Streptomyces Pactum Act12 Assisted Phytoremediation Of Cd And Zn Contaminated Soils By Potherb Mustard (Brassica Juncea,coss.) And Investigation Of Its Mechanisms

Phytoremediation is considered to be an environmentally friendly,non-invasive,energy-saving and cost-effective technology,which is used to remediate medium and low levels of heavy metal polluted sites.How to strengthen the remediation of heavy metals or improve the growth rate/biomass of plants has become a hot issue for researchers.The work of inoculating microorganisms into heavy metal contaminated soil to enhance phytoextraction has attracted extensive attention.As one of the three major microbial groups in soil,actinomycetes are often used in the study of plant growth promotion and biological control of plant diseases and insect pests;however,the research of their application in soil remediation is rarely reported.This research systematically studied the tolerance and enrichment characteristics of potherb mustard for Cd and Zn,the phytoextraction of Cd-and Zn-contaminated soils?physiological and biochemical responses of plant,physical and chemical properties of soil,soil microbial community and remediation efficiency?and the preliminary mechanisms of phytoremediation technology enhanced by Act12,which provided the basic theoretical for the remediation of Cd-and Zn-contaminated sites by potherb mustard,and provided scientific and technical reference for microbe assisted phytoremediation.The main results are as follows:?1?The tolerance,accumulation and translocation characteristics of Cd and Zn in potherb mustard were studied by seed germination under different concentration gradients of Cd and Zn and the growth of potherb mustard under different cultivation methods?soil culture and sand culture?.The results showed that the germination rate of potherb mustard seed was inhibited by Cd and Zn stresses,which was between 18.3%and 37.8%.The lower concentrations of Cd and Zn stresses could promote the growth of potherb mustard lateral root,but the plant biomass decreased continuously with the increasing Cd and Zn concentrations.The content of Cd and Zn in potherb mustard linearly increased with the increasing Cd and Zn concentrations in growth medium.The highest concentrations of Cd and Zn were 79.9 and 3318 mg kg^-1 in soil culture,while they were 129 and 5195 mg kg^-1 in sand culture,respectively.?2?The phytoextraction efficiency of potherb mustard assisted by EDTA,citric acid?CA?and oxalic acid?OA?was studied.The results showed that the bioavailabilities of Cd and Zn in soil were improved by chelation,and the contents of Cd and Zn in potherb mustard were increased by 1.70,2.15 times?leaf?and 1.93,2.70 times?root?respectively,while the dry weights of plant leaf and root were significantly reduced,which were 4.13-9.91 and0.21-0.77g pot^-1,respectively.Chelating agent treatments increased the activities of phenylalnine ammonialyase,polyphenol oxidase and catalase,but decreased the activity of peroxidase;in general,the application of chelating agent improved the efficiency of potherb mustard in remediation of smelter-polluted soils,and follow the order:EDTA>EDTA+CA?EDTA+OA>CK.?3?The effects of Act12 and compost on soil fertility,Cd/Zn availability and phytoextraction efficiency of potherb mustard were studied.The results showed that the application of compost decreased the p H value of soil,increased the electrical conductivity?7.0 times?,available phosphorus?10.8 times?,available potassium?2.81 times?,dissolved organic carbon?5.22 times?,organic matter?4.93 times?,urease?4.39 times?,dehydrogenase?45.0 times?and alkaline phosphatase?123.9 times?,and Act12 inoculation increased the soil fertility,improve the solubility of Cd and Zn in the soil,and thus increased the concentrations of Cd and Zn in plant.The application of compost and Act12 synergistically improved the phytoextraction efficiency,and the metal extraction amounts of Cd and Zn were 9.64 and 11.4 times higher than that of the control.?4?In pot experiment,the biomass and chlorophyll content of potherb mustard were increased after Act12 inoculation,but the fresh weight of leaf,chlorophyll a and chlorophyll b content decreased by 57.8%,38.2%and 40.7%respectively after sulfur treatment.The application of sulfur promoted the production of malondialdehyde,which was increased by18.4-33.6%compared with the control.The activities of peroxidase and superoxide dismutase were increased,while catalase was inhibited by Act12.The activities of soil urease?20.4%?and dehydrogenase?58.5%?were significantly increased,and the alkaline phosphatase?68.0%?activity was decreased?P?0.05?.The concentrationsof Cd nd Zn in the shoot and root were significantly increased by the combination of sulfur and Act12.The order of organic acid production from high to low was formic acid>oxalic acid>malic acid>propionic acid.The composition of rhizosphere soil bacterial community had the same trend in the classification of phylum and genus levels,and sulfur treatment increased the relative abundances of Proteobacteria,Bacteroides,Thiobacillus?12.3 times?,Pseudomonas?2.47 times?and Pedobacter?1.03 times?.?5?In pot experiment,Act12 combined with Hoagland nutrient solution,humic acid and peat significantly increased the contents of chlorophyll and soluble protein in potherb mustard,and thus promoted plant growth.The activities of polyphenol oxidase and peroxidase decreased by 28.2-41.4%and 15.2-59.4%respectively,and the activity of catalase decreased with the addition of Hoagland nutrient solution and humic acid while increased by peat.The order of metal extraction amount was Act12+Hoagland nutrient solution>Act12+peat>control>Act12+humic acid.The soil enzyme activities under Hoagland nutrient solution,humic acid and peat treatments were 0.53-0.62 times?urease?,2.11-5.68 times?dehydrogenase?and 1.21-4.69 times?alkaline phosphatase?higher than that of the control,respectively.High-throughput sequencing showed that 9 phyla and 20 genera are dominant rhizosphere bacteria,among which Proteobacteria and Sphingomonas have the highest relative abundances,while Verrucomicrobia and Nitrospira have the lowest relative abundances.?6?The germination rate of potherb mustard seeds treated with Act12 fermentation broth increased from 37.8%to 50.0%.The biomass of potherb mustard increased compared with the control,especially for the promotion of root growth and development.With the continuous increase of Cd and Zn stress,the germination rate of potherb mustard seeds treated with Act12 fermentation broth first increased(reached the highest level in Cd 5 mg L^-1 and Zn 250 mg L^-1 treatment,63.3%)and then decreased,which increased the resistance of potherb mustard seed to Cd and Zn stress.The phytoextraction ability of Act12fermentation broth to Cd and Zn in polluted soil was better than that of liquid culture medium solution,but it was inferior to that of 1M NH₄OAc solution,indicating that Act12could improve the bioavailability of Cd and Zn in soil.Overall,it was concluded that Act12 was a potential bioremediation agent for Cd and Zn contaminated soil,and combined application of Act12 and fertilizers could effectively strengthen the remediation of Cd and Zn contaminated soil by potherb mustard.