| Polybrominated diphenyl ethers(PBDEs)are a typical class of brominated flame retardants,which are difficult to degrade and easily accumulate and migrate in the environment,causing a series of environmental and ecological safety problems,so it is urgent to develop effective methods to eliminate PBDEs from the polluted environment.In this paper,2,2’,4,4’-tetrabromodiphenyl ether(BDE-47)was used as a representative contaminant of PBDEs.A novel microbial consortium QY2 was obtained by long-term screening and acclimatization as the experimental object.This study took the perspectives of biostimulation to systematically study the effects of external carbon sources(methanol)and carbon materials(biochar)on the degradation performance,degradation pathways and toxicity defense mechanisms of QY2.In addition,the community composition and abundance,diversity,ecological network characteristics and interspecific interactions of QY2 under the biostimulation of methanol/biochar were investigated to reveal the succession pattern and ecological interactions of keystone taxa under the influence of biostimulation.And the feasibility of biochar combined with functional microbial consortium QY2 for bioremediation of actual BDE-47 contaminated soil was preliminarily explored.The main research results are as follows:(1)The microbial consortium QY2,which can use BDE-47 as the only carbon source for growth and metabolism,was constructed through long-term and successive acclimation from indigenous microorganisms.When 2.5 m M methanol was used as the co-metabolized substrate,QY2 could completely degrade 0.25 mg/L BDE-47 in 7 days(30℃,p H 7,1 g/L QY2).Four debromination products(BDE-28,BDE-17,BDE-15 and BDE-8),four hydroxylation products(6-OH-BDE-47,5-OH-BDE-47,2’-OH-BDE-28 and 4’-OH-BDE-17)and two brominated phenols(2,4-BDP and 4-BP)were detected during degradation.The addition of methanol significantly accelerated the debromination,hydroxylation and benzene ether bond breaking of BDE-47 by QY2.The community analysis showed that QY2 mainly contained the genera Chitinophaga,Methylobacterium,Cupriavidus,Achromobacter,Microbacterium,Burkholderia-Caballeronia-Paraburkholderia and Sphingomonas.The contribution of methanol on the assembly of QY2 was least,but it was important in regulating interspecific interactions and the abundance of keystone species(Methylobacterium,Cupriavidus,Ochrobactrum,Sphingomona,and Microbacterium),which played essential roles in maintaining the functional and structural stability of QY2 during co-metabolic degradation.(2)The physiological response and detoxification mechanism of QY2 to methanol as well as BDE-47 during the co-metabolized degradation process were clarified.The results of the toxicity assessment experiments showed that methanol and BDE-47 exhibited a low-promoting and high-inhibiting concentration effect on microbial activity.Low doses of methanol(< 5 m M)were able to stimulate microbial cell growth and activity to alleviate the oxidative stress response of QY2 in response to BDE-47 stress,promoting the biodegradation of BDE-47.In contrast,high doses of methanol(≥ 5 m M)produced toxic synergism with BDE-47,stimulating excessive intracellular reactive oxygen species(ROS)production and inducing intracellular lipid peroxidation,resulting in 20.7% apoptosis and 11.3% cell death.In addition,the toxicity of the co-metabolized degradation products was assessed using Escherichia coli(E.coli),and the effects of the degradation products on ROS content,cell membrane potential,superoxide dismutase(SOD),catalase(CAT),malondialdehyde(MDA),and apoptosis in E.coli cells were found to be less than those of BDE-47.This indicates that the co-metabolism of BDE-47 by QY2 is a detoxification process.(3)The mechanism of co-metabolic degradation of BDE-47 by biochar-stimulated QY2 was further investigated.The results showed that biochar at different temperatures(300℃,500℃and 700℃)had a high adsorption capacity for BDE-47,which was dominated by physisorption.Meanwhile,biochar provided attachment sites for QY2 cells to form biofilms,thus shortening the distance between microorganisms and BDE-47.The biofilm formed on biochar contained redox-active extracellular polymeric substances(EPS).The strong interaction between EPS and biochar promoted the electron transfer capacity of the biofilm and enhanced the further debromination,hydroxylation and phenyl ether bond cleavage of BDE-47.Moreover,potential degraders(Methylobacterium,Sphingomonas,Microbacterium)and electrochemical bacteria(Ochrobactrum)were selectively enriched by biochar,whose role as keystone bacteria may participate in biofilm formation and redox-active EPS secretion.The biostimulatory effect of biochar also significantly increased the complexity of microbial ecological networks(MENs)in biofilms,making the interaction between the microbes in the microbiota more closely and positively correlated,which played an important role in biochar-enhanced bacterial consortium degradation and metabolism of BDE-47.(4)The effects of biochar combined with bacterial consortium QY2 on the dissipation of BDE-47 in contaminated soil and on blocking the transport transformation in the soil-plant(Brassica juncea)system were investigated.Biochar enhanced the adsorption and immobilization of BDE-47 to soil and reduced the bioavailability of BDE-47,thus limiting plant uptake and microbial biodegradation.However,the free QY2 could promote 44.9% of BDE-47 degradation in the soil and had limited effect in in blocking plant uptake.The combination of biochar and QY2(BM)not only promoted both biodegradation of BDE-47(61.5%),but also reduced the bioavailability of BDE-47,which in turn reduced the BDE-47 concentration in the edible part of plant by 82.0%.In addition,the BM treatment significantly increased soil p H,improved soil physicochemical properties and nutrient conditions,and soil activity in dehydrogenase,catalase,urease and acid phosphatase increased by 58.7%,69.3%,54.2% and 74.7%,respectively.Biochar in combination with QY2 changed the soil microbial community structure,enhanced interspecific interaction(cooperation),and reorganized core keystone taxa.Keystone taxa had a direct and significant positive effect on BDE-47 degradation,soil nutrients and soil enzyme activity.The achievements in this study provided some theoretical basis for the degradation mechanism,detoxification mechanism and interspecies interaction of BDE-47 by functional bacterial consortium QY2.Furthermore,biochar combined with bacterial consortium is an effective and feasible bioremediation strategy for contaminated soil.The results of the study will be of great theoretical and practical significance for the development and application of PBDEs removal technology in the environment. |
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