Coordination Mobilizing Of Soil Cadmium By High Siderophore-producing Bacteria

Cadmium（Cd）is one of the potentially toxic trace elements with high toxicity and long half-life.Cd in soil can be transferred through the food chain,posing a serious threat to the safety of agricultural products and human health.Stabilization remediation has been considered as one of the practical methods to control Cd pollution soil.However,metabolites of microorganisms widely exist in soil,such as organic acids and siderophores,etc.,might result in the secondary release of stabilized Cd into the soil,thereby reducing the repair effect of stabilization by passivators.In this study,pseudomonas fluorescence（P.fluorescens）with the ability of high-producing siderophores was selected by CAS detection method,the relationship between Fe³⁺and the siderophores generated by the strain were tested,and Cd resistance of the strain as well.Meanwhile,in order to evaluate the ability of the strain to mobilize different Cd forms in soil,a flask-shaking experiment was carried out on the mobilized soil Cd（cadmium carbonate,cadmium phosphate,goethite bound cadmium,weathered coal bound cadmium,montmorillonite bound cadmium and palygorskite bound cadmium）by the strain.The rusults would be found that when Cd was present in the solution environment,the siderophore levels generated by P.fluorescens significantly decreased（p<0.05）.Thus,the extracellular solutions,which the strain mobilized palygorskite bound cadmium in the MSA culture medium,were analysed by the untargeted metabolomic research,based on ultra high performance liquid chromatography mass spectrometry.From the perspective of metabonomics,to reveal Cd how to influence the strain to produce siderophores.Further,the responding mechanism of between behavior of producing siderophores of the strain and the mobilization of palygorskite bound cadmium,are cleared.The main results of this study are as follows:（1）In the experiment of Fe³⁺and P.fluorescens,it can be found that the strain can produce a large number of siderophores to ensure the normal life activities of themselves.In addition,the exogenous addition of 20μmol/L Fe³⁺can completely inhibit the strain to generate siderophores.Combining with the adsorption characteristics of materials of soil cadmium,the Fe³⁺of 200μmol/L was added as the control experimental group without producing siderophores.From the results of Cd tolerance test of strains,when Cd²⁺（5-40 mg/L）was present in the solution environment,the capability of siderophores generated by P.fluorescens significantly decreased.（p<0.05）（2）The mobilization of different materials of soils Cd（cadmium carbonate,cadmium phosphate,weathered coal bound cadmium,montmorillonite bound cadmium and palygorskite bound cadmium）by P.fluorescens showed that there was no significantly difference in pH of extracellular fluids,with and without siderophores production.From the mobilization of flask-shaking experiments,the pH of extracellular fluids were respectively 4.24、3.77、4.78、8.76、5.86,and the contents of Cd mobilization were respectively 31.12、30.53、0.91、0.52、20.41 mg/L with siderophores secretion.Meanwhile,in the corresponding control group,the pH of extracellular fluids were respectively 4.07、3.70、4.95、8.66、6.04,and the concentrations of Cd mobilization were respectively 32.36、29.99、0.77、0.27、16.32 mg/L without siderophores production.Therefore,The siderophores produced by the strain do not mobilize cadmium carbonate and cadmium phosphate,and the function of organic acid,secreted by the strain,was the main factor leading to Cd re-release.The siderophores generated by P.fluorescens has no significant effect on the mobilization of weathered coal bound cadmium,while the siderophores has a significant effect on the mobilization of Cd forms of montmorillonite bound cadmium and palygorskite bound cadmium,and their mobilization rates of Cd by siderophores were 48.3%and 22.0%,respectively.（3）According to Cd adsorption properties of weathered coal,montmorillonite,and the palygorskite and their corresponding mobilizaition results of flask-shaking experiment by P.fluorescens,the following preliminary conclusion can be obtained.When the Cd contaminated soil is repaired by means of stabilized remediation,weathered coal as the passivator,can play a long-term stability of repairing effect,but it can not improve the pH in soil.The most suitable passivator is montmorillonite,because it not only has a good adsorption performance,but also the siderophore has little effect on the mobilization of montmorillonite bound cadmium,and can significantly improve the pH in soil,so it has a long-term repair effect.But it is best to improve the Cd adsorption property of montmorillonite through modification,to further enhance the repair ability of stabilization by montmorillonite in Cd polluted soils.（4）the results of flask-shaking experiment,P.fluorescens mobilizes palygorskite bound cadmium,show that the Cd mobilization by strain producing siderophores is 22.0%higher than that of strain that does not produce siderophores（p<0.05）.The mobilization of palygorskite bound cadmium in turn significantly reduces siderophores contents of P.fluorescens by 25.1%（p<0.05）.The numbers of metabolites significantly up-regulated and down-regulated are 9 and22 in mobilized groups of palygorskite bound cadmium compared with the group without palygorskite bound cadmium,respectively.The results of metabolomics analysis reveal that the mobilized Cd affects the signal transduction pathway and primary metabolic processes of strain,reduces the metabolic capacity of pentose phosphate pathway,glycolysis and TCA pathway,thereby inhibited the ability of strain to biosynthesize amino acids during the mobilization process,further reducing the capacity of P.fluorescens to produce siderophores.This study provides useful information on how to select soil cadmium-stabilizing materials in a targeted manner and how to avoid cadmium re-mobilization by siderophores,which secreted by microorganisms in soil.