Shifts Of Microbial Community With Anaerobic Dechlorination Of Polychlorinated Biphenyls In Lake Taihu Sediments

Polychlorinated biphenyls(PCBs)are typical organochlorine contaminants with hydrophobic,non-degradable and long-distance transport properties,which are easily adsorbed in soil and sediment.PCBs in the sediments have adverse effects on human health through bioaccumulation in aquatic systems.Previous studies revealed that certain microorganisms are able to ultilize PCBs as their terminal electron acceptor to obtain energy.This results in PCB dechlorination.However,microbial dechlorination of PCBs is relatively site specific.Therefore,the study of the function and mechanism of dechlorination microorganisms in natural environment is one of the key steps for in-situ bioremediation of PCB contaminated sediments.In the present study,we investigated microbial dechlorination of PCBs under three different geochemical conditions in Lake Taihu sediment microcosoms.The experiments were set as group 1 with spiked PCBs(recorded as T-1,also as methanogensis condition of the Lake Taihu sediment microcosms),group 2 with spiked PCBs and Na2SO4(recorded as T-S,also as sulfate reduction condition of the Lake Taihu sediment microcosms),and group 3 with spiked PCBs and FeOOH(recorded as T-Fe,also as ferric iron reduction condition of the Lake Taihu sediment microcosms).Microcosms were set up in triplicates to explore the dechlorination of PCBs.Over a time course of 66-weeks,111 samples were collected at 13 time points.Gas chromatography analysis was performed according to the EPA method 8082 after the extraction and purification of PCBs in sediment slurries and the daughter PCBs were quantified indivually.DNA of samples was extracted for high-throughput sequencing and qPCR analysis.The shifts of microbial communities along with dechlorination were carefully examinated.Moreover,target microorganisms/genes were screened to identifiy the suitable biomarkers for PCBs dechlornation.The findings of this study are as follows:(1)The dechlorination rates were T-1>T-Fe>T-S.Competing electron acceptor partially inhibited PCB dechlorination and the inhibition by sulfate was more than that of ferric iron.After 66 weeks,the total concentrations of PCBs in T-1,T-Fe and T-S decreased from 49.56 ± 0.38 mg/kg to 37.21 ± 1.43 mg/kg,39.80 ± 0.45 mg/kg,43.43 ± 0.63 mg/kg,respectively.The dioxin-like toxic equivalency of PCBs decreased about 80%in the T-1 group after 15 weeks of incubation,in the T-Fe group at the end of 36 weeks,in the T-S group after 66 weeks of incubation,which was consistent with the order of PCBs dechlorination rates.(2)The results of high-throughput sequencing of samples of week 0,week 6,week 24 and week 66 revealed that the percentage of Chloroflexi over total bacteria,especially the percentage of Dehalococcoides increased with the dechlorination process.The known microorganisms,which can dechlorinate PCBs,only accounted for a small proportion of the microbial community.Among all the analyzed samples,Dehalococcoides accounted for 4.13%of the total bacteria at most.Bacteria such as Clostridiales,δ-Proteobacteria,that do not have the dechlorination function were dominant in the microbial communities.(3)The 16S rRNA gene of Chloroflexi(CHL),Dehalococcoides(DHC),reductive dehalogenase genes ardA,rdh12 were suceesuflly used to indicate the dechlorination in T-1 and T-Fe group but not in T-S group.The 16S rRNA gene of Chloroflexi(CHL),and reductive dehalogenase genes ardA,rdh12 were able to be linked to the rapid stage of dechlorination in the T-S group.Compared with Chloroflexi and Dehalococcoides,these two reductive-dehalogenase genes were more sensitive as dechlorination biomarkers.(4)The ortho dechlorination of PCBs was confirmed by gas chromatography analysis,but the concentrations of known ortho dechlorinating bacteria o-17/DF-116S rRNA gene were very low.This suggested that there might be other unknown microorganisms with ortho dechlorination function in Lake Taihu sediments,which need to be studied further.The study of dechlorination microorganisms is crucial for comprehensive understandings of the dechlorination processes of PCBs,as well as the development of in-situ bioremediation technology.