Study On Detoxification And Remediation Of Pakchoi Seedlings Under Cr(?) Stress By A Cr(?) Reducing Bacterium And Preparation Of Its Bacterial Agent

Chromium(?)is a highly toxic,non-essential metallic element.Heavy metal chromium(?)pollution in farmland is one of the important factors restricting plant growth.The toxic effect of chromium not only reduces the yield and quality of crops,but more seriously,once excessive chromium is ingested through the food chain,it will cause varying degrees of damage to the body and threaten human life and health.Therefore,the development of repairing technology for farmland chromium pollution has become an urgent problem to be solved in ecological and environmental studies,and microbial repair technology has attracted extensive attention due to its characteristics of small soil and water disturbance,low cost,simple operation and less secondary pollution.This experiment mainly studied the growth and reduction characteristics of a strain isolated from the activated sludge in the sewage treatment system of electroplating plant with high reduction capacity for Cr(?),and established a hydroponics pot culture system to study its effects on the growth,physiological and biochemical reactions and heavy metal absorption of Pakchoiunder the stress of Cr(?).Finally,the survival rate of the strain under different carrier ratios was studied,and the optimal compound ratio was selected to make it into solid bacterial agent.The results showed that strain E was gram-positive,facultative anaerobe,and the shape and length of bacteria varied with temperature.The colonies are small,round and orange.The surface of the colonies are smooth and shiny,and there is a bulge in the middle.The optimum growth temperature of strain E is 30?,and the optimum p H is 10.By 16S DNA sequencing analysis,the homology between strain E and bacillus wasup to 99%.Combining with the morphological and physiological and biochemical characteristics of the bacteria,it was determined that strain E was Exiguobacterium sp..The reduction characteristics of strain E showed that strain E had a strong tolerance to high concentration Cr(?).In the basic nutrient medium,the minimum inhibitory concentration of Cr(?)on strain E was 350 mg/L.Under the experimental conditions,strain E had a highly effective reduction ability for Cr(?),with a removal rate of 99.96%at 24 h for 20 mg/L Cr(?)and 99%at 48h for 30 mg/L Cr(?).Initial p H had an important effect on the reduction ability of strain E,and had a good reduction ability for Cr(?)under the condition of p H value ranging from 7 to 10.The reduction of Cr(?)by E strain was significantly enhanced by adding carbon succinic acid,sucrose and glucose also promoted the reduction of Cr(?)by bacterial strain E.The addition of metal ions Zn²⁺,Fe³⁺,Pb²⁺and Cu²⁺promoted the reduction,while Mn²⁺inhibited it.Further studies on the resistance mechanism of strain E showed that under the experimental conditions,the removal of Cr(?)by strain E was mainly a reduction effect,and the changes in the permeability of bacterial cell wall membrane would make Cr(?)toxic to cells to some extent,and strain E detoxified Cr(?)mainly by secreting reductase to the outside of cells.The results of hydroponics experiment on Pakchoi showed that the higher the concentration of Cr(?),the more toxic effect it had on the growth and development of Pakchoi.The toxicity of Cr(?)significantly reduced the growth and the synthesis of protein and chlorophyll of Pakchoi,and increased the content of MDA,SOD and Cr in Pakchoi leaves.In the presence of Cr(?),the inoculation of strain E could reduce the absorption of Cr by roots,stems and leaves of Pakchoi,so as to improve the tolerance of Pakchoi to Cr(?).In addition,inoculation of strain E can also reduce oxidative stress and alleviate the toxic effect of Cr(?)on Pakchoi by improving enzyme activity and non-enzyme antioxidant activity.Strain E was prepared as a solid bacterial agent,and the study on the ratio of composite carrier material showed that the bacterial survival rate was the highest when the composite ratio of peat soil:attapulgite soil:diatomite was 3:1:3,indicating that this ratio was the optimal carrier composite ratio.