Toxicities And Mechanism Of Multi-walled Carbon Nanotubes On The Growth And Reproduction Of Xenopus Tropicalis

Carbon nanotubes?CNTs?,a new type of nanomaterials with special physical and chemical properties was widely used in many fields.However,with the global investment in carbon nanotechnology and the increasing production of carbon nanomaterials,CNTs will inevitably release into the aquatic environment,and the possible ecotoxicity has caused widespread concern.Presently,some studies have pointed out that CNTs have toxic effects on aquatic animals such as large fleas and zebrafish,but there are few reports on the toxic effects of CNTs on amphibians.In this study,the Xenpous tropicalis embryo was used as the test animal and the semi-static exposure method was used.The mortality rate,hatching rate,malformation rate,malformation type and probability,heart rate and body length of the embryos were used as the end points of toxicity test.Acute toxic effect of MWCNTs suspension on embryos at different developmental stages and its mechanism of action were studied.The results showed that 25 and 50 mg/L MWCNTs suspension had toxic effects on the X.tropicalis embryo,which increased the mortality and malformation rate to a certain extent,and reduced its hatching rate,but had no significant effect on heart rate and body length?p>0.05?.Among all types of deformities,cardiac edema accounted for the largest proportion?28.7%?,and the notochordal curvature followed.Embryos of different developmental stages were different sensitivity to MWCNTs,from strong to weak in order:gastrointestinal stage,blastocyst stage,nerve stage,and tail bud stage.The main mechanism of MWCNTs may be indirectly related to the adhesion of MWCNTs to the outer glial membrane of the embryos.In addition,the acute toxicity and recovery tests of different concentrations of MWCNTs suspension on X.tropicalis tadpoles were also explored.The results showed that different concentrations of MWCNTs suspensions?0.5,1,2,4,8 mg/L?all had toxic effects on tadpoles,which increased their mortality rate to some extent,induced irregular heart rate,and showed a certain dose-effect relationship.Observation of individual tadpoles by optical microscopy and field emission scanning electron microscopy revealed that tadpoles were very easy to ingest MWCNTs when they lack food,and enriched target organs such as gills and digestive tract,causing different degrees of toxicity to tadpoles.In addition,the low concentration experimental group?0.5 mg/L?had 80%of the living tadpoles,only 43.3%survived after 4 days recovery,while the high concentration experimental group?8 mg/L?all died.There was a direct correlation between the amount of enrichment that exceeds its own tolerance and difficult to excrete.After calculation,the LC₅₀of MWCNTs for tadpoles at 72h was 2.53 mg/L.Finally,the chronic toxicity effects and potential reproductive toxicity of low-concentration MWCNTs on sexually mature X.tropicalis were also explored.The results show that long-term exposure to the aquatic environment of MWCNTs would reduce the weight of tropical clawed frogs and the quality of fat,testes,ovaries and other tissues,but had no effect on the quality of the heart and lungs;it could induce the number of spermatogonia and oocytes and their activity,increased the formation of lung air cannon,but did not significantly proliferate the connective tissue of the heart or liver;MWCNTs could also be ingested by breathing,and enriched in the lungs.In addition,long-term exposure to the aquatic environment of MWCNTs would reduce the fertilization rate and survival rate of the X.tropicalis embryo,and had reproductive toxicity.Based on the above experimental results,MWCNTs have certain toxic effects on X.tropicalis at different developmental stages.The results of this study provide a scientific basis for the rational evaluation of the toxic effects of MWCNTs on aquatic animals.