Bioaccumulation And Biotransformation Of 6:2 Fluorotelomer Sulfonic Acid In Earthworms(Eisenia Fetida)

6:2 fluorotelomer sulfonic acid(6:2 FTSA)is a novel perfluorooctane sulfonic acid(PFOS)substitute,which is widely used in Aqueous Film Forming Foams(AFFFs).6:2 FTSA has been detected at high level in environmental matrices,such as soil.It has been confirmed that 6:2 FTSA can be transformed into other more toxic intermediate products and perfluorocarboxylic acids(PFCAs)with different carbon chain length in environmental media.However,the bioaccumulation and transformation mechanism of 6:2 FTSA in terrestrial invertebrates is still unclear.Therefore,earthworms(eisenia fetida)were exposed with 6:2FTSA by conducting lab-based microcosm studies to examine the bioaccumulation and biotransformation of 6:2 FTSA by the terrestrial organisms after in vivo and in vitro exposure.(1)In order to explore the toxic effects of 6:2 FTSA and short-chain PFCAs on earthworms,earthworms were exposed to soil containing 6:2 FTSA and PFCAs,respectively.Compared with the controls,significant increases of the activities of superoxide dismutase(SOD),peroxidase(POD),catalase(CAT)and glutathione transferase(GST)were observed in both 6:2FTSA and short-chain PFCAs treatments,respectively,and significant effects were observed in malondialdehyde(MDA)contents between 6:2 FTSA treatments and controls except for short-chain PFCAs treatments,suggesting 6:2 FTSA and short-chain PFCAs have toxic effects on earthworms.Otherwise,compared with PFCAs(of the same or much shorter perfluorocarbon chain length as 6:2 FTSA)treatments,the changes of SOD,POD,CAT and GST activities and MDA contents were much higher in 6:2 FTSA treatment groups,demonstrating much higher toxic effects of 6:2 FTSA in earthworms.(2)The uptake,elimination and biotransformation of 6:2 FTSA in earthworms were investigated after in vivo exposure.The uptake rate constant(k_u)and biota-to-soil accumulation factor(BSAF)of 6:2 FTSA in earthworms were 0.185 g_oc/g_ww/d and 0.685 g_oc/g_ww,respectively,indicating high bioaccumulative ability of 6:2 FTSA in earthworms.Five terminal perfluorocarboxylic acids(PFCAs)metabolites,including trifluoroacetic acid(TFA),perfluoropropionic acid(PFPrA),perfluorobutanoic acid(PFBA),perfluoropentanoic acid(PFPeA)and perfluorohexanoic acid(PFHxA)were observed in in vivo earthworms,with TFA as the predominant metabolite.However,no perfluoroheptanoic acid(PFHpA)was observed in the present study.The elimination rate constants(k_e)increased in the order:6:2 FTSA(0.057/d)<TFA(0.058/d)<PFPrA(0.071/d)<PFBA(0.084/d)<PFHx A(0.182/d)<PFPeA(0.193/d).(3)To further confirm the roles of CYP450 and GST in 6:2 FTSA biotransformation,in vitro exposure of the whole earthworm homogenates and crude enzyme extracts to 6:2 FTSA were conducted.Biodegradation of 6:2 FTSA in the earthworm homogenates,CYP450 enzyme solutions and GST enzyme solutions fitted well with the first order kinetics.The biotransformation rate constants(k)were in the following order:homogenates(0.012/h)>CYP450(0.009/h)>GST(0.007/h),implying that CYP450 and GST were involved in biotransformation of 6:2 FTSA in earthworms.To further investigate the ability of bacteria associated with the gut of the host earthworm to biotransform 6:2 FTSA,the bacteria isolated from worm gut was incubated with 6:2 FTSA in both aerobic and anaerobic environments to simulate the environment of gut section.However,the concentrations of 6:2 FTSA weren't changed by the bacteria isolated from earthworm gut in both aerobic and anaerobic environments,suggesting that gut bacteria did not contribute to 6:2 FTSA biotransformation in earthworms.The results of this study confirm that the transformation of 6:2 FTSA in earthworms is mediated mainly by enzymes rather than by gut microbes.