4. CONCLUSION The evaluated fungal strains exhibited pronounced tolerance to both alkaline and saline stress, with A. niger and P. oxalicum demonstrating growth at pH 10 and 11, respectively, and showing moderate tolerance at 10% w/v NaCl. Although P. oxalicum remained viable at higher alkaline conditions before fungicidal effects occurred, it consistently displayed lower neutralization capacity than A. niger under comparable alkaline and saline environments. In both species, alkaline stress enhanced organic acid production, thereby increasing their capacity for pH neutralization, whereas salinity stress significantly reduced this response. Tolerance to BR was influenced by the combined effects of pH, salinity, mineralogical properties, and the chemical composition of the residue. While both strains withstood relatively high residue concentrations, further improvements in tolerance are essential to enable processing of larger waste volumes and to optimize the simultaneous recovery of critical metals through bioleaching. Future work should focus on assessing the bioleaching capacities of these strains under diverse operational configurations and identifying optimal conditions for maximizing metal solubilization and residue remediation.
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