The Screening Of Cadmium-mobilized Rhizosphere-promoting Phosphorus-solubilizing Bacteria And Its Mobilization On Cadmium

The potential hazards caused by cadmium pollution in the Pearl River Delta region to the ecological environment and human health has been attracted widely attention.The methods of repairing heavy soil contaminated farmland was recognized to remove and reduce the heavy metal content,plant extraction and repair technology is one of the most promising technical approach.Rhizosphere microorganisms,especially rhizosphere bacteria can secrete organic acids,surface active agent,iron carriers and other substances to mobilize soil heavy metals.In this paper,the Phosphate-Solubilizing bacteria?PSB?,which are traditionally used for rhizosphere promoting action,are used to mobilize cadmium in soils.The cadmium-mobilized bacteria are screened out and to study its mobilization effect on cadmium.And than combinated with Cd super-cumulative plant Solanum nigrum to remediate the cadmium farmland soil.The results are as follows:?1?Twenty-five strains of PSB were screened from the rhizosphere soil of amaranth,and the dissolved phosphorus were range among 133.21²21.42 mg·L^-1;Seven strains of PSB were obtained from the microbial libraries,and the dissolved phosphorus were range between 253.40⁵46.95 mg·L^-1.The most effective PSB was Acinetobacter,which dissolved phosphorus could reach 546.95 mg·L^-1;Next one was Pseudomonas aeruginosa,the amount of dissolved phosphorus was 422.70 mg·L^-1;And Fluorescence pseudomonas was 341.70 mg·L^-1.Bacillus bacillus and Agrobacterium could not effectly dissolve Ca₃?PO₄?₂,and the dissolved phosphorus were 148.27 mg·L^-1 and 134.33 mg·L^-1,respectively.?2?Selected the PSBs which could effectively dissolve Ca₃?PO₄?₂,and the one which was not effectively dissolve Ca₃?PO₄?₂ to mobilized CdCO₃,respectively.The results showed that Enterobacter,Acinetobacter,Escherichia,Pseudomonas fluorescens and Klebsiella could effectively mobilized CdCO₃ and the concentration of Cd²⁺were between 27.65³8.23 mg·L^-1,and the flux of cadmium were between 70.89%⁹8.02%.Through metabolomics analysis,it was found that the strong ability of mobilization of Pseudomonas fluorescens was mainly owe to the contribution of gluconic acid,the concentration of gluconic acid in the extracellular secretion reached 75.3 mg·L^-1,gluconic acid mainly contributed?42.4%?to Cd dissolution.The content of organic acids in Bacillus cereus extracellular secretion was lower than in the Pseudomonas fluorescens,and the main organic acids,pyruvate acid and glycolic acid,contributed 23.6%and15.4%to Cd dissolution,respectively,lead to its inefficiently to mobilized Cd.The mobilization experiments of Pseudomonas fluorescens and Bacillus cereus on the actual cadmium-contaminated soils also confirmed that Pseudomonas fluorescens could secrete gluconic acid,which could effectively mobilized cadmium in the soil and Pseudomonas fluorescens could mobilized Cd 0.39 mg·kg^-1,which accounted 20.2%of total soil Cd(1.935 mg·kg^-1),and 74.3%of activated(0.286 mg·kg^-1)and carbonate-bound Cd(0.239 mg·kg^-1).?3?The results of pot experiment of rhizosphere promoted PSB and cadmium super-cumulative plant solanum nigrum showed as follows,the rhizosphere-promoting PSB could promote the uptake of phosphorus in the stem and leaf of solanum nigrum,the same as the absorption of Cd.Phosphate-solubilizing bacteria could promoted Cd content in the stem and leaf and higher than the control group 1.02-2.15 times,promoting effect as the order:Pseudomonas fluorescens?GI?>Pseudomonas aeruginosa?K3?>Acinetobacter?AP?>Klebsiella?X2?>Bacillus cereus?HL?>Escherichia coli?EN?>Enterobacter cloacae?EC?>Pseudomonas aeruginosa?PC?>CK.The Cd content in root were higher than in the control group 1.10-1.87 times,promoting effect as the order:GI>EN>X2>EC>PC>HL>AP>K3>CK.Inoculation phosphate-solubilizing bacteria can promote cadmium form changed in soil,exchangeable content had increased,and affect the content and species of organic acids in rhizosphere soil solution,thus promote the accumulation of cadmium.?4?Through high-throughput sequencing macro genomics analysis,the results showed that the microbial diversity and species abundance has changed,Acinetobacter and Enterobacter treatmentss increased the microbial diversity in the rhizosphere soil of solanum nigrum.The relative abundance of Acinetobacter and Enterobacter in the rapid growth and maturation stages showed that Acinetobacter and Enterobacter were successfully colonized in the rhizosphere soil of solanum nigrum,but did not become a dominant bacteria.