Zinc Ions Affect Siderophore Production by Fungi Isolated from the Panax ginseng Rhizosphere

Although siderophore compounds are mainly biosynthesized as a response to iron deficiency in the environment, they also bind with other metals. A few studies have been conducted on the impact of heavy metals on the siderophore-mediated iron uptake by microbiome. Here, we investigated siderophore production by a variety of rhizosphere fungi under different concentrations of Zn²⁺ ion. These strains were specifically isolated from the rhizosphere of Panax ginseng (Korean ginseng). The siderophore production of isolated fungi was investigated with chrome azurol S (CAS) assay liquid media amended with different concentrations of Zn²⁺ (50 to 250 μg/ml). The percentage of siderophore units was quantified using the ultra-violet (UV) irradiation method. The results indicated that high concentrations of Zn²⁺ ion increase the production of siderophore in iron-limited cultures. Maximum siderophore production by the fungal strains was detected at Zn²⁺ ion concentration of 150 μg/ml except for Mortierella sp., which had the highest siderophore production at 200 μg/ml. One potent siderophore-producing strain (Penicillium sp. JJHO) was strongly influenced by the presence of Zn²⁺ ions and showed high identity to P. commune (100% using 18S-rRNA sequencing). The purified siderophores of the Penicillium sp. JJHO strain were chemically identified using UV, Fourier-transform infrared spectroscopy (FTIR), and matrix-assisted laser desorption/ionization time-of-flight mass spectrometer (MALDI-TOF-MS) spectra.