Isolation and characterization of native phosphate-solubilizing microorganisms from Eppawala rock phosphate and associated soil

Abstract

Phosphorus is a crucial macronutrient in plant growth and development. The lower bioavailability of phosphorus in agricultural lands causes phosphorus deficiency in plants leading to plant growth retardation. As a remedy, rock phosphate is incorporated into phosphorus fertilizers. However, the direct application of rock phosphate as a phosphorus fertilizer is not recommended due to its lower solubility. Hence, the focus of the present study was to isolate and characterize the phosphatesolubilizing microorganisms as a cost-effective greener solution to solubilize insoluble rock phosphate. In the present study, phosphate-solubilizing microorganisms were isolated from rock phosphate and associated soil from the Eppawala rock phosphate deposit in Anuradhapura, Sri Lanka. The Pikovskaya's medium was used for the isolation of microorganisms and their ability to solubilize Eppawla rock phosphate was evaluated using phosphate solubilizing index. The quantification of available phosphate was carried out using the molybdenum blue spectrophotometric method with three replicates and the most efficient isolates were subjected to 16S rRNA and ITS molecular identifications. Statistical analysis was conducted using two-way ANOVA in Minitab, followed by Tukey's pairwise comparison test to identify significant differences between treatments at a 0.05 significance level. Based on the results, fifteen bacterial isolates and one fungal isolate were identified as phosphate-solubilizing microorganisms, with phosphate-solubilizing indexes ranging from 2.12 ± 0.03 to 3.00 ± 0.02. When available phosphorus levels were quantified, four isolates with the highest significant (p < 0.05) available phosphorus concentrations; 36.15 ± 0.64 mg L⁻¹, 33.39 ± 0.53 mg L⁻¹, 29.27 ± 0.65 mg L⁻¹ and 28.87 ± 0.27 mg L⁻¹, were identified. They were identified as Pseudomonas monteilii, Aspergillus niger, Paraburkholderia bannensis, and Burkholderia vietnamiensis respectively. Their phosphate solubilizing indexes were recorded as 2.50 ± 0.05, 2.12 ± 0.03, 3.00 ± 0.02, and 2.33 ± 0.03, respectively. Additionally, a significant reduction (p < 0.05) in the pH in media compared to the control suggests the production of organic acids, a key mechanism used by microorganisms for solubilizing insoluble phosphate. This study confirms the potential of native phosphate-solubilizing microorganisms to solubilize Eppawala rock phosphate, demonstrating its effectiveness as a cost-efficient and environmentally friendly phosphorus source. The results provide key insights into the development of biofertilizers, contributing to sustainable agricultural practices.