Structural Fe(Ⅱ) Oxidation in Biotite by an Ectomycorrhizal Fungi Drives Mechanical Forcing

摘要

Microorganisms are essential agents of Earth's soil weathering engine who help transform primary rock-forming minerals into soils. Mycorrhizal fungi, with their vast filamentous networks in symbiosis with the roots of most plants can alter a large number of minerals via local acidification, targeted excretion of ligands, submicron-scale biomechanical forcing, and mobilization of Mg, Fe, Al, and K at the hypha-biotite interface. Here, we present experimental evidence that Paxillus involutus-a basidiomycete fungus-in ectomycorrhizal symbiosis with Scots pine (Pinus sylvestris), is able to oxidize a substantial amount of structural Fe(Ⅱ) in biotite. Iron redox chemistry, quantified by X-ray absorption near edge spectra on 13 fungi-biotite sections along three distinct hypha colonizing the [001] basal plane of biotite, revealed variable but extensive Fe(Ⅱ) oxidation up to ～2 μm in depth and a Fe(Ⅲ)/Fe_(total) ratio of up to ～0.8. The growth of Fe(Ⅲ) hydroxide implies a volumetric change and a strain within the biotite lattice potentially large enough to induce microcrack formation, which are abundant below the hypha-biotite interface. This Fe(Ⅱ) oxidation also leads to the formation of a large pool of Fe(Ⅲ) (i.e., structural Fe(Ⅲ) and Fe(Ⅲ) oxyhydroxides) within biotite that could participate in the Fe redox cycling in soils.