Effects And Mechanism Of Enhancing Phytoremediation Of Vanadium Ore Contaminated Soil By Endophytic Bacteria

Multiple heavy metals co-contamination of V,Cr and Cd in soil due to mining and smelting activities of vanadiferous ores is an increasingly serious problem.At present,there are only a few of research on the remediation of these contaminated sites.In this study,we intend to adopt the strategy of endophyte-assisted phytoremediation to remediate vanadium industry contaminated soils.We conducted a series of screening tests,pot and hydroponic experiments,combining with high-throughput sequencing,biolog plate,SEM-EDS and HPLC,to explore the effects and mechanism of enhancing phytoremediation of vanadium ore contaminated soil by endophytic bacteria.Firstly,the strains PRE01 and PRE05 identified as Serratia marcescens and Arthrobacter ginsengisoli by 16SrDNA were isolated and screened out from wild-type Pteris vittata grown on stone coal smelting district.The two strains exhibited ACC deaminase activity,siderophore production and high indoleacetic acid production and solubilizing mineral phosphate potential.Meanwhile,they both had strong V,Cr and Cd resistance,especially PRE01 up to 30,6 and 4mmol/L,respectively.Moreover,PRE01 was capable of alleviating metal phytotoxicity by the strong ?(?)and Cr(?)reduction and Cd(?)adsorption,specifically assimilating 97.6%of Cd(?),reducing 81.1%of ?(?)and 84.4%of Cr(?).The biolog Gen ? microplate test showed that the two isolates had competitive advantages as bioinoculants for effective phytoremediation purpose.Pot experiments showed that endophytic bacteria PRE01 and PRE05 could promote host plant(P.vittata)and non-host plant(Brassica juncea)growth and uptake of heavy metals.PRE01 could increase the total accumulation of V,Cr and Cd by 25.4%,14.3%and 41.3%in P.vittata,and by 31.0%,18.0%and 35.2%in B.juncea,respectively.The physiological and biochemical analysis revealed that enhancing the activity of antioxidant enzymes in plants are important cooperative mechanisms for endophytic bacteria to alleviate oxidative damage of heavy metals and promote plant growth.Furthermore,the mechanism of heavy metal detoxification and mineral dissolution by endophytic bacteria was studied.The results showed endophytic bacterium PRE01 was effective in enhancing the detoxification process of vanadium in P.vittata.This is mainly attributed to the fact that the strain can increase the vanadium adsorption at the root surface,strengthen the reduction of V(?)in root tissue and promote the secretion of phytic acid,oxalic acid,and malic acid from roots to alleviate vanadium phytotoxicity.It's also found that endophytic bacteria could accelerate dissolving the V-Fe complex in the rhizosphere by secreting microbial siderophore and facilitating the secretion of phytic acid from the roots,thereby promoting the absorption of Fe and V by plants.Finally,we investigated the impacts of endophyte inoculation on rhizosphere and endosphere microecology and its enhancement on phytoremediation of vanadium industry contaminated soils.Inoculation with PRE01 or PRE05 significantly increased organic matter content and bioavailability of heavy metals in rhizosphere soils.The test strains significantly affected the specific composition and diversity of endophytic bacterial communities in roots,with no significant effect on rhizosphere bacterial communities.Despite the endophyte inocula failed to flourish as stable endophytes,they could greatly increase plant growth promotion-related biomarkers in the endosphere,especially those associated with Pseudomonas and Microbacterium genera,via competition with other indigenous bacteria in earlier stages.This study will provide a reference for the construction of endophyte-assisted enhancing phytoremediation method and provide novel insight into the mechanism study of endophyte enhancing phytoremediation.It is of great significance to promote the research and practical application of phytoremediation of vanadium ore contaminated soil.