| Antibiotics pollution and its impact on human health are receiving widespread attention.As an important intertidal wetland,mangrove has a significant function of intercepting and purifying antibiotics,but more attention has been paid to sediment pollution and resistance genes.The microalgae in the mangrove wetlands,which have a large specific surface area and biological activity,play very important for the function of mangrove interception and purification of antibiotics.Diatoms are the dominant group of microplankton in mangroves.However,no related research reports have been reported so far.Based on this,the toxicity test,algae removal ability evaluation experiment and rapid adsorption-desorption experiment based on radioactive tracer were carried out to evaluate the ecotoxicity of selected antibiotics Sulfadiazine(SD),Sulfamethoxazloe(SMX),and Sulfamethazine(SMZ)to Thalassiosira weissflogii(T.weissflogii),and the ability of microalgae to remove antibiotics.The main contents and results are as follows:(1)The results of ecotoxicity of single antibiotic to T.weissflogii showed that the cell concentration decreased with the increased of concentration of three sulfonamides(SD,SMX,SMZ)under five treatment concentration gradients(1～100 mg/L),showing a dose-dependent manner,indicating that antibiotics have a certain toxicity.But the EC50 were all more than 100 mg/L,indicating that the toxicity was generally low.Furthermore,the EC20 value was obtained by fitting the dose-response curve according to the logistics model,and the relative toxicity of the three antibiotics was evaluated as follows:SD(lower than 0.50 mg/L)>SMX(2.66 mg/L)>SMZ(7.19 mg/L).The toxic mechanism of sulfonamides on T.weissflogii not only inhibit cell growth,but also affect the chlorophyll content,photosynthetic activity and so on.The change of chlorophyll content with concentration was similar to that of algae cells.The change of photosynthetic activity parameters with the concentration showed a tendency of "low concentration promotion,high concentration inhibition",indicating that antibiotics may not only inhibit photosynthetic activity,but also promote photosynthesis.(2)The results of mixed toxicity test of SMX and SMZ showed that the EC20 of antibiotics mixture(SMX,SMZ)predicted by concentration addition model(CA)to T.weissflogii was 4.93 mg/L,which was significantly higher than that of logistics model fitting value(0.16 mg/L),indicating that different antibiotic mixtures have toxic synergistic effect on microalgae.Under the treatment of five mixed concentrations(0.1,0.5,1,2 and 5 TU),the cell concentration and chlorophyll content of T.weissflogii were 1.37×104～2.59×104 cells/mL and 199.95～373.93μg/L respectively,which were significantly lower than those of the control,and the change trend of chlorophyll content with concentration was positively correlated with algae cell concentration.Fv/Fm was lower than that of the control,which further indicated that mixed antibiotics could produce greater toxicity and significantly inhibit the efficiency of light energy conversion.(3)The removal efficiency of antibiotics was evaluated in three culture systems:dark without algae,light without algae,and light with algae,which represented hydrolysis,photolysis and microalgae remove respectively.The results showed that the three pathways had an effect on the removal of antibiotics,and the order by hydrolysis rate and photolysis rate was:SMX(0.0%,0.0%～3.3%)<SD(2.2%～18.7%,26.0%～33.5%)<SMZ(7.5%～25.8%,51.8%～85.5%)and by microalgae removal rate:SMX(11.2%～21.6%)<SD(7.9%～24.3%)<SMZ(37.7%～71.1%).It can be concluded that microalgae effectively promoted the removal process of SMX,while the removal rates of SD and SMZ were lower than the photolysis removal rate.In general,photolysis was greater than microalgae removal,and both were significantly larger than hydrolysis.And the removal efficiency was affected by the type of antibiotic.(4)Taking SMZ as the representative,the rapid adsorption process of antibiotics by microalgae(inactivated)was carried out based on radioactive tracer method.The results showed that both the quasi-first-order kinetic model and the quasi-second-order kinetic model fit well with the dynamic adsorption process of T.weissflogii to SMZ,indicating that chemical adsorption may be the main adsorption mechanism.Fitted by Langmuir and Freundlich isotherm models,the model parameters separation factor(RL)was 0～1,Kf and 1/n were 300.82 and 0.76,respectively,which indicated that T.weissflogii had a good adsorption of SMZ.And the adsorption mechanism of algae on SMZ was not limited to monolayer adsorption,but involved the mixing mechanism of heterogeneous surfaces.Further comparison revealed that the RL value decreased as the initial concentration increased,indicating that higher concentrations were more beneficial to adsorption.Comparing the removal capacity of living and inactivated algae to SMZ in each batch treatment at the same concentration,living-algae was 3.45×10-9～2.78×10-8μg/cell,while inactivated algae was 2.53×108～2.34×10-7μg/cell,which indicated that inactivated algae had a better rapid removal effect on antibiotics. |
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