A mold as a gold collector

Soil-living Fusarium mushroom enriches gold particles in its mushroom furs

The fungus Fusarium oxysporum can accumulate gold from the soil and stores it as gold particles on its fungal filaments, as this electron micrograph shows. © CSIRO
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Surprising discovery: A mold found in the soil can accumulate gold in its fungal threads, researchers have discovered in Australia. The fungus dissolves gold from the ground and forms tiny gold particles, which it stores in its tissue. Thus, this fungus turns out to be a hitherto unrecognized actor in the biogeochemical cycle of gold, as researchers report in the journal "Nature Communications".

Gold has been a coveted precious metal for millennia, but there are still a few puzzles left. So far, it has only been partially clarified how the great gold deposits of the earth were created. Earthquakes, arsenic, or the presence of oil and uranium ore have helped alleviate some of these occurrences, while others have helped minerals rich in minerals to enrich the gold. Also some tree species can evidently loosen gold from the soil and store it in their leaves.

Search for traces in the "Golden Triangle"

Now researchers have discovered another "gold collector" organism - a mold fungus. "Molds are known to degrade and recycle organic matter and also to absorb metals such as aluminum, iron, manganese or calcium, " explains Tsing Bohu of the CSIRO's mineral research center. "But gold is so chemically inert that the interaction of the fungus with it is unusual and surprising."

However, this is exactly what happens in the so-called "Golden Triangle" in Western Australia. This area about 100 kilometers southeast of Perth is known for its gold deposits, in some cases the gold-bearing strata reach up to five meters to the earth's surface. As a result, the soil also contains increased amounts of dissolved gold and gold compounds. To find out if and how soil organisms use them, Bohu and his team have studied soil samples and the microorganisms they contain.

Mold with gold particles

The amazing result: it was not exotic bacteria, but a common fungus that seemed to use and enrich the gold in the soil. Wherever Fusarium oxysporum was present in the soil, there were also elevated gold grades. Microscopic analyzes revealed that the filiform hyphae of this fungus were covered with minute gold particles over and over. display

But how had this gold got into the fungi? To find out, the researchers let the mold grow on gold-enriched nutrient media. "After two weeks, a ring of gold-depleted nutrient medium had formed around the fungal colony, " report Bohu and his team. At the same time, the central fungal mass was heavily enriched with gold. "The amount of gold in the center was about 6.5 times higher than in the impoverished, oxidized zone, " the researchers said.

Scheme of Gold Transformation at the Fungus Fossils: Superoxide dissolves colloidal gold and oxidatively converts it to gold ions. These then form complexes with the intracellular produced ligands and are attached to the sites. Bohu et al / Nature Communications, CC-by-sa 4.0

Oxidized gold

Further analysis revealed that the mold can oxidize metallic gold to dissolved gold ions and then incorporate the gold into its tissues. However, the fungus apparently uses a combination of a strong oxidant and suitable molecular binding partners: "The fungus growing in the gold-bearing soils apparently uses gold-specific ligands to stabilize the ionic gold in its system, " mutma The scientists say.

Interesting too: The mold seems to profit from the gold enrichment in its tissue. Fungus colonies that grew on gold-bearing nutrient media developed faster and better than control colonies on substrate without gold, as Bohu and his team noted. "Gold is therefore probably not a mere inactive element, but a crucial abiotic factor for the fungal ecosystems on such gold-bearing soils, " they state.

Actor in the earthly gold cycle

"Our study shows that fungi, a major component of the soil microbiome, can propel gold oxidation on the earth's surface, " said Bohu and his team. This is the first evidence that fungi also play an important role in the biogeochemical cycle of gold. Perhaps Fusarium oxysporum could even be used to indicate hidden gold occurrences.

Biorecycling could also be another practical benefit: The oxidative capabilities of the Fusarium mushroom could potentially be used to recover gold from electronic waste and other waste. Because the fungus naturally achieves what previously required strong and toxic solvents. (Nature Communications, 2019: doi: 10.1038 / s41467-019-10006-5)

Source: CSIRO

- Nadja Podbregar