Toxicological Effects Of Heavy Metal(Cu²⁺,Hg²⁺) Stress On The Ecology Of Diatom（Halamphora Veneta ）

In recent years,the water environment pollution has received widespread attention.Heavy metal pollution is one of the major problems in aquatic environment pollution.The intensification of heavy metal pollution will not only affect the growth of diatoms,but also cause ecological disfunction of diatoms.A diatom-micronilium was successfully isolated through laboratory cultivation and microtubule separation.The cell growth,changes of the content of chlorophyll a and the SOD enzyme activity under the exposure of the two heavy metals Cu²⁺ and Hg²⁺ were investigated of the Halamphora veneta（Kützing）Levkov.In addition,the effects of the heavy metals on the morphology and the expression levels of the heavy metals on SOD genes of H.veneta were analyzed.These results will provide valuable information for the evaluation of heavy metal environmental risk of freshwater bodies and the usage of freshwater diatoms for bioremediation,providing basic data for the future toxicological studies of freshwater diatoms.The main conclusions of this study are as follows:1.CSI culture medium was selected as the better diatom medium after the different culture mediums and culture conditions test.2.With the exposure of Cu²⁺ for 24 h、48h、72h、96 h,the EC50 values of H.veneta were 6.63mg·L^-1,4.88mg·L^-1,2.45mg·L^-1,1.49mg·L^-1.With the exposure of Hg²⁺ for24 h、48h、72h、96 h,the EC50 values of H.veneta were 12.97mg·L^-1、9.30mg·L^-1、7.13mg·L^-1、4.97mg·L^-1,respectively,which indicated the toxicity of the copper on the H.veneta was higher than that of the mercury.3.The cell density during the initial stress period was inhibited by Cu,while it was decreased gradually with the increase of Hg²⁺ stress concentration and the prolongation of exposure time.4.The effect of heavy metal stress on the chlorophyll a content of H.veneta showed that the content of chlorophyll a in 2.0mg L^-1 of Cu²⁺ stress was lower than that in control group.The reason might be that Cu²⁺ destroyed the structure of the plasma membrane and the balance between organelle membrane and ion,which resulted in the decrease of chlorophyll a content.Under Hg²⁺ stress,chlorophyll a content increased first and then decreased.It is assumed that low concentration of Hg²⁺can slightly promote chlorophyll synthesis.Our results indicated that the chlorophyll a content could be used as a biomarker of diatoms to heavy metal stress.5.Compared to the control group,with the stress of Cu²⁺,SOD enzyme,activity in the H.veneta was more facilitated,and its stress concentration firstly increased and then decreased with the extension of exposure duration.With the stress of Hg²⁺,SOD enzyme,the activity was firstly progressed and then suppressed with the extension of exposure duration.As a result,SOD enzyme activity in the H.veneta can be regarded as the antioxidant marker.6.The gene expressions showed that Cu²⁺stress could suppress the expressions of SOD genes.SOD gene expressions could be significantly induced at the lower Hg²⁺stress concentration.With the stress of Hg²⁺,the SOD expression is not significantly changed.The heavy metal stress affected the antioxidant enzyme activity of the diatom.7.The changes of SOD enzyme activity and the difference of SOD gene expression induced by Cu²⁺and Hg²⁺stress were closely related to the treatment concentration and stress time.The results showed that there was inconsistency between the activity of SOD and the expression of SOD gene,and the possible reason was that the transcription and translation were not synchronized.However,the regulation mechanism of the enzyme expression at gene level and protein level under heavy metal stress needed to be further explored.8.The effects of heavy metal stress on the cellular morphology of H.veneta were determined by optical microscope and scanning electron microscope.Our results showed that the malformation of the cellular morphology of the H.veneta was observed under Hg²⁺stress,which was not fund under Cu²⁺stress.