Dynamic And Chemical-microbial Coupling Mechanisms Of Pentachlorophenol Reductive Dechloriantion Under Anaerobic Condition

Pentachlorophenol (PCP) is a highly toxic, mutagenic and carcinogenic polychlorinated organic compound which is widely used as a wood preservative and versatile insecticide in the industry and agriculture. Under anaerobic conditions, reductive dechlorination is thought to be coupled to biogeochemical processes driven by electrons flow from hydrogen and carbon substrates to PCP, which acts as an electron acceptor. Usually, reactions of various ionic species in soils such as NO3-, Fe(Ⅲ), Mn(Ⅳ) and SO42- that serve as terminal electron acceptors which are active redox reactions in anoxic conditions. Thus, the dechlorination of PCP could be influenced by interaction of alternative terminal electron accepting processes.PCP was selected as the target compound in our study. The dymatic processes and microbial community influences during PCP reductive dechlorination in the soil environment under anaerobic condition was studied. The detailed contents mainly include the potential pathways of PCP reductive dechlorination, the influence of PCP on the biogeochemical cycles (NO3-, Fe(Ⅲ), Mn(Ⅳ) and SO42-) in the soil environment, the influence of PCP on the soil microbial community structures and the composition of the functional microorganisms during PCP dechlorination. The main experiments and results were as follows:(1) Pure culture system:An Fe(Ⅲ) reducing bacterium, Clostridium beijerinckii Z, was isolated from paddy slurries, and shown having the ability to dechlorinate pentachlorophenol (PCP). About fifty percent of added PCP was removed by strain Z alone, which increased to 83% in the presence of both strain Z and ferrihydrite after 11 days incubation. In abiotic experiment, the surface-bound Fe(Ⅱ) also abiotically transformed more than 40% of the added PCP. This implied that the biotic dechlorination by C. beijerinckii Z is a dominant process causing PCP transformation, and that the dechlorination rates can be impoved by simultaneous reduction of Fe(Ⅲ). An electron transfer coupling process between sorbed Fe(Ⅱ) produced by strain Z and reductive dechlorination is a possible mechanism.(2) Slurry incubation system:Carbon isotopic analysis and molecular-based methods were used in combine with geochemical data sets to assess the transformaiton of pentachlorophenol when coupled to biogeochemical processes in a mangrove soil that had no prior history of anthropogenic pollution. The PCP underwent 96% dechlorination in the soil amended with acetate, compared to 21% transformation in the control soil. Carbon isotope analysis of the residual PCP showed an obvious enrichment of 13C (εC,-3.01±0.1%). Molecular and statistical analyses implied that PCP dechlorination and Fe(III) reduction were synergistically coupled electron-accepting processes. Microbial community analysis further indicated mat enhanced transforamtion of PCP during Fe(III) reduction was mediated by the multifunctional family of Geobacteraceae. In contrast, PCP significantly inhibited the growth of SO42- reducers, which, in turn, facilitated the emission of CH4 by diversion of electrons from SO42- reduction to methanogenesis.(3) Microbial community system:The aim was to investigate the influence of pentachlorophenol reductive dechlorination oin the microbial community structures in an uncontaminated soil environment under anaerobic conditions. Accordingly, an experimental study of slurry incubation was carried out in which bacterial and archaeal communities in incubated soil samples were detected by MiSeq amplicon sequencing. Obviously, acetate, as the common substrate for microbial metabolism during the anaerobic food chain, was significantly accelerated PCP dechlorinating processes. The phylum of Bacteroidetes was predominated during the earlier PCP dechlorination period and then was progressively replaced by Proteobacteria, Firmicutes and Chloroflexi at the later stage when PCP was mostly transformed into 2,3,4,5-TeCP and 3,4,5-TCP. Heatmap and hierarchical cluster analysis based on the relative abundances of the top 100 OTUs between different treatments revealed the involvement of Clostridium-like, Desulfuromonas-like and Dehalococcoides-like organisms in PCP reductive dechlorination processes. The archaea profiles were less complex than the bacteria profiles, the addition of PCP dramatically decreased the abundance of Crenarchaeota, but promoted the growth of Euryarchaeota. Sequencing analysis showed the methanogens that were as mainly attributed from to Euryarchaeota were enriched in PCP treatments in both soil layers and was much more active when following adding acetate addition. The prominent methanogen in the PCP treatment was a Methanosarcina-like acetotrophic microorganism in both soil layers.(4) Specific microbial community incubation system:DNA-stable isotope probing technology (DNA-SIP) was used to study the influcne of PCP on the microbial community strucetures when used labeled 13C-acetate as the microbial substrate. During long anaerobic incubation,13C was labeled into some microbes, which was selected as the targeted microorganisms to study. The genera of Bradyrhizobium, Burkholderia and Acinetobacter that related to the fixation of nitrogen and phosphorus were enriched in the labeled layer and the relative abundances of these genera were decreased when PCP was added. Similarly, methylotrophs of Methylobacterium was also enriched in the acetate amended treatment but decreased in the PCP treatment. Compared to the above genera, the relative abundance of Fe(Ⅲ) reducing bacteria such as Comamonas, Bacillus, Geobacter, Pelobacter, Clostridium, Pesudomonas changed little under the PCP toxicity. Beisides, the largest increased magnitude in the PCP treatment of the genus was the syntrophic bacterium of Candidates Contubernalis, which can growth through acetate oxidation in a syntrophic association with hydrogenotrophic microorganisms.