induced significantly during the initial period but decreased rapidly later at all Zn²⁺ concentrations. The changes of SOD and CAT activity might indicate that normal functions of antioxidant defense system have been disturbed by a large amount of active oxygen produced from metals bioaccumulation. The degree of induction by (Cd²⁺+Zn²⁺) was lower than that of Cd²⁺ or Zn²⁺ individually, which might indicate that mixture of these two metals had little additive (potentially antagonism) effect compared with single metal toxicity.7. Effects of Cd²⁺, Zn²⁺ and (Cd²⁺+Zn²⁺) on the metallothionein content in Phascolosoma esculentaA Cadmium-Saturation Method was used to investigate the inducement of MT synthesis by Cd²⁺, Zn²⁺ and (Cd²⁺+Zn²⁺), respectively. The results indicated that the MT content in the intestines changed significantly (p<0.05) among the treatments, and a peak value appeared during the experimental time while no significant changes were found in the control (p>0.05). The maximum content appeared in 29 days, 25 days, 25 days and 29 days in four concentrations of Cd²⁺, respectively, then decreased with prolonged exposure and represented a dose and time relationship during a certain experimental period. The MT content increased significantly and changed significantly with the increasing of Zn²⁺ concentrations and prolongation of exposure time(p<0.05). Therefore, metals (Cd²⁺, Zn²⁺ and Cd²⁺+Zn²⁺) exposure would increase the content of MT in the intestines of P. esculenta. The degree of induction by (Cd²⁺+Zn²⁺) was lower than that of Cd²⁺ or Zn²⁺ individually, which might indicate that mixture of these two metals had little additive (potentially antagonism) effect compared with single metal toxicity.