| With the acceleration of economic development,the problem of Cd pollution in vegetable fields in China has become more and more serious.Excessive levels of Cd in vegetables have seriously harmed human health.At present,bacterial passivation remediation in vegetable field Cd pollution remediation has become a research hotspot due to its advantages such as low cost,no secondary pollution and good treatment effect.It was found that Bacillus subtilis,a film-producing bacterium,could effectively inactivate Cd in soil to reduce the absorption of Cd by vegetables,but the mechanism is still unclear.In order to further improve the B12 strains passivation mechanism of soil Cd,this study from B12 strain secreted by fat peptide compounds as the breakthrough point,trying to find B12 strain helps relieve the stress of Cd and regulating the microbial community passivation mechanism of Cd,so as to clarify bacillus passivation soil Cd,Cd provide theoretical basis for the mechanism of absorption and reduce vegetables test base.This paper first explored the mechanism of alleviating Cd stress by strain B12 from the aspect of lipopeptide compounds secreted by strain B12.The results showed that the dry weight of B12 biofilm was significantly increased by 44.8-70.0% in Msgg medium containing 0-1.0 mg/L Cd.Cd stress promoted the secretion of lipopeptide compounds by strain B12,and surfactin content increased.When surfactin was added into Msgg medium containing 0.5 mg/L Cd,the dry weight of Bacillus subtilis B12,Sinorhizobium meliloti Mr40 and Bacillus amyloliquefaciens P29 biofilm were significantly increased by84.1-126.9%,the number of plankton cells was significantly decreased by 46.9-55.3%,and the Cd removal rate of the three strains was significantly increased by 23.0-36.1%.The Cd adsorption ratio of plankton cells was significantly decreased by 50.1-69.5%,and that of biofilm was significantly increased by 46.8-68.1%.In conclusion,surfactin secreted by strain B12 can promote the formation of bacterial biofilm to absorb more Cd.In order to further investigate whether surfactin is the key factor of Cd stress in strain B12,a surfactin-deficient mutant B12△srf AA(△S)was constructed and its characteristics were determined.The results showed that △ S growth of the mutant had no significant change compared with the wild strain B12,but the biofilm was thin and fragile,and the dry weight,protein content and polysaccharide content of the biofilm were significantly reduced by 44.44%,53.08% and 61.23%,respectively.Compared with the B12 treatment group,the number of plankton cells and the Cd adsorption ratio of △ S strain were significantly increased by 45.21% and 139.25%,respectively,while the Cd adsorption ratio and the biofilm adsorption ratio of Cd were decreased by 71.81% and 58.52%,respectively.Compared with CK,B12 and △S strain significantly reduced the content of available Cd by 26.6% and 18.4% in non-sterilized soil.The results showed that the mutant△S was significantly less capable of adsorbing Cd in solution and passivating Cd in soil than the wild strain B12.In this study,the mechanism of soil Cd passivation by strain B12 was explored through soil passivation test.The test has a total of six treatment groups(1)CK: sterile deionized water;(2)Inactivated B12: inactivated B12 suspension;(3)B12:B12 bacterial suspension;(4)△S: △S bacterial suspension;(5)SF: surfactin solution with a concentration of 0.5 mg/L;(6)△S+SF: △S suspensions containing 0.5 mg/L surfactin.It was found that in15-60 days of non-sterilized soil,compared with CK,the content of exchangeable Cd in soil of B12,△S,SF and △S+SF treatment groups was significantly reduced by 12.01--38.66%,and the content of Fe and Mn oxidation state Cd was significantly increased by4.51--55.78%.The content of carbonate bound Cd in soil of B12,SF and △ S+SF treatment groups was significantly increased by 7.11-35.37%.In sterilized soil,compared with CK,B12,delta,delta △S + SF,three treatment groups can exchange state Cd content in soil was significantly decreased 12.13 25.81%,B12 and △S + SF,two treatment groups of soil carbonate combined with state iron and manganese oxidation state Cd content were significantly increased by 5.90-15.60% and 1.36 20.80%,thus,B12 strains in sterilized soil and the effects of sterilization of Cd in soil were significantly better than that of the delta△S strain,and B12 strains in the passivation in sterilized soil Cd significantly superior to the effect of sterilization in the soil.Community composition analysis showed that the abundance of Actinobacteria,Chloroflexi,Bacillus and Lysinibacillus in the soil of SF treatment group increased compared with CK.Lysinibacillus and Bacillus were negatively correlated with the content of exchangable Cd,while Lysinibacillus was positively correlated with the content of Fe-Mn oxidation state and carbonate-bound Cd.The abundance of Stenotrophomonas,Ensifer,Pseudoxanthomonas,Phenylobacterium,Bacillus,Rhizobiales,Brevibacillus increased in the B12 treatment group,and the 7 genera were negatively correlated with the content of exchangable Cd,among which Rhizobiales and Brevibacillus were positively correlated with the content of Fe-Mn oxidation state Cd.Compared with the B12 treatment group,the abundance of Stenotrophomonas,Ensifer,Pseudoxanthomonas,Phenylobacterium,Rhizobiales,Microvirga,Brevibacillus,Rhizobiaceae decreased,and the 8 genera were negatively correlated with the content of commutable Cd,among which Rhizobiales and Brevibacillus were positively correlated with the content of Fe-Mn oxidation state Cd.The pot experiment results showed that compared with CK,the fresh weight of Chinese cabbage in B12,△S,SF and △S+SF groups had no significant change,but the content of available Cd in shoot,root and rhizosphere was significantly decreased by15.10-56.08%,14.65-55.14% and 13.92-44.42%.The rhizosphere bacteria number and soil polysaccharide content significantly increased by 84.27-177.19% and 93.56-206.17%.The results showed that the effect of B12 strain on reducing Cd absorption in Chinese cabbage was significantly better than that of △S strain.In conclusion,under Cd stress,strain B12 can secrete more surfactin to promote the formation of biofilm,thus adsorbing more Cd.In addition,strain B12 can also inactivate more Cd by risking functional microflora in the soil. |
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