Oxalate production by wood-rotting fungi growing in toxic metal-amended medium

In this report, we have identified oxalic acid as an important metabolite elaborated in the response of wood-rotting fungi to toxic metal stress. The formation of oxalate crystals by white rot fungi (Bjerkandera fumosa, Phlebia radiata and Trametes versicolor) and the brown rot fungus Fomitopsis pinicola, grown on media containing high levels of toxic metal ions has been visualized using scanning electron microscopy (SEM) with energy-dispersive X-ray micro-analysis (EDXA) and HPLC. There were no significant differences between the growth of controls (metal-free) and on the 0.5% CaCO3, Co3(PO4)2 or Zn3(PO4)2-amended plates. ZnO inhibited the growth of all strains. Crystals were not detected in Zn3(PO4)2-amended plates. The four examined strains displayed the formation of crystals on ZnO, Co3(PO4)2 and CaCO3-amended plates.