Toxicity And Bioaccumulation Of Perfluorooctanoic Acid And Perfluorooctane Sulfonate In Marine Algae Chlorella Sp.

Perfluoroalkyl acids(PFAAs),represented by perfluoroalkyl acid(PFOA)and perfluorooctane sulfonic acid(PFOS),are widely used in leather,paper,packaging,fabric and furniture because of their special physicochemical properties such as thermochemical stability,hydrophobicity and lipophobic properties.After use,PFAAs can be discharged into aquatic environments mainly through product degradation,surface runoff and wastewater discharge,causing harmful effect on aquatic organisms due to the persistence and biological accumulation.The ocean is an important sink for PFAAs,but the toxic mechanisms of PFOA and PFOS to marine organisms,especially low trophic level algae,have not been clearly studied.Therefore,this study selected Chlorella sp.to explore the bioaccumulation and toxic mechanisms of PFOA and PFOS on Chlorella sp.The main findings are as follows:(1)The effects of PFOA and PFOS on the growth rate of Chlorella sp.were studied by growth inhibition experiments.It was found that PFOA of<40 mg/L and PFOS of<20 mg/L stimulated algal reproduction,this may be due to the overcompensation effect caused by the reversal of homeostasis,and high doses can inhibit the algal growth.The 96 h-EC50 for Chlorella sp.were 127.35 mg/L for PFOA and 77.62 mg/L for PFOS,indicating that PFOS was more toxic than PFOA to Chlorella sp.(2)The photosynthetic damage caused by PFOA and PFOS to Chlorella sp.was investigated by studying the changes of photosynthetic system.The decreased maximal quantum yield(Fv/Fm)and relative electron transfer rate(rETR)indicated the destruction in photosystem Ⅱ function.The mRNA levels of psbA and psbC were down-regulated,confirming that photosystem Ⅱ energy transfer was disturbed,and photoreaction was impaired.Reduced carbohydrate content and expression levels of carbon sequestration genes(rbcL and cah2)suggested impaired carbon assimilation in dark reactions.However,the chlorophyll a content increased along with the up-regulation of its encoding genes(psbB and chlB),probably due to an overcompensation effect of marine algae under PFOA and PFOS stress.(3)The mechanism of toxicity of PFOA and PFOS to Chlorella sp.was revealed by exploring oxidative stress,cell membrane function and metabolic level.PFOA and PFOS enhanced the ROS production in algal cells,and increased SOD activity,CAT activity and GSH content,causing oxidation and antioxidant imbalance,resulting in oxidative stress.ROS and PFAAs jointly attack cell membrane,resulting in membrane damage and metabolic disorder.(4)The bioaccumulation characteristics of PFOA and PFOS in Chlorella sp.were analyzed.The uptake and adsorption of PFOA and PFOS by algal cells were positively correlated with the exposure dose,and the adsorption amount was greater than the uptake amount.The bioconcentration factor(BCF)range of 82 to 200 confirmed the bioaccumulation of PFOA and PFOS in Chlorella sp.,but the bioaccumulation capacity decreased with increasing dose.The BCF of PFOS was greater than PFOA,indicating that PFOS were more likely to accumulate in organisms and transmit through the food chain.In summary,PFOA and FPOS accumulated in marine algae Chlorella sp.,disturbed photosynthesis,triggered oxidative stress,damaged cell membrane and inhibited the algal growth.PFOS showed higher toxicity and bioaccumulation than PFOA.Interestingly,the growth of Chlorella sp.followed a hormetic pattern in response to PFOA and PFOS exposure,and the chlorophyll a content increased with the reduced photosynthesis.Both were not reported in previous algae studies.This study will provide more comprehensive information on the toxicology and ecological impact of PFAAs.