Toxicity And Mechanism Of Nanosilver To Nostoc Sp.

Nowadays, silvernano particles(Ag NPs) with superior performance becomes one of the most widely used nano materials and it has been applied in the fields of industrial production, health care and environmental protection. However, in the process of producting, using and recycling of Ag NPs, some of them will be released into environment inevitably. Nanosilver could be transported into water environment through various ways, and it poses potential threat to non-target aquatic organisms. There have been many researches about the effects of Ag NPs on aquatic microalgae and microanimals. However, there is no report about the effects of Ag NPs on filamentous nitrogen-fixing cyanobacterium. Moreover, the toxic mechanism of Ag NPs on microalgae is still unclear. Therefore, in the present study, we studied the toxicity of Ag NPs and silver ion(Ag+) towards Nostoc sp. to try to clarify the mechanism of Ag NPs on microalgae. The primary results are listed below:In the acute toxicity experiments(4d), the morphology of silver changed immediately after Ag NPs and Ag+ treatment with the alga. In 0~48h, the dissolved Ag concentration in culture media decreased gradually when treated with Ag NPs and Ag+ and reached balance concentrations at last. For Ag NPs treatment, in 0~6h, little particle Ag concentration falled dramatically, and reached balance after 24 h. However, for Ag+ treatment, little particle Ag concentration unchanged in the first 12 h and increased significantly after then and reached the highest concentration after 24 h treatment. Treatments with high concentration of Ag NPs and Ag+ caused algal cell plasmolysis, contraction, intracellular materials disorder, and cell wall crack. All Ag NPs and Ag+ treatments can result in the decrease of algal dry weight, Chla content, total carbon content(TC), C/N, and nitrogenase activity contrasted with CK, whileas an increase in algal total nitrogen content(TN). When treated Ag NPs, C/N of culture media was significantly higher than that in CK, whileas with 0.04 mg·L-1 Ag+, it was significantly lower than that in CK. However, there was no significant difference in C/N of culture media between 0.06 mg·L-1 Ag+ treatment and CK. After treated with Ag+ for 4d, the value of parameters including Fm, Fv/Fm, ?0, ?E0, and PIABS increased significantly contrasted with CK. Especially, the value of ABS/RC and DI0/RC showed a notable increase(97% and 126%, respectively). However, with 0.6 mg·L-1Ag NPs treatment, F0, Fm and ?D0 showed significant decrease and Fv/Fm, ?0, ?E0 and PIABS showed significant increase. And there was no significant difference in all parameters between 0.4 mg·L-1Ag NPs treatment and CK. It showed that short time treatment with Ag+ and Ag NPs showed significant stress on the alga. However, the negative effects of Ag NPs on PSII was lower than Ag+.In the subacute toxicity experiments(9d), only the higher concentration Ag NPs and Ag+ treatments can result in the significant decrease of algal dry weight and Chla content contrasted with CK. All Ag+ treatments can result in the significant decrease of algal C/N, but Ag NPs treaments had no effects on the algal C/N. 0.6 mg·L-1 Ag NPs treatment lead to the significant increase of nutrient solution C/N, algal soluble protein and phycobiliprotein. The other treatments had no obvious changes. Ag+ can result in the significant decrease of algal nitrogenase activity and Ag NPs had no effects on algal nitrogenase activity. After nine days of treatments with Ag NPs and Ag+, more than 200 different protein points were recorded from the algal cell protein two-dimensional electrophoresis. 132 protein points which were different over 2.5 times than the CK were selected to analysis qualitatively. Only 51 proteins are matched successfully, there were 12 proteins were repeated, so 39 proteins are analysis finally. 26 proteins became significantly increase and 6 proteins became significantly decrease after treated with 0.06 mg·L-1 Ag+. 22 proteins became significantly increase and 3 proteins became significantly decrease after treated with 0.6 mg·L-1Ag NPs. There were 10 increased proteins and 1 decreased proteins were same after treated with 0.6 mg·L-1 Ag NPs and 0.06 mg·L-1 Ag+. Transketolase which is a key protein in Calvin cycle was recorded after treated with 0.6 mg·L-1 Ag NPs and 0.06 mg·L-1 Ag+. And 0.6 mg·L-1 Ag NPs can significantly increase it, 0.06 mg·L-1 Ag+ can significantly decrease it. Meanwhile, Ag+ can inhibit nitrogenase activity and Ag NPs had no obvious effects on the nitrogenase activity which is the same as the result measured with acetylene reduction assay. Ag NPs can increase APC concentration and Ag+ had no effects on it which is the same as the result measured by spectrophotometry. In addtion, Ag+ can increase SOD concentration and Ag NPs had no effects on it. After treated with Ag+ for 9d, the value of parameters including F0, Fm, Fv/Fm, ?0, ?E0, PIABS and ET0/RC increased significantly contrasted with CK. Especially, the value of ABS/RC and DI0/RC showed a notable increase 70%. However, with 0.4 and 0.6 mg·L-1Ag NPs treatment, F0 and Fm showed significant increase, other parameters were no significant difference compared with CK. These were in contrast to the result in the acute experiment. The results showed that Ag NPs and Ag+ had obvious different effects on the photosynthesis, nitrogen fixation and protein synthesis. Moreover, the results showed that Ag concentration in algae became increased with the concentration of Ag NPs and Ag+ becoming increased. Ag concentration in algae was the highest with 0.6 mg·L-1 Ag NPs treatment. The Ag concentration in algae with the same treatment on the tourth day was lower than the result on the ninth day.Combining with the growth, physiological and biochemical data, it indicated that Ag NPs and Ag+ had significant toxic effects on Nostoc sp. But the alga generated considerable adaptation for Ag NPs and Ag+ from the result on the ninth day, Nostoc sp. had different adaptive strategy to the stress of Ag NPs and Ag+.