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.
Clean-up crew
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
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Very cool, thanks!
Another cool article. Thanks Ed
Do these "simple" fungi have names? Do any of them form edible fruiting bodies,i.e. mushrooms? Are there implications for mushroom hunters?
Ed, this is a terrific description of a fascinating article - now I just need to bust through some paywalls to read the original. Thanks!
cool article, and i'm glad you are making it better known.
but!!
i think the title is very misleading. as you say, one of the concerns about depleted uranium is the residue of radioactive isotopes. sure, DU is primarily 238, but there is still some 235 in there.
and 235 is unstable--radioactively unstable.
when you say that the fungi convert DU into "stable minerals", that sounds as though you are saying that it reduces the amount of unstable minerals in it, where in this context, "unstable" means radioactive", and "stable" means non-radioactive.
and of course, the fungi do nothing of the sort. they facilate some *chemical* changes in the uranium. but they do not make unstable nuclei stable. it would be a cool trick if they did, but they don't.
i'm not even really sure what i think about the claims of *chemical* stabilization, e.g. that the uranium-phosporus compounds "cannot be taken up by plants and worms". why not? and what's to prevent our finding a new plant and a new worm that are just as surprising as our new fungus is?
still, that's not my main point. my main point is that the use of the word "stable" in your headline is really, really misleading. it suggests that the fungi are making the uranium less radioactively unstable. and that just ain't so.
That's a really good point. I really hate writing misleading headlines, so with this in mind, would you accept the following:
"Fungi transform depleted uranium into chemically stable mineral."
dear ed--
thanks for your reply.
it sounds better to me, but i am no expert on this stuff, so whether i will accept it or not is pretty irrelevant.
the question is: would experts in this area, who are thinking about the potential for confusion among non-experts, find this a minimally confusing way to express it?
as a non-expert, i dunno. it looks like a step in the right direction, to me, but me doesn't matter.
I hope they've checked to make sure that the fungi doesn't fractionate the isotopes during uptake.