Isolation, characterization, and evaluation of a high-siderophore-yielding bacterium from heavy metal–contaminated soil

Heavy metal–resistant siderophore-producing bacteria (SPB) with plant growth–promoting traits can assist in phytoremediation of heavy metal–contaminated soil. We isolated siderophore-producing bacteria from Pb and Zn mine soil in Shangyu, Zhejiang, China. The isolate with the highest siderophore production, strain SX9, was identified as Burkholderia sp. Burkholderia sp. SX9 produced catecholate-type siderophore, with the highest production at a pH range of 6.0 to 8.0, a temperature range of 20 to 30 °C and NaCl concentration below 2%. Siderophore production was highest without Fe³⁺ and became gradually lower with increasing Fe³⁺ concentration. Minimal inhibitory concentrations (MIC) of Pb²⁺, Zn²⁺, Cu²⁺, and Cd²⁺ were 4000, 22000, 5000, and 2000 μmol L⁻¹, respectively. The strain had a strong metal solubilization ability: the contents of Cu²⁺, Zn²⁺, and Cd²⁺ in the supernatant were 47.4%, 133.0%, and 35.4% higher, respectively, in strain SX9–inoculated cultures than in the not inoculated controls. The siderophore produced by strain SX9 could combine with Fe³⁺, Zn²⁺, and Cd²⁺ with good effectiveness. The plant growth–promoting traits of the strain included indole acetic acid (IAA) production, 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity, and phosphate solubilization capability. Compared to the uninoculated growth medium and SX9 culture supernatant, the germination rate of Lolium perenne seeds was higher when inoculated with strain SX9 culture. In the experiment of seed germination, adding bacterial culture or supernatant could alleviate the toxicity of heavy metals to L. perenne seed germination. Under Cu²⁺ and Zn²⁺ stress, strain SX9 promoted the germination rate. Taken together, Burkholderia sp. SX9 had properties beneficial in the microbial enhancement of phytoremediation of soil contaminated with heavy metals.