A new report has suggested that fungi may have an important role to play in capturing potentially dangerous depleted uranium, left in the environment after recent war campaigns.
The researchers, from the University of Dundee in Scotland, found evidence that fungi can "lock" depleted uranium into a mineral form that may be less likely to find its way into plants, animals, or the water supply.
Depleted uranium differs from natural uranium in the balance of isotopes it contains. It is the byproduct of uranium enrichment for use in nuclear reactors or nuclear weapons and is valued for its very high density.
Although less radioactive than natural uranium, depleted uranium is just as toxic and poses a threat to people.
The testing of depleted-uranium ammunition and its recent use in Iraq and the Balkans has led to contamination of the environment with the unstable metal.
In the new study, the researchers found that free-living and plant symbiotic (mycorrhizal) fungi can colonize depleted-uranium surfaces and transform the metal into uranyl phosphate minerals.
"While they probably still pose some threat, the fungal-produced minerals are capable of long-term uranium retention, so this may help prevent uptake of uranium by plants, animals, and microbes," said Geoffrey Gadd from the University of Dundee.
"It might also prevent the spent uranium from leaching out from the soil," he added.
According to Gadd, a combination of environmental and biological factors is involved in the process.
First, the unstable uranium metal gets coated with a layer of oxides. Moisture in the environment also "corrodes" the depleted uranium, encouraging fungal colonization and growth. While the fungi grow, they produce acidic substances, which corrode the depleted uranium even further.
Some of the substances produced include organic acids that convert the uranium into a form that the fungi can take up or that can interact with other compounds.
"Ultimately, the interaction of soluble forms of uranium with phosphate leads to the formation of the new uranium minerals that get deposited around the fungal biomass," said Gadd.
"We have shown for the first time that fungi can transform metallic uranium into minerals, which are capable of long-term uranium retention," the researchers said.
"This phenomenon could be relevant to the future development of various remediation and revegetation techniques for uranium-polluted soils," they added.