Long-term Microbial-catalyzed Degradation Of Polychlorinated Biphenyls In Taihu Lake Sediments

Polychlorinated Biphenyls(PCBs)are typical persistent organic pollutants(POPs)in the environment.Due to their high toxicity,long-range transport and bioaccumulation,PCBs have caused great threat to the ecological environment and human health and are of global concern.Sediment is the sink of PCBs,where microbial-catalyzed reductive dechlorination is able to take place.Anaerobic dechlorination is an extremely complex process.The dechlorination mechanisms under different geochemical conditions as well as the microbial processes remain unclear.Moreover,the long-term microbial degradation potential in historically polluted sediments are still inconclusive.Therefore,it is necessary to study the long-term degradation process of PCBs in the sediments.In this study,16 surface sediments were collected from Zhushan Bay and four inflow rivers(Taige Canal,Caoqiao River,Yincun River and Shedu River)of Taihu Lake.Characterization,distribution and source apportionment of PCBs in research area of Taihu Lake were conducted.According to it,anaerobic microcosms were established with the sediment collected from Zhushan Bay,Taihu Lake.The microbial dechlorination process of 9 typical PCB congeners:PCB 5(23-CB),PCB 12(34-CB),PCB 64(236-4-CB),PCB 71(26-34-CB),PCB 105(234-34-CB),PCB 114(234-45-CB),PCB 149(236-245-CB),PCB 153(245-245-CB)and PCB 170(2345-234-CB)were investigated in a time course of 108 weeks.The experiments carried out with four sets:group 1 without PCBs(T),group 2 with spiked PCBs(T-1),group 3 with spiked PCBs and supplementary acetate(T-1-AC)and group 4 with spiked PCBs and supplementary fatty acid mixture(T-1-CS).The long-term degradation of PCBs in sediment microcosms was tracked using instrumental analysis.The influence of supplementary carbon source was also investigated.Meanwhile,the microbial community and the numbers of some dechlorinating microorganisms were studied via molecular biology technology,to reveal the mechanism of long-term degradation of PCBs in Taihu Lake sediments.The main findings are as follows:(1)115 PCB congeners were detected and the concentrations of total PCBs ranged from11.02 ng/g to 84.05 ng/g(d.w.)at the 16 sampling sites.The dominant PCB congeners in Taihu Lake sediments were monochlorobiphenyls and dichlorobiphenyls,accounting for 57.45%.Compared with other rivers and lakes in China,the pollution of polychlorinated biphenyls in Taihu Lake is at a high level.According to the principal component analysis(PCA)of PCB homologue,PCBs in the sediments were mainly originated from leakage of capacitor,transformer oil and sewage discharged from industries such as oil paint and paper mill.(2)During the long-term degradation process(108 weeks),extensive dechlorination of PCBs was observed,and the total amount of PCBs kept decreasing suggesting a potential for further dechlorination.However,the declining trends of parent PCBs were different from those of total PCBs,the curve of total parent PCBs showed a typical lag phase,rapid degradation phase and quasi-plateau phase.Thereafter,dechlorination of parent PCBs restarted and decreased rapidly.The degradation curve flattened again after 51 weeks of incubation.From week 51 to week 108,parent PCBs declined slowly.By analysing each of the individual parent PCBs,it was found that such phenomenon was mainly caused by the high degradation-resistance of PCB 64,PCB 71 and PCB 149.Other PCB congeners had achieved significant degradation,especially for dioxin-like PCB 105 and PCB 114.With the dechlorination of these two PCB congeners,the toxic equivalency(TEQs)in sediment microcosms reduced to as low as 2.64±0.45 pg/g,which was comparable to the current environemental TEQs levels in Taihu Lake(0.006~13.552 pg/g).PCB 13(3-4-CB),the ortho dechlorination product of PCB 25(24-3-CB)was detected in the sediment microcosms,indicating the existence of ortho dechlorinating bacteria in Taihu Lake sediments.(3)The supplementation of carbon source after a long time of incubation did not significantly favor overall PCB dechlorination.However,the addition of acetate apparently enhanced the degradation of PCB 105.Meanwhile,PCB 66(24-34-CB,)the first generation degradation product of PCB 105,greatly dechlorinated to PCB 25.This indicates that acteate could stimulate the dechlorination pathways targeting DF Meta and SF Para chlorine atoms.In addition,the fatty acid mixture could favor the degradation of PCB 64 while both acetate and the fatty acid mixture can enhance the dechlorination of PCB 149 and PCB 153.The dechlorination enhancement by fatty acids addition was relatively congener and/or structure specific.(4)The results of high-throughput sequencing showed that the relative abundance of Chloroflexi gradually increased along with dechlorination and became a dominant phylum.The relative abundance of Chloroflexi in the T-1-AC group and T-1-CS group after 51 weeks of incubation was not significantly different from that in the T-1 group,indicating that the supplementation of carbon source has no significant effect on Chloroflexi.In addition,the percent of Dehalococcoidales,which is known to be associated with dechlorination,also increased.Additionally,the ratio of Dehalococcoidales to Chloroflexi increased.The selective enrichment of Dehalococcoides in the T-1 group partially support that Dehalococcoides was crucial to anaerobic dechlorination of PCBs in Taihu Lake.(5)The results of qPCR revealed that the 16S rRNA gene of Chloroflexi,Dehalococcoides,dehalogenase ardA and rdh12 jumped when dechlorination occurred and these genes could be used as PCB dechlorination indicators,ArdA and rdh12 were more accurate than 16S rRNA gene of Chloroflexi,Dehalococcoides.The levels of these genes in the sediment microcosms remained high after 108 weeks of incubation(between 10~7 and 10~8 copies/ml),which indicates a further dechlorination potential.This research resulted in abundant raw data and crucial preliminary conclusions for screening effective PCB dechlorinating microorganisms,which offered scientific support to future works on the efficient degradation of PCBs through genetic engineering methods.