The countryside around Iraq and the Balkans are still suffering from the ravages of wars fought in the 1990s. The environment is littered with the potentially dangerous remnants of military weapons - depleted uranium.
Depleted uranium is what's left over after 'enrichment', when uranium-235 is separated from natural uranium. This isotope is suitable for nuclear reactors and weapons, and the remainder consists of uranium-238, a less radioactive isotope with a longer half-life. This "depleted uranium" is valued by the military for its high density and is often combined with titanium to produce an alloy used in both armour-piercing weapons and defensive plating.
But penetrating rounds aren't the only potential threat to human health posed by depleted uranium. The substance is still radioactive, can cause heavy metal poisoning and burn spontaneously on impact to produce aerosols of uranium compounds. These potential risks have been downplayed by many reports but they make the use of depleted uranium in munitions highly controversial, especially when locals have to deal with traces that litter the landscape after battle ceases.
Now, a new study shows that very unlikely allies may be helping to clean up these remains. Marina Fomina from the University of Dundee found that several species of fungi can not only thrive on depleted uranium, but also convert it into stable minerals.
Together with a team of British researchers, Fomina found that a large number of different species could happily colonise small wedges of depleted uranium. The fungi covered the wedges with large networks of long, branching cells called hyphae.
The uranium wedges corrode naturally as they react with moisture in the environment to form uranium oxides, whose black and yellow hues were clearly visible. The tangles of fungal hyphae speed this process by trapping even more water and pumping out hydrogen ions and other molecules that acidify the local environment. These conditions enabled the fungi to corrode the surface of the uranium fragments, which lost about 8% of their weight in a 3 month period.
As a direct response to depleted uranium, the fungi also excreted organic acids such as oxalic acid that bind to uranium. It's a strategy that fungi also use to deal with other heavy metals and it converts uranium into a form that the fungi can take up. Indeed, some of the hyphae started turning yellow themselves, a sign that they had started incorporating the uranium salts into their network. Amazingly, about 30-40% of the dry weight of the exposed fungi was made up of uranium.
When Fomina looked at the fungi under the microscope, she found that the hyphae were encrusted by crystalline sheaths made of uranium minerals. The uranyl ions produced by the fungi's corrosive actions had reacted with phosphate ions released by the fungi themselves. These resulting uranium-phosphorus minerals, such as uramphite and chernikovite, formed large crystals that enveloped the hyphae.
In these mineral guises, uranium is much more stable, and is effectively locked away for the foreseeable future. It can't be taken up by plants and worm its way up the food chain. Fomina's study suggests that simple fungi could find themselves recruited into strategies designed to reclaim soil polluted by depleted uranium.
Reference: Fomina et al.: "Role of fungi in the biogeochemical fate of depleted uranium." Publishing in Current Biology 18, R375 -R377, May 6, 2008.
Images: from Current Biology