A barrage of radiation couldn’t kill this hardy life-form

An unassuming lichen harbors a hidden superpower: It’s remarkably resistant to ultraviolet radiation. New experiments on the hardy organism call into question the long-held belief that alien planets bathed in ultraviolet light must be sterile worlds, researchers report June 12 in Astrobiology. The discovery may open up more options in the search for life elsewhere in the universe.

Thousands of exoplanets have been discovered to date, but many of them orbit a type of small, highly active star apt to send off blasts of energetic particles and radiation. And unlike our planet, those worlds likely don’t have ozone in their atmospheres. Ozone is formed from compounds produced by photosynthesis, and researchers haven’t found any evidence of such a process occurring on an exoplanet.

That’s been a worry, because ozone, which consists of three oxygen atoms, functions as a protective blanket blocking the shortest-wavelength — and therefore most damaging — ultraviolet radiation from reaching a planet’s surface. On Earth, hospitals use such radiation, known as UVC, to sterilize instruments.

“It’s meant to kill life,” says Tejinder Singh, an astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Previous work has shown that lichens, a symbiosis of algae and fungi, are remarkably resistant to a barrage of tough conditions. But no one had ever thrown high intensities of extreme ultraviolet light at them for long periods of time to mimic the conditions on an exoplanet, Singh says. “We pushed the limits.”

In 2021 and 2022, Singh and his collaborators collected roughly half a kilogram of Clavascidium lacinulatum, a soil-dwelling lichen, from the Mojave Desert in southern Nevada. Back in the laboratory, the team exposed the dark-colored lichen to UVC radiation.

After three months of exposure at intensities comparable to those received on exoplanets, more than 60 percent of C. lacinulatum’s photosyntheticalgal cells were still viable, the team found. “We couldn’t kill the lichen,” says Henry Sun, an astrobiologist at the Desert Research Institute in Las Vegas, Nev. and a member of the research team.

But roughly one minute in the same conditions was 100 percent lethal to Deinococcus radiodurans, a bacterium known for withstanding radiation. And when the team isolated just the lichen’s algal cells, they suffered the same fate as D. radiodurans.

Singh and his collaborators showed that C. lacinulatum contains chemicals that not only block ultraviolet light but also remove harmful compounds known as free radicals produced by exposure to ultraviolet radiation.

Exoplanets blasted with ultraviolet light therefore aren’t necessarily uninhabitable, the team concluded. Maybe an alien form of lichen could gain a toehold after all.

A similar scenario also likely played out closer to home long ago, the researchers suggest. Before our planet’s ozone layer developed, the earliest life would have needed to withstand UVC radiation, Sun says. “The scenario that we’re hypothesizing actually happened on Earth billions of years ago.”

These results clearly demonstrate that C. lacinulatum is unusually resistant to ultraviolet radiation, says Matthew Nelsen, a biologist at the Field Museum in Chicago who was not involved in the research. But the authors’ assertion that exoplanets bathed in ultraviolet light might harbor similar organisms is a bit of a stretch, he says, since there are other stressors like temperature and water availability to consider. “I think it’s premature to say that lichens can survive on other planets.”