736 Figure 52- P. oxalicum tolerance across the bauxite residue samples: (A) BR-A and BR-B, and (B) BR-C. The red dashed line indicates a tolerance index of 0.5. Asterisks (*) indicate statistically significant differences. Figure 53- pH variation of the culture medium after 18 days of incubation with P. oxalicum under different bauxite residues and concentrations. Values are expressed as ΔpH (1õ3)$5H - 1õ)$)>)5H). Negative ΔpH values represent a reduction in pH (acidification) relative to the initial medium. Means and confidence intervals (95%) are presented. 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. REFERENCES Anahid, S., Yaghmaei, S., & Ghobadinejad, Z. (2011). Heavy metal tolerance of fungi. Scientia Iranica, 18(3), 502-508. doi:10.1016/j.scient.2011.05.015 Babu, A. G., & Reddy, M. S. (2011). Influence of arbuscular mycorrhizal fungi on the growth and nutrient status of bermudagrass grown in alkaline bauxite processing residue. Environmental Pollution, 159(1), 25-29. doi:10.1016/j.envpol.2010.09.032 Bandopadhyay, S. K. R. K. A. (2006). Innovative methodologies for the utilisation of wastes from metallurgical and allied industries. Resources, Conservation and Recycling, 48(4), 301-314. doi:10.1016/j.resconrec.2006.03.003 Borra, C. R., Pontikes, Y., Binnemans, K., & Gerven, T. V. (2015). Leaching of rare earths from bauxite residue (red mud). Minerals Engineering, 76, 20-27. doi:10.1016/j.mineng.2015.01.005 Chauhan, P., Singh, M., Sharma, A., Chadha, P., & Kaur, A. (2025). Plant growth promoting endophytic fungus Aspergillus niger VN2 enhances growth, regulates oxidative stress and protects DNA damage in Vigna radiata under salt stress. BMC Microbiology, 25(1), 585. doi:10.1186/s12866025-04209-8
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