| Phthalate acid esters(PAEs) are a kind of environmental hormone pollutants, and widely used as plasticisers in industry and daily products to enhance the flexibility and technical properties of material. Because of long residual time in environment, PAEs have become the typical persistent organic pollutant and attracted increasing attention in environment science field. The coastal ocean are among the most heavily polluted because they directly receive wastewater and runoff from inland and coastal areas. PAEs have been commonly detected in coastal waters, while the biodegradation was one of the major removal ways. As the important primary producers of coastal ecosystem, marine microalgae were able to influence the fate of organic pollutants in waters. Furthermore, benthic microalgae not only grow within the upper millimeter of the sediment layer, but also resuspend into overlaying water, which also play important role in coastal water purification. However, up to now, the relevant studies were rare.In this study, the tolerance of three marine microalgae including two marine planktonic algae Dunaliella salina and Chaetoceros sp. and one marine benthic alga Cylindrotheca closterium to diethyl phthalate(DEP), di-n-butyl phthalate(DBP) and the mixture of DEP and DBP was investigated. On this basis, the removal of PAEs in culture medium by three marine microalgae and in sediments by C. closterium were further studied, respectively. Furthermore, the mechanism was also discussed. The results were as follows:1. Tolerance experiment studyUnder single PAE condition, the No Observed Effect Concentration(NOEC) at 96 h was in the order of Chaetoceros sp.(100 mg·L-1)>C. closterium(50 mg·L-1)> D.salina(30 mg·L-1) for DEP, and C. closterium(2.5 mg·L-1) > Chaetoceros sp.(1 mg·L-1)> D.salina(0.4 mg·L-1) for DBP, respectively. The half inhibitory effect concentration EC50 at 96 h was in the order of Chaetoceros sp.(194 mg·L-1) >C. closterium(77 mg·L-1)> D.salina(62 mg·L-1) for DEP, and C. closterium(4.2 mg·L-1) > Chaetoceros sp.(3.5 mg·L-1)> D.salina(0.7 mg·L-1) for DBP, respectively. The results indicated that the tolerance of bacillariophyta was higher than that of chlorophyta. Moreover, both NOEC and EC50 of three species to DBP were lower for 1~2 orders of magnitude than that of DEP, which showed that the ecological risk of DBP for marine microalgae was higher. The correspond EC50 of chlorophyll at 96 h for three species was 146 mg·L-1, 56 mg·L-1, 73 mg·L-1 to DEP and 2.5 mg·L-1, 0.6 mg·L-1, 3.3 mg·L-1 to DBP, respectively. Obviously, the synthesis of chlorophyll was more sensitive than growth of microalgal cells.According to time-effect curves, both C. closterium and Chaetoceros sp. had the recovery ability after being inhibited by DEP or DBP, especially for C. closterium. On the contrary, D.salina was hardly able to grow, which was possibly due to difference of cell structure and physiological characteristics.DBP and DEP show a simple additive toxic effect on the three marine microalgae by the additive index method combined effect of aquatic toxicology.2. Removal of PAEs by three marine microalgae in culture mediumThe concentration of DBP or DEP was 0.1 mg·L-1, EC20, EC50, EC75 obtained from the growth inhibition test. The results were as follows:Under the condition of single DBP, the BCF of DBP at 96 h followed the order of C.closterium(4.69?104~9.92?104) > Chaetoceros sp.(8.40?103~5.55?104) > D.salina(4.65?103~7.87?103), with which the order of DBP biodegradation was same. Although three marine microalgae were able to bioaccumulate DBP, the bioaccumulation percentage was below 32%, 10% and 5% for C.closterium, Chaetoceros sp. and D.salina, respectively. The bioaccumulation ability of Chaetoceros sp. decreased with increasing DBP concentration. When the DBP concentration was higher that EC20, C.closterium had the same trend of bioaccumulation with that of Chaetoceros sp., which indicated that the growth condition of these two species had important role in bioaccumulation of DBP. However, the bioaccumulation of D.salina had no obvious relationship with initial concentration of DBP.Under the condition of single DEP, the bioaccumulation of DEP for three species was very low(BCF < 50), indicating the contribution of bioaccumulation in removal of DEP was nearly neglected. When the initial concentration of DEP was 0.1 mg·L-1, the biodegradation percentage of DEP exceeded 90%; however, the biodegradation efficiency fell to 20%, which demonstrated that the growth condition of microalgae played an important role in biodegradation of DEP.Under the condition of mixture of DEP and DBP, bioaccumulation of DBP was higher than that of DEP for three species. Contrarily, biodegradation efficiency of DEP was higher than that of DBP for Chaetoceros sp. and D.salina. For C. closterium, the biodegradation efficiency of DEP was higher than that of DBP only at the initial concentration of(0.1+0.1) mg·L-1; and the biodegradation efficiency of both DEP and DBP exceeded 90% when the initial concentration of DEP and DBP was higher than EC20.For C. closterium, the biodegradation efficiency of PAEs under the condition of the single DBP and mixture of DEP and DBP exceeded 90%, even though the initial concentration of PAEs was higher than EC50. However, the biodegradation efficiency of DEP decreased obviously with increasing initial concentrations, which indicated that initial concentration of medium was more important factor than growth condition of microalgae.For Chaetoceros sp., DEP had inhibitory effect on bioaccumulation of DBP, but had no effect on biodegradation of DBP. For D.salina., DEP had no effect on bioaccumulation of DBP, but had inhibitory effect on biodegradation of DBP. For C. closterium, DEP could promote the biodegradation of DBP when the initial concentration was higher than 0.1 mg·L-1. On the other hand, DBP had inhibitory effect on biodegradation of DEP under the mixture of DEP and DBP of(0.1+0.1) mg·L-1.3. Removal of PAEs in sediments by C. closteriumThe experience about the removal of PAEs in sediments with single bacteria, single microalgae or mixture of bacteria and microalgae was set up, and the results were as follows:For the experiment only with microalgae, the removal percentage of DEP in surface layer(top 0.5 cm) and bottom layer of sediments was 80% and 60%, respectively. The reason for removal of DEP in bottom layer of sediments was possibly due to the release of microalgal extracellular enzymes into deep layer of sediment or upward migration of DEP resulting from the concentration difference between surface and bottom layer of sediments. 13% of DBP was removed obviously in surface of sediments, but not removed significantly in bottom layer of sediments. DBP with high molecular weight and low water solubility was possibly more susceptible to be adsorbed by sediment particles than microalgae.The degradation of DBP in surface layer(0~0.5 cm) and bottom layer( below 0.5 cm) of sediments was fit well to the fist-order kinetic equation and the biodegradation rate constant of DBP was in the order of bacteria- microalgae system(2.098 and 0.309 d-1) > single bacteria system(0.460 and 0.256 d-1) and single microalgae system(0.216 and 0.039 d-1), which indicated that bacteria played an important role in degradation of PAEs and C. closterium could enhance the degradation efficiency of PAEs by bacteria.The results about PLFA indicated that the characteristic of microorganism community structure was clearly changed in the surface layer of sediments with C. closterium, which could provide oxygen for aerobic bacteria and then promoted the degradation of PAEs. Pearson correlation analysis showed significantly positive correlation between PAEs removal efficiency and abundance of total aerobic bacterial PLFAs, suggesting that aerobic bacteria played a key role in degradation of PAEs by C. closterium in sediments. Thus, C. closterium had the potential for the remediation of PAEs in marine sediments. |
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