Abortiporus biennis tolerance to insoluble metal oxides: oxalate secretion, oxalate oxidase activity, and mycelial morphology

The ability of Abortiporus biennis to tolerate and solubilize toxic metal oxides (Cu2O, Al2O3, ZnO, CuFe2O4Zn, CdO, and MnO2) incorporated into agar media was investigated and the growth rate, oxalic acid secretion, and mycelial morphology were monitored. Among the tested metal oxides, formation of clear zones underneath the mycelium growing on Cu2O- and ZnO-amended plates was observed. ZnO, CdO and Cu2O caused the highest rate of fungal growth inhibition. An increased level of oxalic acid concentration was detected as a response of A. biennis to the presence of Cu2O, MnO2, ZnO and CuFe2O4Zn in growth medium. The oxalate oxidase (OXO) was found to be responsible for oxalic acid degradation in A. biennis cultivated in metal-amended media. An increased level of OXO was observed in media amended with Cu2O, ZnO and MnO2. Confocal microscopy used in this study revealed changes in mycelial morphology which appeared as increased hyphal branching, increased septation and increased spore number.