Heavy metals can be a problem.
We’re not talking guitar riffs and volume to make your ears bleed. We’re talking heavy metals that occur in the environment – whether naturally or as a by-product from human activities.
A ‘heavy metal’ describes a metallic element that has a high density and atomic number. They are usually found further down on the periodic table, such as mercury (Hg), chromium (Cr), selenium (Se) and silver (Ag).
Many heavy metals are also toxic, which is why they can be a problem in the environment.
In Hinkley, California, the company Pacific Gas & Electric leaked hexavalent chromium into the local groundwater: a by-product of chemical processes and the result of unlined wastewater ponds. This contamination of soil and water had devastating effects on the people who lived nearby, which resulted in a lawsuit being settled for $333 million (and the award-winning film Erin Brockovich). Laws and regulations do limit levels of toxic metal contamination, but these differ by country and can be tricky to enforce.
That said, many metals are very useful in the right context. The smart phone in your hand, the solar panels on that building, the health tracker on your wrist: these modern advancements, and many others you can’t see, need a range of heavy metals to work, like silver, gold, copper, cobalt, selenium and tellurium.
But these metals are in increasingly short supply, due to high demand. They can also be difficult to mine or extract from the earth, and can contaminate soil and water in the process. These challenges mean that scientists would like to find ways to make extraction more environmentally sustainable and reduce levels of contamination.
However, hope may come from fungi, of all places.
Research has shown that certain species of fungi accumulate some heavy metals from the soil in which they grow [1]. The fungus Boletus, growing in parts of the Yunnan province of China with high mercury concentrate, have been found to have high levels of mercury, as have Leccinum mushrooms in Poland. Although we don't know what effect this is having on the fungi!
Given these findings, Professor Geoffrey Gadd at the University of Dundee believes that the properties of certain fungi could provide a solution to the toxicity problems posed by some heavy metals. He has experimented with fungal species, including Aspergillus niger and Paecilomyces javanicus, to recover valuable metals from soil and water [2].
'Some of the processes by which fungi interact with metals could have potential for treatment of contaminated land,' he says. 'They can alter the chemical state of [a metal] so it can be more easily managed, or make them immobile and less toxic. This is a very common process in microbes, and there’s a lot of different ways they can do it.
'We found that fungi can attack depleted uranium, and in the process, they can make new minerals which lock the depleted uranium into a mineral form. So, they actually change it to a different substance, and this substance is new and more stable than the depleted uranium itself.
'In addition, many fungal processes can be used to recover valuable elements from solution, often as new nanoparticle forms, such as copper, cobalt, selenium and tellurium. These elements are critical in powering the digital age in electronics, computing, renewable energy and environmental biotechnologies.'
Studies into this are still in the early stages, but it’s exciting to imagine that organisms naturally growing in soil could hold the key to a cleaner planet.
Can you tell your metal from your mushroom? Take our quiz!
References
[1] Falandysz J, Zhang J, Wang Y-Z, Saba M, Krasińska G, Wiejak A, et al. (2015) Evaluation of Mercury Contamination in Fungi Boletus Species from Latosols, Lateritic Red Earths, and Red and Yellow Earths in the Circum-Pacific Mercuriferous Belt of Southwestern China. PLoS ONE 10(11): e0143608. doi:10.1371/journal.pone.0143608
[2] Liang, X. and Gadd, G.M. (2017). Metal and metalloid biorecovery using fungi. Microbial Biotechnology 10, 1199-1205.