Specificity of low molecular weight organic acids on the release of elements from lizardite during fungal weathering

The solubilization and mobilization of elements from silicate minerals during fungal weathering are predominantly promoted by acidification and complexation reactions. However, stark differences exist in the release rates of different elements driven by fungal-derived low molecular weight organic compounds (LMWOCs) when acidity maintains constant, raising the question of whether the release of individual element during dissolution is ligand-specific. In this work, we investigate this question by characterizing the release of Mg, Si, Fe, and Ni from lizardite [(Mg, Fe, Ni)3Si2O5(OH)4]. Miniaturized batch reactors (microplate wells of 250 µL) were used in dissolution experiments in the presence and absence of the indigenous fungus Talaromyces flavus. Abiotic chemical weathering experiments with metabolically relevant organics and HCl were also carried out to isolate the vital effects of elemental releases. The initial release rates of Mg, Si, Fe, and Ni were obtained from determining the dissolved elemental concentrations. The results show that T. flavus enhances the release of Mg and Si by a factor of ∼2, but that of Fe and Ni by a factor of >10, relative to the rates measured in the control experiment. Additionally, the measurements show an overexcretion of siderophores and oxalic acid, as well as acidification of bulk solution, during bioweathering. Abiotic chemical dissolution of lizardite confirms the release of Fe and Ni proceeds mainly via a ligand-promoted pathway. In addition, the results indicate that siderophores and oxalic acid are responsible for the solubilization of Fe and Ni, respectively. These findings provide direct evidence that the rate and mechanism of elemental release from silicate during bioweathering are ligand-specific, and both synergistic and inhibitory effects may be involved. Given that siderophore- and oxalic acid-producing fungi are highly active in soils, these results may have the potential to advance our understanding of the critical roles of fungi in rhizosphere geochemistry and ecology.