Bioweathering of Stable Phosphate Mineral by Filamentous Fungi under Different CO₂ Concentrations

The weathering of phosphate minerals plays a pivotal role in the phosphorus (P) cycle. This study investigated the bioweathering of pyromorphite by filamentous fungi (Aspergillus niger) under different CO₂ concentrations (0.04%, 0.4%, and 4%). SEM observations suggested differences in mycelial branching and sporulation patterns under 0.4% CO₂ concentration, although fungal abundance did not differ significantly from that under 0.04% CO₂ concentration. 0.4% CO₂ concentration may accelerate fungal weathering. The release of water-soluble P from pyromorphite was also elevated from 14 to 56 mg kg^-1 substrates when the CO₂ concentration increased from 0.04 to 0.4%. In contrast, 4% CO₂ concentration limited fungal growth. In addition, the 0.4% CO₂ concentration enhanced mineral dissolution, while at 4% CO₂ concentration, precipitation would partially cover the reactive mineral surfaces, thereby hindering further weathering. Focused ion beam transmission electron microscopy (FIB-TEM) results demonstrated that the depletion depth (0.96 μm) of P in the cross section was significantly higher than that in the longitudinal section (0.05 μm). Cross section are hence more susceptible to weathering, which might be related to orientation-dependent crystallographic and microstructural characteristics. These findings elucidate the synergistic mechanism between microbial activity and CO₂ concentration in mineral weathering, providing insights into the biogeochemical cycling of phosphates.