The energy used by most living things is ultimately derived from sunlight, which is converted into simple sugars by plants and passed up the food chain.
Besides plants and photosynthetic bacteria, only a few organisms can fuel their own growth with energy obtained from nonbiological sources.
Last year, for example, scientists discovered bacteria living deep underground in a gold mine that harvest the energy of natural radioactivity.
If fungi containing melanin can do the same thing, the number of "self-feeding" organisms may be far greater than previously believed, the authors said.
"This observation raises the possibility that terrestrial life has another group of organisms capable of making their own food," study co-author Casadevall said.
"Given that fungi are such a large part of the biota, even a small energy-synthesizing capacity could have large effect on Earth's energy cycles," he added.
(See related: "Army of Tiny Fungi Keep Forests Healthy, Study Suggests" [November 21, 2005].)
Lead study author Dadachova also speculated the new source of biological energy could be useful to humans.
Space travelers, for example, could someday harvest foods raised on ionizing radiation, which is prevalent in outer space. Several edible mushrooms contain melanin.
In addition, "the genes from those fungi responsible for melanin production could be put into food-producing plants," she said.
Melanin is widespread in nature and has long been known to provide protection from ultraviolet radiation, which in high exposures can cause skin cancer.
The pigment is present in most fungi and also accounts for skin color in humans.
"Melanin is pretty amazing stuff," said Duke's Vilgalys. "It's not surprising that many fungi have figured out how to use it for protection and possibly as a means to harvest [energy]."
John Dighton is a fungi specialist at Rutgers University in Camden, New Jersey, who has worked with a team of Ukrainian scientists studying fungi at Chernobyl.
He said the findings "confirm some of the thoughts we have been having for a few years."
"Fungi that have been previously exposed to ionizing radiation have a propensity to direct their growth towards sources [of the radiation]," Dighton said.
"We have suspected that cell-wall pigments, such as melanin, may be involved."
For now, lead author Dadachova said, the actual mechanism by which melanin captures and transforms high-energy radiation remains a mystery—though that's to be expected.
"For photosynthesis, it took decades for the mechanism to be understood."
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