Characterization of zinc solubilization potential of arsenic tolerant Burkholderia spp. isolated from rice rhizospheric soil.

In this study, experiments were conducted to isolate, characterize, and evaluate rice rhizosphere bacteria for their arsenic (As) tolerance ability and zinc (Zn) solubilization potential in culture media and soil. Among 20 bacterial isolates recovered, six were found to solubilize inorganic Zn salt(s) efficiently under in vitro culture conditions. 16S rRNA gene sequence-based phylogenetic analysis indicated the affiliation of efficient Zn solubilizing bacteria (ZSB) to Burkholderia vietnamiensis and Burkholderia seminalis. Zinc solubilizing efficiency (ZSE) of the bacteria varied with the concentrations and types of Zn salts used in the experiments. Increasing trend in ZSE of the bacteria was noticed when the percentage of ZnO increased from 0.1 to 0.5 but the same decreased at 1.0%. Increased Zn solubilization was noticed when bacteria were incubated with lower concentration of Zn(PO) and ZnCO. In general, Zn solubilization increased with increasing incubation time in lower volume medium, while some isolates failed to solubilize one or more tested Zn salts. However, enriched concentrated cells of the ZSB in glucose amended medium with 0.5% ZnO showed an increasing trend of Zn solubilization with time and were able to solubilize more than 300 mg/L Zn. This increased rate of Zn release by the ZSB was attributed to marked decline in pH that might be due to the enhanced gluconic acid production from glucose. As evident from the decreased ZSE of the bacteria in the presence of As(V) in particular, it seems arsenic imparts a negative effect on Zn solubilization. The ZSB were also able to increase the rate of Zn release in soil. A microcosm-based soil incubation study amending the enriched bacteria and 0.5% ZnO in soil showed an elevated level of both water-soluble and available Zn compared to un-inoculated control. During Zn solubilization in microcosms, viable cells in terms of colony-forming unit (CFU) declined by the same order of magnitude both in the presence and absence of ZnO that might be due to the nutrients limiting condition aroused during the incubation period rather than Zn toxicity. The bacteria in this study also exhibited plant growth promoting traits, such as growth in nitrogen-free medium, production of indole acetic acid (IAA), and solubilization of potassium and phosphate. Our findings suggested that Burkholderia spp. could be the potential candidates for enhancing Zn dissolution in the soil that might reduce the rate of inorganic Zn fertilization in agricultural soil.