Fungal Transformation and Oxalate-Mediated Mineralization of Heavy Metal Oxides by Aspergillus aculeatus

TL;DR

A fungus called Aspergillus aculeatus can solubilize and precipitate certain heavy metal oxides, potentially aiding in bioremediation.

Contribution

The study identifies a new fungal strain capable of oxalate-mediated mineralization of specific heavy metal oxides.

Findings

Aspergillus aculeatus PTW4 solubilized ZnO, Pb3O4, Cu2O, and MoO3 with halo formation and acidification.

The fungus precipitated zinc, lead, and copper oxalates, but not molybdenum oxalate.

Oxalate-associated biomineralization may reduce metal mobility in soil environments.

Abstract

Fungal transformation is increasingly recognized as an important process influencing metal solubilization and immobilization in soil environments. In this study, a fungal strain (PTW4) isolated from mining-contaminated soil was molecularly identified as Aspergillus aculeatus. The strain was evaluated for its ability to solubilize and transform several heavy metal oxides, including ZnO, Pb3O4, Cu2O, and MoO3. PTW4 produced consistent halo formation across all tested oxides, accompanied by progressive acidification of the culture medium, suggesting organic acid-mediated solubilization. Characterization of extracellular precipitates by SEM-EDS and XRD indicated mineral phases consistent with oxalate-associated biominerals, including zinc oxalate dihydrate (ZnC2O4·2H2O), lead oxalate (PbC2O4), and copper oxalate hydrate (CuC2O4·xH2O). These minerals represent low-solubility phases that may reduce metal mobility in the surrounding environment. In contrast, molybdenum did not precipitate under the experimental conditions, suggesting metal-specific constraints in fungal biomineralization processes. Although organic acid production was not directly quantified, identification of oxalate mineral phases supports an oxalate-associated mineralization mechanism. Overall, the results provide evidence for heavy metal solubilization and selective extracellular precipitation consistent with oxalate biomineral formation by A. aculeatus PTW4, highlighting its potential relevance to fungal-mediated bioremediation and selective bioleaching processes.

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