Microbial Mechanisms Of Humic Substances Enhance Anaerobically Reductive Dechlorination And Mineralization Of Pentachlorophenol In Paddy Soil

Pentachlorophenol(PCP) is one of high toxic and persistent POPs, and has been used as wood preservative, herbicide, pesticide for many years in agriculture production. After prolonged nonprotective usage, PCP enters nontarget uplands, wetlands, and aquatic environments, associating with colloidal and particulate matter and eventually settling in the soils. Also, PCP residues are widespread in living bodies, which would cause cancer, birth defects, blood disorders and nerve damage. Humic substances(HSs) are ubiquitous in aquatic and terrestrial environments, which tend to adsorb or chelate the metal and organic matters. HSs as redox mediators, which are reversibly oxidized and reduced, can participate in microbial metabolism, influence the microbial activity, and have a direct impact on organic biodegradation. Therefore, it is particularly important to elucidate the microbial mechanisms of HSs effect on the anaerobically degradation of PCP in paddy soil. Dechlorination is the first step of PCP transformation, then further mineralization to CO2 and CH4. In this study, anaerobic batch experiments were performed using paddy soil as the inoculum to explore the effect of biochar on reductive transformation of PCP, especially for electron transfer and microbial communities; then, DNA-based stable isotope probing(DNA-SIP) was applied to determine the microorganisms responsible for 13 C label uptake from PCP or the intermediates of PCP degradation under HSs-impacted condition. The main results were as follows:(1) The rape-straw derived biochar was used to investigate how biochar amendment affected microbial communities and PCP transformation in a paddy soil under anoxic conditions. PCP sorption and transformation and HCl-extractable Fe(II) generation were significantly enhanced in biochar amended soils. The results showed that the enhanced transferred extracellular electron in biochar amended soils promoted the PCP transformation while biochar supplement promoted the growth and metabolism of microorganisms in the soil. The relative abundance of dominant microbial population was bearing on biochar dosages in the soil. In addition, quantitative real-time PCR results illustrated that biochar acted as a growth stimulant for iron(III)-reducing and dechlorinating bacteria, which consequently promoted PCP biotransformation. Furthermore, two diversity indices(ACE and Chao 1) derived from T-RFLP analyses had positive exponential relationships with the percentages of added biochar, which indicated biochar could accelerate in the species diversities.(2) The DNA-based SIP was applied to investigate the PCP mineralization microorganisms in paddy soil in presence and absence of lactate under anaerobic condition. The results revealed that a clear increase in the 13CO2 and 13CH4 producued afer 10 days incubation was observed, indicating the utilization of PCP(ring-clevage) by the microorganisms. The increased production of 13CH4 and 13CO2 indicated that the addition of lactate enhanced the rate of biodegradation and mineralization of PCP. It was fonund that Dechloromonas was responsible for the anaerobic PCP mineralization with or without the presence of the additional carbon source of lactate in the paddy. However, the archaea of Methanosaeta were detected only in the soil treatment with lactate, which was favorable for the mineralization of PCP or its breakdown products.(3) The SIP was applied to investigate the influence of humic acid(HA) derived from different soil on the microbial community involved in PCP anaerobic mineralization. The results showed all humic acids accelerated microbial degradation of PCP and the highest rates were observed in humic acids with high carbon contents, extracted from peat soils from Yunnan. The Illumina sequencing revealed that the HA had a significant impact on the microbial communities and several phylotypes were enriched following PCP mineralization in the 13 C heavy fractions compared to the initial soil and the 12 C heavy fractions. The results showed that Methanosarcina and OP11 were the dominant PCP degraders in microcosms amended with CBHA, whereas in PSHA and YNHA-amended microcosms, Burkholderia and Methanobacterium were the key PCP degraders.