In Vitro Siderophore Production and Zinc Solubilisation by Bacterial Root Isolates from Rice

Iron (Fe) and zinc (Zn) are essential micronutrients for plant metabolism; however, their bioavailability in tropical soils is often limited by low solubility and complex mineral interactions. Root-associated bacteria may enhance micronutrient availability through siderophore production and the solubilisation of insoluble mineral forms. This study aimed to functionally characterise three bacterial isolates from rice roots—Bacillus siamensis TUR07-02b, Priestia aryabhattai SMNCH17-07, and Priestia megaterium SMBH14-02—under controlled in vitro conditions. Siderophore activity was evaluated qualitatively using Chrome Azurol S (CAS) agar, where percentages represent halo-based indices relative to colony diameter, and quantitatively using the CAS–shuttle assay, expressed as percent siderophore units relative to an uninoculated reference. Zinc solubilisation was assessed in solid media as halo-based indices and in liquid media as Zn-equivalent signals (mg L−1) obtained by spectrophotometry. All strains produced siderophores, with P. aryabhattai showing the highest qualitative index (167%), while P. aryabhattai and B. siamensis showed statistically similar activity in liquid medium (~23%). Zinc solubilisation was substrate-dependent: B. siamensis showed the broadest solubilisation spectrum in solid media, whereas P. aryabhattai achieved the highest Zn-equivalent signals for ZnCO3 and Zn3(PO4)2 after 20 days. These results demonstrate strain-specific functional differences and represent a preliminary screening method based on relative in vitro estimations.