Did You Hear about the Gold-munching Fungus?

Gold present in the critical zone originates from dissolved and dispersed primary ores and undergoes a variety of physicochemical interactions¹. Under Earth surface conditions, gold exhibits enigmatic patterns of transformation and translocation^2,3. Various forms of gold⁴, including secondary aggregates, crystalline grains, colloidal nanoparticles, and Au(I/III) complexes are ubiquitous in diverse environments, such as hypersaline waters⁵, iron-bearing nodules⁶, and carbonate-rich accumulations⁷.

Capillary migration, gaseous transport, and bioturbation are the principal mechanisms hypothetically affect gold speciation, complexation, and mobility in the critical zone¹. Prokaryotic gold biogeochemical cycling has been proposed and suggests that through bioweathering and oxidative complexation, bacteria and archaea can liberate gold from minerals to form dissolved gold species⁸. Detoxification-oriented biomineralization immobilizes and precipitates dissolved gold species to form secondary deposits^8,9. Gold transformation subsequently drives changes in the composition of the associated bacterial community¹⁰. Functional transcriptomic and proteomic analyses of Cupriavidus metallidurans, a dominant bacterial species in biofilms on natural gold nuggets, have increased understanding of the role of prokaryotic microorganisms in gold biomineralization^11,12.

Fungi are known to be critical for metals (e.g., aluminum, iron, manganese, calcium, and magnesium) cycling under aerobic Earth surface conditions, influencing essential pedogenic processes, such as rock weathering, soil organic matter degradation, and element distribution¹³. However, little is known about the biogeochemical interaction between fungi and gold. To investigate geomycological contribution to gold cycling in geological records is challenging; partly because it is difficult to empirically verify the fossilized fungi involved in the geological process and distinguish their past activities from those of bacteria or archaea¹⁴. In addition, fungi are usually better preserved through mineralization than prokaryotes, which further makes it difficult to differentiate between the importance of their respective activities¹⁵. Recently, fungi have been found to mobilize gold from electronic waste¹⁶, suggesting they can interact with metallic gold, possibly through biological redox transformations.

Gold is extremely resistant to tarnishing (chemical oxidation). Both, an oxidant and a ligand with high affinity for gold ions are required to solubilize gold¹⁷. Fungi are ideal to simultaneously produce oxidants, e.g., reactive oxygen species¹⁸, and ligands, e.g., organic acids¹⁹, thiosulfate²⁰, and cyanide²¹. These coordinating molecules also serve as exogenous electron donors to active fungal extracellular oxidation-reduction systems²², thereby potentially facilitating colloidal gold redox transformation.