Remediation Of Hg-contaminated Soil Using Plant Growth-promoting Bacteria AN-b15 And Biochar

A plant growth-promoting bacteria(AN-15)with high Hg resistance was isolated from plants from Mercury(Hg)-contaminated farmland soil.AN-15 was identified as speices of genus Pseudomonas.The results showed that AN-B15 could effectively remove Hg²⁺in liquid medium(total removal rate>90%)under the concentration of 5and 20 mg/L Hg²⁺,in which the removal rate of bio-volatilization was approximate 60%,and the removal rate of adsorption and precipitation(approximate 27%).Transcriptome analysis showed that the expression of mer operon in AN-B15 was up-regulated under Hg²⁺stress,and a strong activity of mercury reductase was detected in the cells,indicating that AN-B15 could reduce Hg²⁺to volatile Hg⁰.The phase analysis showed that the mercury was mainly?-HgS,which indicated that AN-B15 could transform Hg²⁺with high bioavailability to HgS with low bioavailability.Microcosm experiments showed that the inoculation of AN-B15 could significantly reduce the content of total and available mercury in Hg-polluted soil,compared to the control.Pot experiments showed the inoculation AN-B15 increased antioxidant enzyme activity(SOD?POD and CAT)and the biomass and chlorophyll content in wheat seedlings.Meanwhile,the inoculation AN-B15 could reduce the uptake of Hg by wheat seedlings.RNA-Seq analysis showed that genes related to plant growh-promoting(iaaM,PqqC,pvdA,speD,speE,TreS,TreY,and TreZ)and biological control(aprA and chiD)were up-regulated,possiblely connecting the reduction of Hg toxicity in the treatment with inoculation of AN-15 for wheat seedlings under Hg stress.Biochar,as a soil improver,can be used for remediation of mercury contaminated soil.The results show that biochar can increase soil pH,TOC,A-P content,reduce NH₄-N content,significantly reduce soil available Hg content,increase plant height,dry weight and chlorophyll content of wheat seedlings,and reduce Hg content in the shoot and root of wheat seedlings.Soil microecological analysis showed that mercury treatment reduced the number,abundance and diversity of OTU in soil samples,and adding biochar was beneficial to reduce mercury toxicity in soil.The analysis of?-diversity showed that 1%biochar was beneficial to the increase of soil microbial richness,and 4%biochar was beneficial to the increase of soil microbial diversity.The correlation analysis of environmental factors and soil microorganisms showed that 11dominant classes,such as Alphaproteobacteria,actinobacteria sphingobacteria and chloroplast,were positively correlated with pH,total Hg,TOC,DOC,NH₄-N and A-P.Among the top 20 dominant genera,massilia,mucilaginbacter,Burkholderia paraburkurkholderia,Sphingomonas,latobacillus and methylbacterium were positively correlated with pH,TOC,DOC,A-P and total Hg,and negatively correlated with DTPA-Hg.In summary,the plant growth-promoting bacteria AN-B15 and biochar have high remediation potential for Hg-contaminated soil.This study can provide a reference for improving the in-situ remediation of Hg-contaminated soil,and has certain practical significance and application value.