Ectomycorrhizal Fungi in the Weathering Environment.

Characterization and quantification of biological weathering processes have been of great interest in several scientific fields and in the last couple of decades, cross-disciplinary approaches have been developed. It has been shown that bacteria and fungi increase chemical weathering rates in various environmental settings. Microbe- and fungal-mineral interactions play a significant role in regulating nutrient uptake of plants, influencing soil formation and water chemistry. However, it is still debated how much fungal weathering actually contributes to weathering budgets of watersheds or globally - and on what time scales.

This presentation summarizes several collaborative studies focusing on biotite weathering by ectomycorrhizal fungi over a range of scales, specifically investigating the fungal hyphae-mineral interface. Scanning and transmission electron microscopy and atomic force microscopy were applied to characterize biotite surface changes, microbial and fungal attachment structures and biofilm coverage. 

The overall results show that ectomycorrhizal (symbiotic) fungi can promote chemical dissolution and help plants take up potassium and magnesium under limiting conditions of these base cation nutrients. Microscopic chemical analyses of biotite cross sections indicate a slight depletion of potassium and magnesium under the fungal hyphae but these changes are not significantly different from the controls. Fungi can also apply physical forces to the chemically weakened biotite surface to produce channels and branching features that are the same diameter as the fungal hyphae. On the other hand, laboratory studies show that abiotic processes can also contribute to similarly sized and shaped channels on biotite surfaces because biotite is easily scratched by sharp, hard objects, such as sand grains. However, branching cannot occur via abiotic processes.

Base cation limitations in forest ecosystems did not promote dense fungal colonization of minerals in buried mesh bags, which contradicts the laboratory results. Therefore further studies are needed to elucidate the contribution of fungal weathering to ecosystem and larger scale processes.