Protection of Metal Artifacts with the Formation of Metal–Oxalates Complexes by Beauveria bassiana

Abstract

Several fungi present high tolerance to toxic metals and some are able to transform metals into metal–oxalate complexes. In this study, the ability of Beauveria bassiana to produce copper oxalates was evaluated. Growth performance was tested on various copper-containing media. B. bassiana proved highly resistant to copper, tolerating concentrations of up to 20 g L−1, and precipitating copper oxalates on all media tested. Chromatographic analyses showed that this species produced oxalic acid as sole metal chelator. The production of metal–oxalates can be used in the restoration and conservation of archeological and modern metal artifacts. The production of copper oxalates was confirmed directly using metallic pieces (both archeological and modern). The conversion of corrosion products into copper oxalates was demonstrated as well. In order to assess whether the capability of B. bassiana to produce metal–oxalates could be applied to other metals, iron and silver were tested as well. Iron appears to be directly sequestered in the wall of the fungal hyphae forming oxalates. However, the formation of a homogeneous layer on the object is not yet optimal. On silver, a co-precipitation of copper and silver oxalates occurred. As this greenish patina would not be acceptable on silver objects, silver reduction was explored as a tarnishing remediation. First experiments showed the transformation of silver nitrate into nanoparticles of elemental silver by an unknown extracellular mechanism. The production of copper oxalates is immediately applicable for the conservation of copper-based artifacts. For iron and silver this is not yet the case. However, the vast ability of B. bassiana to transform toxic metals using different immobilization mechanisms seems to offer considerable possibilities for industrial applications, such as the bioremediation of contaminated soils or the green synthesis of chemicals.

Innovative biological approaches for metal conservation

“…All patinas were characterized using a protocol validated in previous studies . The same analytical methods (colorimetry, electrochemical impedance spectroscopy and scanning electron microscopy) were used before and after treatment as well as during ageing.…”

The dual role of microbes in corrosion

“…However, for metal corrosion microbially induced protection requires better understanding of the processes involved before it can be applied in full practice. One application is known to be successful: Beauveria bassiana produces copper-oxalate complexes, thereby forming a protective coating on copper objects (Joseph et al, 2012). There are few examples, such as the biofilms of Pseudomonas spp.…”

Beauveria bassiana: Biocontrol Beyond Lepidopteran Pests

“…B. bassiana is reported to be endowed with the ability of heavy-metal chelation using oxalates as organic chelators. The beauverial oxalate reportedly chelates a variety of metals, involving iron (iron oxalates were directly sequestered on the fungal hyphae), copper (formation of characteristic "Liesegang rings" due to simultaneous diffusion and precipitation of copper oxalates), and silver (coprecipitation of copper and silver oxalates) (Joseph et al 2012). …”