Biomineralization mechanism of gold by zygomycete fungi rhizopous oryzae

In recent years, there has been significant progress in the biological synthesis of nanomaterials. However, the molecular mechanism of gold biomineralization in microorganisms of industrial relevance remains largely unexplored. Here we describe the biosynthesis mechanism of gold nanoparticles (AuNPs) in the fungus Rhizopus oryzae. Reduction of AuCl ₄ ^- [Au(III)] to nanoparticulate Au ⁰ (AuNPs) occurs in both the cell wall and cytoplasmic region of R. oryzae. The average size of the as-synthesized AuNPs is ∼15 nm. The biomineralization occurs through adsorption, initial reduction to Au(I), followed by complexation [Au(I) complexes], and final reduction to Au ⁰. Subtoxic concentrations (up to 130 μM) of AuCl ₄ ^- in the growth medium increase growth of R. oryzae and induce two stress response proteins while simultaneously down-regulating two other proteins. The induction increases mycelial growth, protein yield, and AuNP biosynthesis. At higher Au(III) concentrations (>130 μM), both mycelial and protein yield decrease and damages to the cellular ultrastructure are observed, likely due to the toxic effect of Au(III). Protein profile analysis also confirms the gold toxicity on R. oryzae at high concentrations. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis shows that two proteins of 45 and 42 kDa participate in gold reduction, while an 80 kDa protein serves as a capping agent in AuNP biosynthesis.