Pigment seems to put radiation to good use
By Davide Castelvecchi
From May 26th, 2007; Vol.171 #21 (p. 325)
Call them the Hulk bugs. Just as they do for the comic book hero, gamma rays seem to make certain microscopic fungi stronger. Researchers have found hints that melanin—the same pigment that’s the natural ultraviolet filter in people’s skin—might enable these fungi to harness the energy of gamma radiation as well as to shield themselves from it.
Microbiologist Arturo Casadevall of the Albert Einstein College of Medicine in New York City recalls learning several years ago that single-cell fungi had been found thriving inside the collapsed nuclear reactor at Chernobyl, Ukraine. He and his colleagues later saw reports that the cooling water in some working nuclear reactors turns black from colonies of melanin-rich fungi.
Nuclear reactors are intense sources of gamma rays, which can zap through living organisms and leave behind trails of destruction. Many microorganisms can survive in extreme environments, but Casadevall thought that something more might be going on. Perhaps the fungi were growing thanks to the radiation, not in spite of it. “The thought was that biology never wastes any energy source,” he says.
Casadevall says that fungi such as Cryptococcus neoformans—which causes grave infections in AIDS patients—have layers of melanin on their membranes. Melanin is rich in radicals—molecules with highly reactive unpaired electrons—that may help fend off attacks by the immune system of any organism that the fungus is trying to infect. But Casadevall wondered whether these layers might also turn gamma ray energy into a form the cell could use.
To test this hypothesis, Casadevall’s team exposed colonies of C. neoformans to gamma rays 500 times as intense as the normal radiation background on Earth’s surface. The colonies grew up to three times as fast as normal. A mutant “albino” form of the fungus, which produced no melanin, grew at a normal pace, the team reports in the May PLoS ONE, an online journal.
But the accelerated growth didn’t prove that the fungi drew energy from the radiation, Casadevall says, so the researchers took a closer look at melanin.
In one experiment, they found that gamma rays induced a four-fold increase in melanin’s ability to catalyze an oxidation-reduction reaction typical of cell metabolism.
They also tested melanin’s response to gamma rays using electron spin resonance, a technique similar to nuclear magnetic resonance spectroscopy. Gamma rays changed the distribution of unpaired electrons in the molecule, says Casadevall’s Albert Einstein colleague Ekaterina Dadachova.
These findings suggest that gamma rays kick some melanin electrons into excited states, initiating a yet-unknown process that would end up producing chemical energy, Casadevall says. This might be similar to the way in which photosynthesis supplies energy to plants, he adds. He speculates that melanin might collect energy not only from gamma rays but also from lower-energy radiation such as X rays or ultraviolet rays. “I think this is only the tip of the iceberg,” he says.
The findings are interesting, says Darrell Fisher, a radiation biologist at the Pacific Northwest National Laboratory in Richland, Wash. However, he says, “one must be careful not to draw unwarranted conclusions.” If radiation enhances the growth of fungi, he says, it’s “important to understand and test the underlying mechanisms.”