Combined Toxic Effects Of Nano-TiO₂ And Organochlorines On Algae In Karst Natural Waters

With the advance of nanotechnology,nanoparticles（NPs）have been widely used,which inevitably increases the risk of its discharge into the environment.Organochlorines（OCs）is lipophilic and anti-degradable,and can be accumulated in organisms easily,posing a potential threat to the growth of aquatic organisms.There is a great chance for NPs and OCs to coexist in the natural environment.Due to the complex hydrochemical properties of natural water,the toxicity of NPs and coexisting pollutants becomes unpredictable.However,there are few reports on the aquatic biotoxicity of NPs and coexisting pollutants in natural water.Therefore,in this study,the combined toxicity effects of TiO₂ NPs and three different OCs-（pentachlorobenzene（PeCB）,atrazine and 3,3,4,4-tetrachlorobiphenyl（PCB-77））to Chlorella pyrenoidosa were assessed in four typical karst natural waters（Aha Lake Reservoir（AH）,Songbaishan Reservoir（BS）,Karst Cave（RD）and Hongfeng Lake（HF））,and the toxic mechanisms of TiO₂ NPs and OCs on algal cells were revealed at the molecular and subcellular structure levels.This study mainly draws the following conclusions.（1）The four kinds of natural waters alleviated the individual and combined toxicity of TiO₂ NPs and OCs to algal cells compared to pure water（CS）.The correlation analysis showed that the toxicity of TiO₂ NPs and OCs to algal cells was mainly related to total organic carbon,ionic strength,Ca²⁺and Mg²⁺in natural water.When TiO₂ NPs was co-exposed with PeCB and atrazine in different waters,the measured toxicity values were greater than the predicted values of independent action model（IA）and concentration addition model（CA）,indicating that it was a synergistic effect.When TiO₂ NPs and PCB-77 were co-exposed,the measured toxicity values were less than the predicted toxicity values of CA and IA models,indicating that was an antagonistic effect.The presence of TiO₂ NPs in different water bodies increased the bioaccumulation of OCs by algal cells.At the same time,PeCB and atrazine both increased the bioaccumulation of TiO₂ NPs by algal cells.The difference was that PCB-77 reduced the bioaccumulation of TiO₂ NPs by algal cells.As the increased toxicity of PCB-77 to algal cells could not offset the reduced toxicity of TiO₂ NPs,the combined toxicity of TiO₂ NPs and PCB-77 was reduced.（2）It was concluded from infrared differential spectrum analysis that TiO₂ NPs and OCs affected the synthesis of carbohydrates in algal cells,and the lipids,polysaccharides,protein and nucleic acid in algal cells were damaged to different degrees.The observation of algal cell structure by transmission electron microscope showed that the exposure of TiO₂ NPs and OCs both caused severe plasmolysis of algal cells,resulting in smaller organelles,damaged algal cell structure,and electron dense body or vacuole structure appeared in some groups containing TiO₂ NPs.The presence of TiO₂ NPs and atrazine significantly increased the oxidative pressure of algal cells.（3）The presence of both TiO₂ NPs and OCs in natural water affected the metabolism of algal cells to different degrees.When TiO₂ NPs were exposed alone or in combination with OCs,the changes in the content of differential metabolites in algal cells affected metabolic pathways as glucose metabolism,amino acid metabolism,fat metabolism,and nucleotide metabolism,resulting in impacts on the synthesis of protein,DNA,and RNA.The changes of the content of differential metabolites in pentose phosphate cycle and tricarboxylic acid cycle affected the synthesis of ATP,an energy substance in algal cells,thus interfering with the normal physiological activities of algal cells.In summary,under the combined exposure of TiO₂ NPs and atrazine,the content of differential metabolites in algal cells was significantly changed and their metabolic processes were significantly affected,which was basically consistent with the toxic effect.