Solubilisation of Phosphate Rock by Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans Bacteria in Pure Cultures and in Association with Bacillus megaterium and Pseudomonas fluorescens

Abstract Phosphate rock (PR) is a natural source of phosphorus (P) that can be used as an alternative to chemical fertilizers. Traditional methods of applying phosphorus fertilizer cannot address phosphorus deficiency problems. Once that ore material is a nonrenewable natural resource, it is imperative to find alternative, environmentally compatible, and economically viable strategies to address phosphorus scarcity. Phosphorus-solubilizing bacteria can convert insoluble phosphorus into usable P that can be directly absorbed by plants. The organic acids produced by microorganisms have been proposed as a biotechnological tool for releasing phosphorus from sparingly soluble ores. This study examined new insights into the solubilisation of P from a PR concentrate using Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans individually or in association with Bacillus megaterium and Pseudomonas fluorescens during liquid medium fermentation. The (A) ferrooxidans , (B) megaterium , P. fluorescens , and A. thiooxidans strains were used to solubilize phosphorus from concentrate ore (fluorapatite) containing 15.6% of P. The influence of the initial pH of the culture medium (pH 2.0 and pH 4.0), the evaluations of the rock concentration (2, 3.5, and 5 g/L) with ferrous sulphate addition (16.7 g and 33.3 g/L), and the fermentation with nutrient broth medium with ferrous sulphate addition of 33.3 g/L were studied. Additionally, the liquid fermentation with bacterial associations among the studied strains was evaluated. To study the solubilisation results, the quantification of soluble phosphorus, biomass concentration, and organic acids was determined using a specific methodology and pH measurement. At an initial pH of 2.0, A. thiooxidans presented solubilisation rates 34.5% higher than those obtained at pH 4.0. The A. ferrooxidans exhibited similar behaviour to the studied pH conditions, yielding the best results for assays with pH 2.0 initially controlled (51.28% solubilisation). A. ferrooxidans exhibited the best efficiency when cultured alone. A higher solubilisation efficiency was achieved at 2 g/L of PR (312 mg/L of P). The efficiency of the tests with A. thiooxidans in a nutrient broth medium with added 33.33 g/L of ferrous sulphate was 54.2% higher than in the 9 K culture medium. In the tests using a combination of A. ferrooxidans and P. fluorescens , the solubilisation of phosphorus was 31.44%. In the association of (A) thiooxidans with (B) megaterium , the P solubilisation efficiency rose to 57.75%, while the combination of A. thiooxidans and P. fluorescens yielded a P solubilisation of 51.33%. Combining (A) ferrooxidans and (B) megaterium , the solubilisation of phosphorus was 48.07%. This study demonstrated that the highest solubilisation of 88.22% was obtained with a fermentation medium pH of 2.72, using the strain A. ferrooxidans with 2.0 g/L of ore and ferrous sulphate (33.3 g/L). This study demonstrates an innovative strategy for phosphorus production relevant to the agricultural industry.