Abstract
P-solubilizing microorganisms are a promising alternative for a sustainable use of P against a backdrop of depletion of high-grade rock phosphates (RPs). Nevertheless, toxic elements present in RPs, such as fluorine, can negatively affect microbial solubilization. Thus, this study aimed at selecting Aspergillus niger mutants efficient at P solubilization in the presence of fluoride (F−). The mutants were obtained by exposition of conidia to UV light followed by screening in a medium supplemented with Ca3(PO4)2 and F−. The mutant FS1-555 showed the highest solubilization in the presence of F−, releasing approximately 70% of the P contained in Ca3(PO4)2, a value 1.7 times higher than that obtained for the wild type (WT). The mutant FS1-331 showed improved ability of solubilizing fluorapatites, increasing the solubilization of Araxá, Catalão, and Patos RPs by 1.7, 1.6, and 2.5 times that of the WT, respectively. These mutants also grew better in the presence of F−, indicating that mutagenesis allowed the acquisition of F− tolerance. Higher production of oxalic acid by FS1-331 correlated with its improved capacity for RP solubilization. This mutant represents a significant improvement and possess a high potential for application in solubilization systems with fluoride-rich phosphate sources.
Highlights
Phosphate fertilizers are used intensively in agriculture for improving crop production
Mutagenesis of A. niger Twenty-nine mutants showing higher and lower solubilization halos than the wild type (WT) were obtained in National Botanical Research Institute’s phosphate growth medium (NBRIP)-F medium after exposure to UV light
Three strains were selected for further studies according to the following criteria: FS1-555 showed the highest increase in P solubilization from Ca3(PO4)2 with F2 (67%); FS1-331 showed the highest increase in P solubilization from Araxa rock phosphates (RPs) (64%) and FS1375 showed decreased solubilized P for both sources
Summary
Phosphate fertilizers are used intensively in agriculture for improving crop production. The ability of PSM to solubilize P is mainly associated with the release of metabolites with chelating or complexing properties, such as organic acids [3,6]. Aspergillus niger is a PSM with high P solubilization activity due to its capacity of medium acidification and production of organic acids with high metal complexation activity [3]. Aspergillus niger has been shown to solubilize either synthetic or natural apatites, i.e. RPs [3,8,9,10]. The chemical characteristics of RPs can interfere with the production of organic acids by PSM [4,11,12] and elements released during the solubilization may be toxic to microbial metabolism [4,13]. Given the ubiquitous distribution of F2 in RPs, most microbial
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