Dose-Dependent Antagonistic Effects of Montmorillonite on Cadmium Removal Efficiency and Metabolic Defense of Aspergillus niger.

Cadmium (Cd) pollution threatens global ecosystems and human health owing to its high toxicity, long-term persistence, and dual irreversibility of environmental geochemical behavior and biological toxicological effects. The interaction between phosphate-solubilizing fungi (PSF) and clay minerals governs the biogeochemical transformation, chemical speciation, and migration fate of soil Cd. However, the dose-dependent mechanisms underlying Cd2+ removal by Aspergillus niger (A. niger) and montmorillonite remain unclear. In this study, three Cd2+ stress levels were simulated, and four A. niger-montmorillonite composite systems were established. Key physicochemical and metabolic indicators, including pH, available phosphorus (AP) concentration, oxalic acid concentration, acid phosphatase (ACP) activity, and Cd2+ removal rate, were determined. The results showed that the single A. niger treatment achieved the optimal Cd removal performance among all composite treatment groups. Severe Cd2+ stress induced abundant oxalic acid and ACP secretion, which simultaneously mobilized phosphorus and facilitated Cd removal via phosphate precipitation and oxalate complexation. Low-to-medium montmorillonite dosage temporarily alleviated Cd2+ toxicity, whereas high montmorillonite dosage restricted fungal metabolic exudates and, together with acid-mediated mineral-Cd interactions, triggered antagonistic rather than synergistic Cd removal. This study clarifies how montmorillonite dosage reshapes the metabolic defense of A. niger, providing theoretical references for optimizing microbe-montmorillonite composite remediation formulations.