Casadevall and his colleagues, however, have a theory. Based on experiments with three different types of fungi, they believe the melanin-containing breeds absorb the high levels of energy in ionizing radiation and somehow turn it into a biologically useful (and benign) form, akin to a dark and dangerous version of photosynthesis. "We were able to see significant growth of the black ones relative to the white ones in a radiation field," he says. "That is the observation. How you interpret it is where the interesting speculations come in."
In a paper published online in PLoS One, Casadevall and his colleagues report that ionizing radiation changes the electron structure of the melanin molecule and that fungi with a natural melanin shell (the soil-dwelling Cladosporium sphaerospermum and yeastlike Wangiella dermatitidis varieties), which were deprived of other nutrients, grew better in the presence of radiation. They also report that fungi induced to produce a melanin shell (the human pathogen Cryptococcocus neoformans) grew well in such levels of radiation, unlike those sans pigment. Further, an albino mutant strain of W. dermatitidis failed to thrive as well as its black cousin when exposed to 500 times the normal amount of ionizing radiation (still well below the level of radiation necessary to kill tough fungal forms).
"The presumption has always been that we don't know why truffles and other fungi are black," Casadevall says. "If they have some primitive capacity to harvest sunlight or to harvest some kind of background radiation a lot of them would be using it."
Melanin drinks in ultraviolet rays, acting as a natural sunblock for human skin. "Melanin is very good at absorbing energy and then dissipating it as quickly as possible," says Jennifer Riesz, a biophysicist at the University of Queensland in Brisbane, Australia. "It does this by very efficiently changing the energy into heat."
But Casadevall and his colleague Ekaterina Dadachova, a nuclear chemist at Einstein, speculate that the melanin in this case acts like a step-down electric transformer, weakening the energy until it is useable by the fungi. "The energy becomes low [at] a certain point where it can already be used by a fungus as chemical energy," Dadachova argues. "Protection doesn't play a role here. It is real energy conversion."
Mycologists and biophysicists find the notion both intriguing and potentially plausible. "Since melanin is used commonly by fungi—and other organisms—to protect themselves against UV radiation, it is perhaps not surprising that melanin would be affected by ionizing radiation,'' says Albert Torzilli, a mycologist at George Mason University in Virginia, adding that "the subsequent enhancement of growth, if true, is a novel response."
Riesz, for one, is skeptical. "It does not surprise me that fungi protected with higher levels of melanin might grow better when exposed to [ionizing radiation], since the nonprotected fungi are more likely to be harmed by the radiation," she says. "However, I find the claim that melanin is involved in energy capture and utilization to be unlikely."
More study is needed to confirm whether fungi will be able to add the ability to grow by harvesting radiation to their list of seeming superpowers, but it does raise the question of whether edible fungi—like mushrooms—have been harboring this function undiscovered for years. If true, melanin could be genetically engineered into photosynthetic plants to boost their productivity or melanin-bearing fungi could be used in clothing to shield workers from radiation or even farmed in space as astronaut food. The group plans further tests to see if fungi with melanin are converting other wavelengths of the electromagnetic spectrum into energy, as well.
"[Melanin] doesn't reflect any light; it's all going into it. Is it all disappearing into a black pigment and has no use whatsoever? Biology is incredibly inventive," Casadevall argues. After all, extremophile microbes thrive in the heat and acid of hydrothermal vents below the sea or live off the radiation of decaying radioactive rocks deep inside Earth's crust. "It's not that outlandish," Casadevall says, for fungi to harvest the energy in ionizing radiation with the help of melanin. But it is unexpected and strange.