Above: Eucalyptus trees (© istockphoto.com/Turnervisual)
Did you know? Eucalyptus tree leaves produce an essential oil that has strong disinfecting properties. This is one reason koala bears eat so many eucalyptus leaves: it helps keep them healthy.
This illustration shows the root system of a eucalyptus tree growing above the Freddo gold deposit in Australia, as well as the concentrations of gold found in the tree leaves. Click on the image to view the full-sized version on nature.com..
Although gold is normally mined deep underground, small traces of the mineral are also found in the leaves of eucalyptus trees, which grow in Australia and on some nearby islands. By studying how the gold gets there, Australian scientists are helping develop new methods of finding gold and other mineral deposits.
Until recently it wasn't clear where the gold in eucalyptus leaves was coming from. However, when researchers tested the soil, it became clear the source was underground—trees growing over gold deposits had more gold in their leaves.
There’s actually a good reason for gold to collect there. Gold is toxic to plants, and the leaves are the part of a tree most easily replaced. So it’s a much better idea to store toxic particles there than have them build up in the roots or trunk, where they could cause more harm.
The gold comes from deep underground, more than 40 meters below the surface. It gets absorbed into the tree roots along with other minerals and water. The water and minerals (including gold) are then transported up the roots, into the trunk and, finally, into the leaves.
The leaves use water and some of the minerals drawn from the ground, along with sunlight and carbon dioxide (CO2), to make sugar. This process is called photosynthesis. However, the gold just stays in the leaves, accumulating there over time.
So does this mean you can get rich by picking eucalyptus leaves? Not really. There isn't much gold at all in the leaves, less than 0.000005% by weight. That's a lot of zeros, equivalent to about six drops of water in a full tank of gasoline. It would take all of the leaves from 500 trees to make a single wedding ring. After all, if the amount of gold were significant, it would harm the tree. Nevertheless, by studying exactly how and why gold gets into the leaves of the eucalyptus trees, scientists can help mining companies and geoscientists (researchers who study the earth and the rocks that make it up) find gold deposits. It could ultimately make gold exploration much faster and cheaper. The same process could also be used to locate deposits of other metals, like copper and zinc. There has also been some research into whether other tree species in other parts of the world could provide similar information.
So although you won’t be able to strike it rich by collecting eucalyptus leaves, a better understanding of the process that causes gold to collect there may help increase supply and make gold a bit more affordable.
Did you know? The chemical symbol for gold is Au. This stands for Aurum, the Latin word for gold.