© Peter Pichler
October 2017 − Exploring Life
Heavy metals burden the soils of our planet. Researchers have identified more than 500 super plants that independently clean the earth and may even generate raw materials.
Nickel, lead, zinc, highly toxic cadmium – all over the world, scientists across all disciplines are searching for methods to reclaim the contaminated sites of former mines and industry for renewed use. Decontamination using classic methods such as exchange or heating of the soil to 800 degrees centigrade is costly. However, help is on the way, and it originates in nature itself. Heavy metal contamination has finally met its match, and super plants are the new focus. “They are capable of concentrating toxic metals to amounts which are a hundred to many thousand fold above the levels found in other plants”, says Ute Krämer. The plant physiologist at Ruhr University in Bochum (Germany) researches the DNA of Arabidopsis halleri (see info box).
Hoover for heavy metals
Super plants take up the heavy metals with their roots like a biological vacuum cleaner. Plant cells push the toxic cargo further to the surface, where it is then deposited inside the vacuole, a cellular component, in the outer layers of the leaves. In the case of Arabidopsis halleri, a specific gene appears to be responsible for this logistical masterpiece: nicotianamine. It was discovered by molecular biologists at the University of Bayreuth during experiments involving zinc. If the gene is inhibited, the contaminants remain sequestered in the roots. So far, more than 500 different super plants have been identified, raising scientists’ hopes twofold: decontamination of industrial wastelands, also known as phyto-remediation, as well as the extraction of valuable raw materials, aka phyto-mining. If these plants, also called hyperaccumulators due to their capacity for storage, are burned, the respective accumulated heavy metals are left behind in the ashes and may be eluted with the help of acids. Mining with plants instead of pickaxes and mallets.
The plant as a miner
Farmers in Albania already benefit from green mining: on the banks of Lake Ohrid, they harvest Alyssum murale. The ashes of the weed consist of twenty percent nickel. An even higher concentration was found in a giant tree in New Caledonia by the Dutch ecologist Antony van der Ent. If the bark of Pycnandra acuminata is scratched, blue-green plant juice will emerge which consists of 25 percent nickel.
In contrast, storage of especially high amounts of cadmium, a raw material used in cell phone batteries, is a strength of Noccaea caerulescens, which also blossoms in the alpine meadows of the European alpine upland. The current joker among the magic plants is the South African Berkheya coddii. It is capable of extracting up to ten-fold concentrations of four metals simultaneously from the ground: nickel, cobalt, platinum and palladium. Platinum is indispensable in the automotive industry as well as in medical engineering – the noble metal is a component of every catalyst and pacemaker.
Heavy metals as a defense mechanism
What prompts the super plants to subsist on such heavy fare? Professor Krämer speaks of a natural defense mechanism: “They use the heavy metal concentrations as a defense against predation by animals.” Some magical plants benefit from their heavy-metal content in yet another way: by securing their habitat. Once the contaminated leaves fall on the ground, no other plant will thrive.
Many questions remain open, however. Why do some plants display a preference for heavy metals, while others do not? Why are they impervious to the toxic substances? Ute Krämer: “We want to understand the mechanism behind the extremely high accumulation of metals in these plants, and what this means for the pathways on the molecular and physiological level.” It is still a long way before any lucrative commercial exploitation of the hyperaccumulators will be feasible.
The search for Arabidopsis halleri
In the past several years, Professor Ute Krämer and her team have traveled more than 40,000 kilometers across Europe in order to collect specimens of Arabidopsis halleri from industrial wastelands. This effort resulted in a collection of approximately 3,000 plants, kept at Ruhr University in Bochum (Germany). At this time, it is not clear why this plain weed is able to tolerate toxic heavy metals at such elevated concentrations. The researchers do know, however, that it is the plant itself, rather than the ground concentration levels, that determines how much lead, zinc or cadmium it will accumulate. “This is due to certain small but momentous genetic differences. We keep identifying more and more things, and we are investigating the functional context in detail”, says Ute Krämer. In addition, different strains of Arabidopsis halleri are interbred, and isolated DNA is transferred to the target plant. Professor Krämer: “Both approaches are important in order to gain a basic understanding and in order to perform initial tests which may lead to future applications.”