Heavy metal tolerance and oxalate crystallization by wood-rotting fungi

Abstract

This study was conducted to select wood-rot fungal strains which showed high metal tolerance and were able to remove heavy metals by precipitation as metal oxalate crystals. The result showed that among sixty fungal isolates, only five isolates which included three brown-rot fungal isolate MR40, KYO and WR4, and two white-rot fungal isolate WR4 and WR5, were able to tolerate and produce the metal crystals in the agar medium amended with various concentrations of heavy metals. Especially brown-rot fungal isolate KYO was able to grow at high concentration of heavy metals and produced numerous metal crystals. Metal crystals formed by wood-rotting fungi consist mainly of oxalic acid which indicated that oxalic acid as an important metabolite elaborated in the response of wood-rotting fungi to toxic metal stress. Oxalate production could result in the transformation of zinc sulfate into zinc oxalate hydrate (C2O4Zn.2H2O), copper sulfate into copper oxalate hydrate (moolooite)(C2CuO4.xH2O), cadmium sulfate into cadmium oxalate hydrate (C2CdO4.2.5H2O and C2CdO4.3H2O), and lead nitrate into lead oxalate (PbC2O4), which were resistant to further solubilization and less inhibitory effect on the fungal growth. An experiment was conducted to determine the effect of heavy metals on fungal growth and heavy metal accumulation by wood-rotting fungi. The result showed that the presences of the metal compounds did not stimulate the pH reduction and/or acid production. Moreover, increasing concentrations of heavy metal compounds in the culture medium caused a significant decrease in the radial growth of the most fungal isolates, whereas the fungal biomasses were significantly increased. The analysis of metal content in the fungal mycelia cultivated on media containing heavy metals showed that the accumulated heavy metals significantly increased when the concentrations of heavy metals increased. These results provide the evidence that these wood-rotting fungi could tolerate high concentrations of heavy metals and able to remove heavy metals from the metal-amended media by precipitation as metal oxalate crystals. Moreover, these fungi were capable to accumulate heavy metals within their biomass during immobilization of the soluble metal compounds therefore fungal biomasses should be applied as biosorbent for heavy metals which may provide potential for metal removal and recovery of valuable elements.