Toxicological Effects Of Heavy Metal Cadmium Cd²⁺ And Lead Pb²⁺ Stress On Two Diatoms

Although the rapid development of industrial technology has brought about the progress of human civilization,industrial wastewater containing a large amount of heavy metal ions was discharged into the rivers,lakes,rivers and ocean waters,causing serious pollution to soil and water bodies.Heavy metal ions which accumulation and transmission in aquatic animals and aquatic plants was not only seriously affected the growth and development of aquatic animals as well as plants,but also entered the food chain,and endangering the human health.Diatom plants are a source of food and oxygen for heterotrophic organisms in aquatic habitats and are important primary productivity that directly affects the cycle of the biological chain.Heavy metal ions and other contaminants enter the water body in various ways,which has a bad effect on the primary producers such as diatoms.In addition,they can enter into the food chain to affect the entire aquatic ecosystem and human health.In this study,the freshwater benthic diatoms Halamphora veneta(Kützing)Levkov and Surirella crumena Brébisson were treated with heavy metal cadmium(Cd2+)and lead(Pb2+)after purification.Acute toxicity test was used to determine the growth trend,chlorophyll a content,antioxidant enzyme activities(SOD,CAT,MDA),rbc L,psb A gene expression and morphological changes under Cd2+ and Pd2+ exposurees.The results are as following:1.After heavy metal Cd2+ and Pb2+ treatment,Halamphora veneta(Kützing)Levkov exposurecell density exhibited a promoting trend at the initial stage of exposureexposure,but the cell density decreased significantly during the long-term exposure.Meanwhile,the cell density of Surirella crumena Brébisson of decreased significantly.2.After Halamphora veneta(Kützing)Levkov was under the exposure of heavy metals Cd2+ and Pb2+,the content of chlorophyll a increased first and then decreased under the 24 h-48 h exposure.After the exposure time was extended to 72 h,the chlorophyll a content decreased sharply;After Surirella crumena Brébisson was under the exposure of heavy metals Cd2+ and Pb2+,the chlorophyll a content fluctuated rapidly,and the overall process showed a downward trend.These results may be due to the long-term heavy metal exposure affected the normal metabolism and the physiological and biochemical processes of algae,destroying the chloroplast function of algae,resulting in decreased chlorophyll content and decreased photosynthesis.3.Under the exposure of heavy metals Cd2+ and Pb2+,the activity of SOD and CAT enzyme levels in Halamphora veneta(Kützing)Levkov were inhibited,while the content of MDA was increased.In addition,under the exposure of Cd2+ and Pb2+,the activity of SOD in Surirella crumena Brébisson was increased,and the activity of CAT is inhibited,and the content of MDA was increased.4.After Halamphora veneta(Kützing)Levkov Cd2+ and Pb2+ exposure,the expression levels of rbc L and psb A m RNA genes showed a “decline-rise” trend,while the stress reaction of microalgae was stimulated under the long-term exposure;After Cd2+ and Pb2+ exposure,the expression levels of rbc L and psb A m RNA increased,indicating that the algae cells start the repair mechanism,which increases Protein synthesis when stimulated by harmful substances.5.After Halamphora veneta(Kützing)Levkov was treated by Cd2+,the morphology of cells was not change seriously.Meanwhile,the cells were deformed after the treatment of Pb2+.The back margin was slightly convex.The abdominal margin was concave,and the ends were slightly concave and convex.After Cd2+ treatment,the surface of algae cells in Surirella crumena Brébisson showed nearly elliptical,and the spinous processes disappeare.However,after Pb2+ treatment,the spinous processes of the cell morphology were not change obviously.Our results showed that the growth trend,chlorophyll a content,antioxidant enzyme activity,morphological characteristics and gene expression of Halamphora veneta(Kützing)Levkov and Surirella crumena Brébisson were changed after heavy metal exposure.Therefore,these two diatoms can be used to monitor the degree of contamination of heavy metals Cd2+ and Pb2+ in water environments,and to evaluate the toxic effects of heavy metals Cd2+ and Pb2+.