Calcium Oxalate Biomineralization by Piloderma Fallax in Response to Calcium and Phosphorus in Solid Medium

Many fungi from forest litter exhibit calcium oxalate crystals on their hyphae. Among them is the ectomycorrhizal fungus Piloderma fallax which is associated with several varieties of conifer and hardwood species. We examined the formation of calcium oxalate crystals by P. fallax in response to calcium (0, 0.1, 0.5, 1, and 5 mM) and phosphorus (0.1 and 6 mM) in solid medium. Both calcium and phosphorus bioavailability significantly affected the amount of calcium oxalate formed. There was a strong positive linear relationship between Ca level and calcium oxalate but only under limited P availability. Calcium oxalate monohydrate (COM, whewellite) was identified at higher Ca concentrations and limiting P by X-ray diffraction analysis. This was confirmed by the presence of elongated narrow flat crystals, characteristic of COM, on the surface of fungal hyphae observed by scanning electron microscopy. The biomineralization of calcium oxalate by fungi may be an important step in the translocation and cycling of Ca in soil.