| Selective serotonin reuptake inhibitors（SSRIs）are the most commonly used antidepressants in the treatment of depression.Due to their extensive use and incomplete removal in wastewater treatment plants,SSRIs are constantly entering the aquatic environment,posing a threat to the health and stability of aquatic ecosystems.Fluoxetine（FLX）and sertraline（SER）are two most widely used SSRIs.As one of the producers in aquatic ecosystems,microalgae are important indicator organisms for assessing the ecological risks of contaminants.At the same time,microalgae can effectively remove a variety of environmental pollutants.Microalgae have the advantages of carbon sequestration,environmental friendliness and high economic added value,making them highly promising for application in wastewater treatment.However,few studies have focused on the interaction of microalgae with SSRIs.In this study,C.pyrenoidosa was selected as the test algal species.The single and combined effects of FLX and SER and their removal mechanism by microalgae were investigated in order to better understand the ecotoxicological effects of FLX and SER and to provide a theoretical basis for the development of wastewater treatment technology for SSRIs by microalgae.The main results were as follows:（1）There was no significant effect of low concentrations of FLX（≤20 ug/L）and SER（≤5 ug/L）alone on the growth of C.pyrenoidosa,while high concentrations of FLX and SER alone exerted concentration dependent effects on the growth of C.pyrenoidosa.The 96 h half maximal effective concentrations(EC50)of FLX and SER alone were calculated to be 493 ug/L and 61.1 ug/L,respectively.The low concentrations of mixtures（≤13.9 ug/L）significantly promoted the growth of C.pyrenoidosa,while high concentrations of mixtures inhibited the growth of C.pyrenoidosa.The 96 h EC50of combined antidepressants was 271 ug/L.The combined toxicity of FLX and SER was judged to be additive by the independent addition,concentration addition,toxicity unit and additive index.（2）The effects of single and combined FLX and SER exposures on the C.pyrenoidosa photosynthetic system were evaluated by measuring photosynthetic pigment content and chlorophyll a fluorescence parameters.On the 4th day of exposure,chlorophyll a and carotenoid contents decreased in the combined exposure group compared to the corresponding single exposure group,indicating that combined exposure exhibited a stronger inhibition of photosynthetic activity.The inhibitory effects of FLX and SER alone on the synthesis of photosynthetic pigments elevated with increasing exposure time.On the 11th day of exposure,Fv/Fmin the single and combined exposure groups returned to normal level or even higher than that in the control group,indicating that photosynthetic activity recovered over time.（3）The single and combined effects of FLX and SER on the antioxidant system of C.pyrenoidosa were evaluated by measuring superoxide dismutase（SOD）activity and malondialdehyde（MDA）content.On the 4th day of exposure,SOD activitys in all single and combined exposure groups were higher than those in control group,indicating that C.pyrenoidosa resisted oxidative damage by inducing SOD.The MDA content in the combined high exposure group was significantly higher than that in the corresponding single FLX and SER exposure groups,indicating that combined exposure caused more severe oxidative damage to C.pyrenoidosa.On the 11th day of exposure,the SOD activity in single FLX group recovered to normal level,while the SOD activity in the single SER and combined exposure groups remained at high level,indicating that the presence of SER promoted the production of SOD in C.pyrenoidosa;the MDA content in the single and combined exposure groups did not change significantly or even decreased,suggesting that the oxidative damage disappeared with the extension of expoure time.（4）The removal of FLX and SER by C.pyrenoidosa gradually increased with exposure time.The removal rates of single FLX and SER by C.pyrenoidosa ranged from 67.6-99.1%and 94.9%-99.1%,respectively.The combined exposure inhibited the removal efficiency of C.pyrenoidosa on FLX and SER.The removal rate and speed of both antidepressants were significantly lower when compared to single exposure.Biodegradation and bioaccumulation were the main mechanisms of FLX and SER removal by C.pyrenoidosa,while abiotic degradation had a very limited effect on the removal of antidepressants.The proportion of biodegradation was negatively correlated with the exposure concentration,while bioaccumulation was positively correlated with the exposure concentration.The proportion of biodegradation increased with exposure time,while the proportion of bioaccumulation decreased.The combined exposure promoted the bioaccumulation of FLX and SER in C.pyrenoidosa and inhibited the biodegradation compared to the single exposure.（5）FLX could be metabolized to form NFLX and TFMP through N-desmethylation and O-dealkylation,respectively.NFLX was easily accumulated in C.pyrenoidosa cells,while TFMP in algal cells was mostly excreted into the culture medium.The ratios of NFLX to FLX in algal cells and the ratios of TFMP to FLX in the culture medium in the combined exposure group was lower than those in the single exposure group;indicating that the combined exposure inhibited the metabolism of FLX by C.pyrenoidosa.NSER was generated by N-demethylation of SER,while the amion groups of NSER and SER may be oxidized by monoaminases to generate SEK.The ratios of NSER to SER in algal cells in the combined exposure group were higher than those in the single exposure group,while the ratios of SEK to SER were the same in single and combined exposure groups.However,the ratio of SEK to SER concentration in the culture medium of combined exposure group was lower than that in the single exposure group.These results indicated that the combined exposure affected the metabolism of SER by microalgae and the process of metabolite excretion. |
