In 1987, a group of researchers in France discovered something peculiar. When they protected single-celled organisms from background radiation—the sort that comes from cosmic rays and radioactive rocks—the creatures’ growth was stunted. Colonies that receive a background dose of radiation actually grew more quickly than their shielded brethren. That’s radiation—not vitamins, not nutrition, not anything people generally suggest you should get more of.
Was background radiation somehow required for life?
The dangers of radiation loom large in our cultural memory. With good reason: From the melting, cancerous jaws of factory workers exposed to radium-infused paint in the 1920s, to the gruesome after-effects of the atomic bombs dropped on Japan, the results of exposure speak vividly against taking radiation lightly.
On the cellular level, radiation’s particles wreak havoc on DNA, ripping through the double helix like bullets and causing the creation of corrosive free radicals. Irradiated cells form strange structures in their nuclei as they attempt to replicate themselves, the genetic material twisting and clumping sickeningly. The general rule is the less radiation exposure, the better: In this view, “there is no safe amount of radiation,” says New Mexico State University researcher Hugo Castillo.
Separately, however, since life’s beginnings—long, long before humans began toying with things like uranium—all organisms have been exposed to what is known as background radiation. This is naturally occurring, with some of it coming from the sky, in the form of cosmic rays, and some of it coming from the Earth, from isotopes like potassium-40 in the soil and rock. The amounts are not huge—something like having a mammogram every couple of months—but they exist.
You’d think that when you protect cells from the natural background radiation, they’d do better—no radiation means no stress, no damage, faster growth. But as the French experiment suggested, some level of radiation may be beneficial.
Since the 1987 paper, a small community of researchers have worked to understand what happens to cells in low-radiation environments. In the Gran Sasso Cosmic Silence experiments, under a mountain in Italy, scientists have found that cells grown under such conditions respond more poorly than controls to damaging radiation. And at France’s Laboratoire Souterrain de Modane, a team has been studying the evolutionary effects of very low radiation. “We wanted to quantify as precisely as we could what effect radiation is having on cells,” says Nathanael Lampe, a graduate student there. But there are limited places on the planet where it’s even possible to contemplate such experiments.
Castillo, his supervisor Geoffrey Smith, and their colleagues at NMSU have such a set-up: In a salt mine deep beneath the New Mexico desert, they are investigating background radiation’s importance to life in a very unusual series of experiments. In a paper published this February in Frontiers of Microbiology, they report that, indeed, bacteria grown in an extremely sheltered environment there—at levels of background radiation nearly 80 times lower than at the surface—appear to be stressed. It’s the same kind of reaction you get if you starve cells or heat them, but in this case, it’s because of the lack of radiation. That implies that there is something cells have evolved to expect, over the eons, from the background.
Castillo and Smith’s New Mexico group works out of a small lab in a shipping container more than 2,000 feet underground, in a man-made system of caverns created for storing nuclear waste. The facility, called the Waste Isolation Pilot Plant, has socked away thousands and thousands of containers bearing strongly worded labels since 1999, in chambers chiseled from a salt formation. Entering the WIPP facility is not for tourists, and even the scientists are few. The team only has access for a few months out of the year, and on the way down, they crowd in a small elevator. “After 20 seconds, all the light is gone,” Castillo says, recalling his first trip down. “I’ve never seen such darkness before in my life.”
When the researchers disembark at the bottom of the shaft, they are cocooned in a bubble of salt, exceptionally poor in natural radioisotopes, and so far down that cosmic rays can’t penetrate. They make their way to the lab space, where their neighbors are physicists doing similarly sensitive experiments. Finally they come to their most important piece of equipment: A steel vault, built by the military before World War II, that does not have the trace radiation that steel made afterwards acquired as a result of the fallout when the atomic bombs went off. That means that it does not have the trace radiation that everything aboveground acquired as a result of the fallout.
Within this vault, hidden in the salt bubble, more than 2,000 feet underground, the background radiation level is already ten times less than at the surface. The group has erected even more shielding around an incubator, bringing down the level to 79 times less than normal. And there they have grown two kinds of microbes: Shewanella oneidensis and Deinococcus radiodurans, both stalwarts of radiation biology experiments. In a control incubator with identical bacterial cultures nearby, they have added back in an amount of radiation comparable to the surface.
It’s delicate work, designing an experiment where you know the effects are from the lack of background radiation, and not from, say, myriad other small differences that can creep in. You must account for differences in air pressure, if the cells are grown in different places; too, the cells aboveground and below may grow more genetically distinct over time, making it harder to tell what alterations are radiation-related. The best course of action, if possible, is making the level of radiation the sole difference between control and treatment, as the New Mexico team has done.
In the shielded incubator, D. radiodurans grows more slowly, its numbers lagging behind controls throughout the duration of the experiment, they report, in line with what’s been found by prior groups. S. oneidensis, however, after a short delay, matches the growth numbers of the controls.
Delving into this, the group checked the organisms’ expressions of 10 genes involved in coping with stress. They found that D. radiodurans was laboring along with few of these genes expressed any differently from the norm, and apparently suffering for it. S. oneidensis, on the other hand, had at least doubled the expression levels of some of them. “This is a subtle effect,” Castillo says. “But it appears the cells can turn some genes on and off to compensate for this lack of radiation.” Moving the microbes to the control incubator with more radiation erased both the growth problems and the change in gene regulation, confirming the connection with radiation.
The genes that S. oneidensis upregulated are associated with particular kinds of genetic repair. Specifically, they help mend damage caused by free radicals, which can be produced by radiation exposure, but also can be made by cells themselves as waste products. Since they certainly aren’t getting free radicals from radiation, this suggests that S. oneidensis may be responding to an over-abundance of free radicals produced internally, Castillo hypothesizes. Perhaps the usual steady dose produced by background radiation on the surface serves as a signal to the cell to keep free radicals low—like a finger pushing on a button. Maybe without those regular prompts from the environment, cells accumulate the molecules to an unhealthy level and then struggle as a result.
“A Billion Detectors”
Still, this opens up an even greater mystery. Background radiation is sparse stuff, just a few particles bouncing through a very large volume of space. And cells are tiny—the vast, vast majority of the cells in a dish grown on the surface are never struck by a particle at all, says Lampe, from the French group, who has been modeling the number of mutations arising from the background. “This is why observations of different biological effects in underground labs are so fascinating,” he says. If in the normal course of life cells are never struck by a radiation particle, how in the world does the presence or absence of such particles register? Perhaps the answer lies in incredibly swift and complete communication between cells. “If you have a cell population, you effectively have an array of a billion detectors,” Lampe proposes. “They can aid each other in figuring out what’s going on with the radiation background.” And perhaps it takes only a very small, conditioning dose of free radicals—or something else, something still unknown—to keep closely communicating cells at the surface in working order.
Then, though, there are the effects observed by the Italian group, led for many years by Luigi Satta of the Italian National Institute for Nuclear Physics, which has been working on the problem since the late 90s. In those experiments, yeast, hamster, and human cells were grown in low-radiation environments underground and also in a lab on the surface. Then, the underground cells were brought up and both sets were given challenging doses of radiation. The team found that cells grown below had more DNA damage afterwards, measured by the appearance of twisted wads of DNA in the nucleus. “If the cells had been living for a few months in an environment in which the background radiation was normal,” says Massimo Pinto, a radiation scientist who worked on the experiments, “these cells were more ready to respond to an insult in the form of a challenge dose. The sister cells that had an artificially reduced background radiation level were not as capable.”
It’s a peculiar finding—why would being without radiation make cells more vulnerable? One theory is that very low doses of radiation can cause cells to keep their repair machinery switched on, and those without it are unprepared, like a runner who skipped one too many workouts before a marathon. But the answers are not clear yet. Satta says that the next step would be to try similar experiments with fruit flies, to get a sense of whether the effect occurs in more complicated creatures. “All this suggests that these experiments should go on,” he says, “but due to budget limitations, this plan is unfortunately not sure.”
Still, despite the difficulty of finding dependable funding for such esoteric experiments, “we’ve got a very fascinating phenomenon on our hands,” Lampe says. Life has evolved to take advantage of the light shining down on Earth, by developing photosynthesis. It’s evolved to take advantage of an oxygen-rich atmosphere, using it to help generate energy at the cellular level. It’s evolved to be comfortable at a specific range of temperatures commonly available on our planet. Is it any wonder that there may be another, less noticeable aspect of our environment that we’ve missed the significance of until now?