| Zhangye City is located in the middle part of Hexi Corridor of Gansu Province.There are two national nature reserves "Qilian Mountain" and "Heihe Wetland" in Zhangye,which plays an important role in maintaining the ecological security barrier in western China and even the whole country.Zhangye City is rich in mineral reserves,especially the iron ore reserves and output ranking the top in the province,so the iron ore mining activities are very frequent.There is no report on the extent of the impact of iron tailing discharge on the local soil environment.Therefore,in order to have a thorough understanding of the soil environmental quality and bacterial community features of the iron tailing areas in Zhangye region,the Houertou iron tailing pond in Linze county was selected as the research object in this study.The "Five-point sampling method" was adopted to collect 24 surface soil samples with a depth of 0～20 cm from8 sampling sites in the iron tailing reservoir and its surrounding areas.Through calculation and analysis of the Nemerow integrated pollution index,geo-accumulation index and Hakanson potential ecological risk index,and by using the high-throughput sequencing technology based on the 16 S r RNA gene sequences and the bioinformatics analysis,the soil physicochemical properties,nutrient content,types and pollution levels of heavy metals,and soil bacterial community structure have been studied thoroughly.Moreover,through the microbial separation and purification technology,a batch of functional heavy-metal resistant bacterial strains have been screened,and mechanism of heavy metal adsorption by the cadmium and lead resistant strains has been studied by Scanning electron microscopy-Energy dispersive spectroscopy(SEM-EDS)and Fourier transform infrared spectroscopy(FTIR).Thereby,these research would provide important basis for pollution control and ecological restoration of iron tailing environment in northwest China.The main results are as follows:(1)The soil of Houertou iron tailing pond and its surrounding area was alkaline(p H7.66～9.02).The soil organic matter content was less than half of the background value of surface soil in Gansu Province,and the content of total nitrogen and total potassium were also very low,so the soil environment were extremely barren.The iron tailing pond was contaminated by a high concentration of multiple heavy metals,including Pb,Mn,As,Cd,Zn and Fe.Among them,concentrations of the highly ecotoxic Cd,Pb and As were 18.5-60.8 times,24.6-250.8 times and 4.2-76.2 times of the soil background value,respectively.The calculation results of both Nemerow integrated pollution index and geo-accumulation index showed that Pb was the most polluted heavy metal and Ni was the least polluted one.The results of Hakanson potential ecological risk index showed that the comprehensive effects of various heavy metals resulted in high risks to regional ecological environment,especially Cd and Pb in soil.(2)The main bacteria phyla and their relative abundance in the soil of Houertou iron tailing pond and its surrounding areas were as follows: Actinobacteria 39.4%,Proteobacteria 25.5%,Chloroflexi 10.0%,Firmicutes 5.1% and Bacteroidetes 3.8%.The diversity of soil bacterial communities in the seriously polluted tailing area were significantly lower than those in the control area.The relative abundance of Actinobacteria in soil was positively correlated with the comprehensive pollution level of heavy metals at each sampling site.The proportion of Chloroflexi and Proteobacteria in the soil of tailing area was significantly lower,while that of Blastococcus was significantly higher.The results showed that the dominant bacterial groups and diversity of the iron tailings had definite indicative effect on the soil environmental quality.In addition,there were 11 environmental factors significantly affecting the composition and distribution of soil bacterial communities in this region,including p H,CEC,OM,TP,TK,Cd,Pb,Ni,Zn,As and Mn,among which,heavy metals had the greates impacts.(3)A total of 62 culturable heavy-metal resistant bacterial strains were isolated and purified from soil samples of the Houertou iron tailing pond.The heavy metal resistance of all strains were different.The minimum inhibitory concentrations of cadmium,lead,manganese and iron resistant strains were 10-100 mg/L,800-2500 mg/L,200-700 mg/L and 100-500 mg/L,respectively.The 62 heavy-metal resistant strains belonged to 3 phyla and 12 genera.The bacterial groups isolated and identified by the pure culture method were significantly different from the dominant bacterial groups analyzed by the high-throughput sequencing technology based on the 16 S r DNA sequences.(4)Under the laboratory liquid culture condition,the growth of Cd resistant strains LSC4 and LSC9 was significantly inhibited by 50 mg/L Cd(II)stress,and the maximum Cd(II)removal rates of the LSC4 and LSC9 strains were 62.8% and 36.6%,respectively.Under 1000mg/L Pb(II)stress,both LSP29 and LSP34 strains showed good resistance,and the maximum Pb(II)removal rates of LSP29 and LSP34 strains were 99.5% and 98.7%,respectively.In the LB liquid medium with an initial concentration of 50 mg/L Cd(II),the adsorption capacity of LSC4 and LSC9 strains to Cd(II)were 22.1 mg/g and 16.8 mg/g,respectively.Under the same condition,the adsorption capacity of LSP29 and LSP34 strains to Pb(II)were 173.5 mg/g and127.3 mg/g,respectively,in the LB liquid medium with an initial concentration of Pb(II)of1000 mg/L.In general,the growth curves and adsorption properties of each strain under stress exhibited different characteristics.(5)SEM-EDS analysis showed that after treatment with 50 mg/L Cd(II)for 60 h,the extracellular heavy metal adsorption of the LSC4 strain was weak,and only 0.13%(wt)Cd existed on the surface of the LSC4 strain,which might transfer Cd(II)into cells and isolate it in specific areas.While the cell surface of the LSC9 strain was homogeneous with 2.94%(wt)Cd distribution,indicating that it had certain extracellular adsorption capacity of Cd(II)in the environment.After treatment with 1000 mg/L Pb(II)for 24 h,a large amount of granular precipitates could be observed on the surface of the LSP29 and LSP34 strains,and the distribution of Pb were 33.75%(wt)and 16.56%(wt),respectively,indicating that both LSP29 and LSP34 strains had strong extracellular adsorption capacity for Pb(II),but the former had stronger adsorption capacity.FTIR analysis showed that hydroxyl,amide,hydrocarbon,nitro and organophosphorus groups on the cell surface of the four resistant strains were extensively involved in the adsorption process of Cd(II)/Pb(II).The results indicated that extracellular adsorption pathway was one of the important mechanisms of cadmium and lead resistance for the four resistant strains,and these strains would have further research value in the study of bacterial resistance mechanism and microbial remediation of heavy metal polluted environment.In conclusion,the soil environment of Zhangye Houertou iron tailing pond and its surrounding areas was extremely barren,and there was high concentration of multiple heavy metals in the tailing area,resulting in extremely high ecological environmental risks.The diversity of soil bacterial community and the dominant bacterial groups could clearly indicate the degree of heavy metal pollution in the tailing area.62 heavy metal resistant strains have been isolated from the soil of the tailing area,and it was found that extracellular adsorption was one of the important mechanisms for the cadmium and lead resistant strains to adapt to the polluted tailing environment.The results of this study would provide important support for environmental pollution control and ecological restoration of iron mine tailings in Zhangye region and even in northwest China. |
PDF Full Text Request