Calcium oxalate biomineralization by Piloderma fallax in response to various levels of calcium and phosphorus

Piloderma fallax is an ectomycorrhizal fungus commonly associated with several conifer and hardwood species. We examined the formation of calcium oxalate crystals by P. fallax in response to calcium (0.0, 0.1, 0.5, 1, and 5 mM) and phosphorus (0.1 and 6 mM) additions in modified Melin-Norkrans agar medium. Both calcium and phosphorus supplementation significantly affected the amount of calcium oxalate formed. More calcium oxalate was formed at high P levels. Concentrations of soluble oxalate in the fungus and medium were higher at low P levels. There was a strong positive linear relationship between Ca level and calcium oxalate but only under conditions of phosphorus limitation. Calcium oxalate crystals were identified as the monohydrate form (calcium oxalate monohydrate [COM] whewellite) by X-ray diffraction analysis. Prismatic, styloid, and raphide forms of the crystals, characteristic COM, were observed on the surface of fungal hyphae by scanning electron microscopy. P. fallax may be capable of dissolving hyphal calcium oxalate under conditions of limited Ca. The biomineralization of calcium oxalate by fungi may be an important step in the translocation and cycling of Ca and P in soil.