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

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 similar to 2 mu m in depth and a Fe(III)/Fe-total ratio of up to similar 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.