Jun 1, 2002
The French have always had a penchant for fungi, but one day you may be more likely to find a cep cleaning up after a terrorist bomb or a nuclear accident than being served up in a wild mushroom soufflé.
Edible mushrooms might not be the most obvious choice for cleaning up after a nuclear accident or the explosion of a so-called “dirty” bomb, a conventional explosive carrying radioactive material. But, French scientists reckon that a wild mushroom might soak up radioactive caesium-137 ions just as easily as it can olive oil. Caesium-137 was released in vast quantities by the explosion at the Chernobyl nuclear power station eleven years ago and could be a major contaminant from a terrorist dirty bomb.
Removing metal and radioactive contaminants from exposed land is a crucial task. Aside from the immediate threat to the environment and the health of those living on or near such land, toxic metal ions can be carried into the food chain by vegetation growing on the land. One clean-up solution, known as bioremediation, involves deliberately planting plant species that might absorb the metals from the soil and then harvesting plants for safe disposal.
When it comes to the alkali metal caesium, however, there are no plant species that thrive on soil contaminated with it. So, if not a plant, why not a fungus?
Anne-Marie Albrecht-Gary and her colleagues at the Louis Pasteur University and the University of Strasbourg think they have found the solution in the unlikely form of the tasty bay boletus, Xerocomus badius. “Fungi often exhibit a remarkable ability to accumulate a large variety of elements, ranging from the heaviest of the transition metals such as lead or mercury, to the alkali metals, including radioisotopes like caesium-137,” explains Albrecht-Gary in a recent issue of Chemical Communications. But, she adds, little is known about how these fungi take up such metal ions.
She and her colleagues have studied the chemistry of the two pigments that give the inside of the bay boletus cap its bright yellow colour – norbadione A and badione A. These chemicals can act like molecular crab claws grabbing hold of metal ions in a pincer movement known as chelation. The yellow colour of the pigments provided the team with the means to test how well each latches on to metals, such as caesium. They exploited the pigments’ strong absorbtion of ultraviolet wavelengths to record a spectrum of the free pigment molecules and their spectra in the presence of caesium ions are markedly different. UV spectroscopy coupled with chemical analysis revealed that norbadione A in particular can bind to radioactive caesium very strongly. In fact, so strong is the norbadione claw that it can bind two caesium ions, whereas its weaker sibling badione A only has the strength to grip one at a time.
Albrecht-Gary and her colleagues believe that norbadione gets its strength from a so-called allosteric effect. When one caesium ion enters the claw, the molecule’s chemistry changes slightly so that a second gripping position opens up to accept another caesium, working like a double claw. Badione A, on the other hand has only one possible grip.
The researchers believe that norbadione A makes the bay boletus so good at sequestering radioactive caesium ions from the soil in which it grows that it should be the bioremedial agent of choice for removing this hazardous metal from contaminated land. “Obviously, fungi can be very efficient at accumulating toxic metals and radioelements and constitute an excellent and elegant tool for soil bioremediation,” says Albrecht-Gary, “However, the limitation on this very potent application is controlled growing of the mushrooms.”
While norbadione A will almost certainly have a role in bioremediation, the team has dashed hopes for its medical use in removing toxic metals, such as radioactive caesium-137 cadmium and nickel, from the body. Its grip on other alkali metals, such as the essential minerals sodium and potassium is just too strong.
Of, course one problem remains: what to do with the radioactive fungi…one can hardly cook them in an omelet.