The Screening Of Siderophore-producing Bacteria And Its Enhancing Effect On Remediation Of Cadmium-contaminated Soil By Solanum Nigrum L.

In recent years,the mining,sewage irrigation,and overuse of chemical fertilizers have caused cadmium(Cd)accumulation in farmland soils,which seriously threaten crop growth and consumption safety of agricultural products.Phytoremediation has been deemed as an important technology for remediation of medium and mild-Cd polluted soils due to its low-cost and good ecological benefits.However,the remediation for Cd-polluted soils using single plants is low efficiency.Siderophore-producing bacteria(SPB),are widely present in the rhizospheric soils and have strong abilities to mobilize undissolved Cd and P,which provide research ideas for microbe-enhanced phytoremediation.In the present study,two strains of SPB were isolated from Cd contaminated soils,and the siderophore-producing capacity and insoluble Cd-P mobilization mechanism were analyzed.Pot experiment was further conducted to investigate the influencing mechanism of exogenous carbon signal molecules on the remediation of Cd-polluted soils by SPB-Solanum nigrum L.(S.nigrum)combination.The main result of study was as follows:(1)Two strains of SPB T1 and Y16 were isolated from the Cd-polluted soils by CAS(Chrome Azurol S)medium,which were confirmed respectively as Pseudomonas aeruginosa and Enterobacter cloacae by combining 16 S r DNA sequence analysis and physio-biochemical identification.The growth curves showed that T1 and Y16 entered into stable period at 14 th h and 13 th h,respectively,and both them exhibited siderophore-producing capacity and strong Cd-tolerance ability.(2)The abilities of the two strains to mobilize insoluble Cd and P were evaluated by a shake-flask culture.The results showed that the soluble Cd content in the iron-rich medium inoculated with T1 and Y16 was 4.6 and 5.5 folds that of not-inoculated treatment(CK),and the soluble P content was 4.3 and 6.5 folds that of CK,respectively,which may be the reason for the medium acidification caused by the organic acid release by SPB(p<0.05).Noteworthily,the medium p H was not significantly decreased in the iron-deficient culture,yet soluble Cd and P contents in the medium were increased(p<0.05).Therefore,the insoluble Cd-P mobilization was mainly attributed to chelate effect of siderophore secreted by the two strains.(3)In the pot experiment,the T1 and Y16 inoculation significantly accelerated the soil Cd uptake by Solanum nigrum L,which increased by 62% and 73% in contrast to not-inoculated treatment,respectively.Additionally,the results of Tessier sequential extraction methods showed that the T1 and Y16 inoculation significantly promoted the transformation of rhizospheric soil Cd from insoluble to exchangeable species.The chemotactic behaviour further confirmed that D-gluconate has chemotaxis effect on Y16 bacteria,and Biology Gen III microplate show that D-gluconate could be used as a carbon source for Y16 bacteria.In the Y16 inoculation treatment,D-gluconate addition promoted biomass and Cd accumulation of Solanum nigrum L,which increased by 14% and 21% in contrast to the treatment without D-gluconate,respectively.Simultaneously,structure and function analysis of microbial communities in the rhizospheric soil indicates that the D-gluconate could increase the abundance of E.cloacae,bradyrhizobium and mesorhizobium in soil,and promote the expression of cell mobility genes and metabolic functional genes.