Role of cadmium-resistant siderophore-producing rhizobacteria in increasing cadmium bioavailability in contaminated soils

The aims of this research are to isolate rhizobacteria from plant roots and rhizosphere soils, screen them for siderophore production and resistance to cadmium toxicity. The role of bacteria in increasing cadmium solubilization or mobilization in cadmium-contaminated soil was investigated. The results showed that 61 isolates were isolated from plant roots and 87 isolates were isolated from rhizosphere soil. A total of 148 isolates were tested for their ability to produce siderophores. It was found that 77 isolates(52%) produced siderophores. Only 3 isolates, designated as SR5, SR6 and TS32, highly produced siderophores and resisted high concentrations of cadmium. The M IC values of cadmium for SR5, SR6 and TS32 were 600, 600 and 400 mg L^-1, respectively. SR5, SR6 and TS32 were identified by 16S rDNA gene sequencing to be Burkholderia, Burkholderia and Pseudomonas, respectively. Burkholderia sp. SR5 produced the highest level of siderophores by 65.24 μM of deferoxamine mesylate equivalent when cultured in M 9 minimal medium at the late-stationary phase of growth. All cadmium-resistant siderophore-producing rhizobacteria had shown positive effects in increasing the concentrations of the bioavailable form of cadmium in contaminated soil. These rhizobacteria could be applied for enhancing the phytoextraction performance of cadmium in polluted soil.