Characteristics Of Phytotoxicity Induced By Perfluorooctanoic Acid And Perfluorooctane Sulfonate In Water

The reuse of urban reclaimed water for agricultural irrigation is an important option to alleviate the water shortage in China.Wastewater treatment plants mainly aim to remove traditional pollutants such as particulate matter and organic nutrients,without the design of the removal of persistent organic pollutants（POPs）.Reclaimed water contains a variety of POPs which may be harmful to the environment and human health.Perfluoroalkyl substances（PFASs）are a class of artificial POPs that are fluorinated at entirely of the carbon atoms(C_nF_2n+1),which are characterized by their environmental persistence,bioaccumulation and high biological toxicity.PFASs are widely detected in urban wastewater and agricultural environment.In the context of agricultural irrigation using urban reclaimed water,the study on the phytotoxicity of PFASs is of great significance for environmental risk assessment.In the present thesis,the toxic effects of two typical PFASs（Perfluorooctanoic acid,PFOA and Perfluorooctane sulfonate,PFOS）in lettuce and the detoxification mechanisms activated in lettuce were systematically studied through hydroponic exposure assays.This study revealed the accumulation characteristics of reactive oxygen species（ROS）,the responses of antioxidant defense systems,the occurrences of oxidative damages,the disturbances of metabolites and metabolic pathways,as well as the detoxification mechanisms based on the metabolic regulations in lettuce under the exposure to PFOA and PFOS.The main conclusions are as follows:（1）In different tissues of lettuce,PFOA and PFOS caused different kinds and levels of ROS accumulation.PFOA caused the accumulation of H₂O₂,^·OH and O₂^·-in leaves and^·OH in roots,respectively.PFOS caused the accumulation of H₂O₂ and O₂^·-in leaves and H₂O₂ in roots,respectively.Under the exposure to 50μg/L PFOA,the contents of H₂O₂,^·OH and O₂^·-were increased by 38.7%,26.4%and 22.8%in leaves（p<0.01）,respectively.In roots,the contents of^·OH were increased by 9.47%（p<0.05）.Under the exposure to 50μg/L PFOS,the contents of H₂O₂ and O₂^·-were increased by 23.8%and 25.3%in leaves（p<0.01）,respectively.In roots,the contents of H₂O₂ were increased by 14.4%（p<0.01）.（2）Different antioxidants were activated in different lettuce tissues to scavenge the ROS induced by PFOA and PFOS.Both enzymatic and non-enzymatic antioxidants were activated in leaves to cope with PFOA,while only non-enzymatic antioxidants were activated to cope with PFOS.Both enzymatic and non-enzymatic antioxidants were activated in roots to cope with PFOA and PFOS.Under the exposure to 50μg/L PFOA,the activities of catalase（CAT）,peroxidase（POD）,glutathione peroxidase（GPX）and the contents of glutathione（GSH）were significantly elevated in leaves（p<0.05）,while the contents of total phenolics were decreased（p<0.05）.In roots,the activities of ascorbate peroxidase（APX）and POD and the contents of GSH significantly increased（p<0.05）.Under the exposure to 50μg/L PFOS,the contents of GSH and the activities of GPX were significantly elevated in leaves（p<0.05）,while the contents of total phenolics were decreased（p<0.05）.In roots,the activities of APX,CAT,POD and GPX and the contents of GSH and total phenolics were significantly increased（p<0.05）.（3）The tolerances of lettuce leaves to PFOA and PFOS were lower than that of roots.Under the exposure to PFOA,lipid peroxidation,soluble protein contents and net photosynthetic rate were elevated in leaves,while intercellular CO₂concentration was decreased.Under the exposure to PFOS,impaired permeability of cell membrane was observed and photosynthetic efficiency was inhibited in leaves.Under the exposure to 5 and 50μg/L PFOA,the contents of malondialdehyde were increased by 57.8%and 50.0%in leaves（p<0.05）,respectively.Under the exposure to50μg/L PFOA,the contents of soluble proteins were increased by 31.2%in leaves（p<0.01）.Under the exposure to 5 and 50μg/L PFOS,the membrane permeabilities were elevated by 88.7%and 58.7%（p<0.05）,respectively;The contents of chlorophyll a were decreased by 41.4%and 55.6%（p<0.01）,respectively;The contents of chlorophyll b were decreased by 38.4%and 41.3%（p<0.01）,respectively;The contents of carotenoids were decreased by 24.6%and 30.1%（p<0.05）,respectively.（4）PFOA or PFOS exposure caused the metabolic disorders in lettuce leaves.The detoxification mechanisms based on the metabolic regulations in lettuce leaves under the exposure to PFOA and PFOS included alteration in plasma membrane,activation of antioxidant systems,increase of tolerance and repair of DNA injury.Under the exposure to 0.5 and 5μg/L PFOA or PFOS,the abundances of lipids,carbohydrates,fatty acids,amino acids and phenolic compounds were regulated in leaves.Key pathways in energy metabolism were disturbed by both PFOA and PFOS,including tricarboxylic acid cycle,glyoxylate and dicarboxylate metabolism and pyruvate metabolism.Amino acid metabolism,e.g.,phenylalanine and tyrosine,was disturbed by PFOA.The metabolism of linoleic acid was disturbed by PFOS.（5）PFOA and PFOS co-exposure altered the primary and secondary metabolisms in lettuce leaves and roots.The metabolic regulation by PFOA and PFOS co-exposure in roots was in a non-dose-dependent manner,while it was dose-dependent in leaves.Under the co-exposure to 0.5-5μg/L PFOA and PFOS（1:1）,the abundances of phenols,lipids,amino acids,fatty acids,carbohydrates,purine and nucleosides were regulated in roots.The metabolic pathways of isoquinoline alkaloid biosynthesis,terpenoid backbone biosynthesis,tyrosine metabolism,purine metabolism and monoterpene biosynthesis were perturbed.In leaves,the abundances of amino acids and peptides,fatty acids and lipids were dose-dependently down-regulated,while purines and nucleosides were dose-dependently up-regulated.The metabolisms of（poly）phenol,involved in shikimate-phenylpropanoid pathway and flavonoid branch pathway,were strengthened,to cope with the PFOA and PFOS co-exposure.Figures,58;Tables,23;References,250.