The Toxicity Effects Of Different Types Of CNTs On Scenedesmus Obliquus

With the rapid development of nanotechnology, the application fields of carbon nanotubes(CNTs) is becoming more and more widespread because of its so many unique physical and chemical properties as one type of nanomaterials. And at the same time, there is an expansion of the potential for release into ecological environment, also, the biological safety problems of nanomaterials including CNTs have attracted high attention from the whole world. CNTs’features of toxicology study are determined by its unique physicochemical properties, so that the safety evaluation and risk assessment of CNTs are becoming a new topic of toxicology in21century.This paper studied the influence of four different types of CNTs stress on Scenedesmus obliquus in fresh water. The paper studied from different physiological and biochemical indexes to assess the safety of CNTs, and to provide theoretical basis for mechanism of nanomaterials inhibiting algae’s growth. The96-h acute toxicity exposure test was designed a series of gradient concentrations for various kinds of CNTs, then the OD650of the cultured algae were recorded every24h and the densities of algal cells exposed to test suspensions were measured and the morphology of these cells was observed. Contents of photosynthetic pigments for each treatment with the activity of SOD and POD were all detected. These detections were done to figure out the toxic effects of these CNTs on Scenedesmus obliquus. The results are as follows:1. The growth inhibition test showed that, at some range of low concentrations, the growth of algae was stimulated by CNTs, and algal number was promoted to raise during a short-term of the test. The same situation was not only found in early period of these low concentration groups, but also in the middle and late period of those moderate concentration treatments. At the high concentration groups, the growth of algae had been highly inhibited during the whole test. The algae in high concentration groups was found to be stressed by CNTs through a microscope. According to the EC50of each type of CNTs, their toxicities were in the following order:SWCNTs> Short SWCNTs>DWCNTs> Short DWCNTs. As the testing time went by, we found that the EC50decreased which suggested that the CNTs had inhibition effect on the growth of Scenedesmus obliquus.2. Results of the testing for contents of photosynthetic pigments showed that, the contents of Chl-a and Chl-b were remarkable raised in these moderate concentration groups, and decreased in these high concentration treatments(>50mg/L). What else, SWCNTs showed more effects on chlorophyll than DWCNTs, which proved again that SWCNTs had a higher toxicity than DWCNTs. Carotenoid was little affected by CNTs on low and moderate concentrations, but its contents had a sudden decrease in high concentration groups.3. Results of protein contents detection showed that, the protein contents of these tested algal cells slowly raised on low concentration levels, reach the top on moderate concentrations and decreased quite a lot in the high concentration groups. It suggested that when the CNTs’ concentration reached some level, it would inhibit the protein synthesis, which showed a dose-effect relation.4. The testing results of SOD and POD activity suggested that, the SOD activity decreased slightly in the low concentration groups, which meant that CNTs can stress on Scenedesmus obliquus even on low concentration levels. All these four types of CNTs showed high inhibition on the activity SOD on high concentration levels. The POD activity showed a trend that it increased when on low concentration levels and decreased when on high concentration levels. It suggested that the SOD and POD activity decreased on high concentration levels which had an oxidation damage on the algae.According to all these results, CNTs would be toxic to Scenedesmus obliquus when its concentration reached some level (>50mg/L), the toxicities of the four types of CNTs used in this test were in the following order:SWCNTs> Short SWCNTs> DWCNTs> Short DWCNTs. The result showed that firstly, the toxic effects of CNTs on Scenedesmus obliquus were connected with the number of graphene layers, SWCNTs had stronger toxicities than DWCNTs; and then, the toxicity of CNTs was associated with the length of the CNTs. The long ones had stronger toxicities than the short ones. This study was fundamental research, in order to figure out the specific toxic reason and the molecular toxic mechanism, further exploration was necessary.