Â鶹ÒùÔº

The question of whether Earth is alone in harboring life has captivated humanity for millennia. In recent years, scientists have turned to Earth-like planets in other solar systems that may show the most promise, but many revolve around stars that emit much stronger solar radiation than our own. Now, a new study offers evidence that life as we know it may be able to thrive on those Earth-like exoplanets.

Published in , the new research demonstrated that lichen found in the Mojave Desert survived for three months under levels of solar radiation previously considered lethal.

The common lichen, Clavascidium lacinulatum, was injured, but able to recover and replicate. The results show that photosynthetic life may be possible on planets exposed to intense solar radiation.

"The study was motivated by a curious observation," said Henry Sun, Associate Research Professor of Microbiology at DRI and one of the lead authors of the study.

"I was just walking in the desert and I noticed that the lichens growing there aren't green, they're black. They are photosynthetic and contain chlorophyll, so you would think they'd be green. So I wondered, 'What is the pigment they're wearing?' And that pigment turned out to be the world's best sunscreen."

Life on Earth has evolved to withstand solar radiation known as UVA and UVB rays, which encompass longer wavelengths of ultraviolet radiation. UVC rays represent the shorter, more damaging wavelengths; these are filtered by Earth's atmosphere and therefore do not reach the surface.

UVC radiation is so lethal to life, in fact, that it is used to sterilize air, water, and surfaces of microorganisms like viruses and bacteria. Even a brief exposure to UVC radiation will damage an organism's DNA and prevent it from reproducing.

Scientists have wondered, then, whether many of the Earth-like planets discovered in recent years could possibly harbor life. Many of these planets revolve around stars known as M and F stars that emit intense UVC radiation, especially during solar flares.

"After the launch of the James Webb Space Telescope, which can see extremely far into space, the excitement shifted from finding life on Mars to these exoplanets," Sun said. "We're talking about planets that have liquid water and an atmosphere."

Sun and his graduate student at the time, Tejinder Singh, collected the lichen from the Mojave Desert near their homes in Las Vegas. They then placed it next to a UVC lamp in a controlled laboratory setting for three months straight. Remarkably, half of the algal cells in the lichen remained viable and replicated when rehydrated.

"In order for a microorganism to persist on a planet, it has to last longer than a day," Sun said. "So, our experiment had to be long enough to be ecologically significant. We also wanted to go beyond just activity and demonstrate viability."

To explore how this was chemically possible, Sun teamed up with chemists from the University of Nevada, Reno. They conducted two experiments that demonstrated how lichen acids are the natural world's equivalent of the additives used to make plastics UV-resistant.

They investigated the lichen's protective layer by cutting a cross-section of it and found that the top layer was darker, like a human's suntan. Lichen is composed of algae or cyanobacteria living symbiotically with fungi; when the algal cells were separated from the fungi and protective layer, exposure to the same UVC radiation killed the cells in less than a minute.

The discovery that lichen has evolved this protective layer to UVC radiation was surprising, Sun said, because it isn't necessary for their survival. Earth's atmosphere was already filtering out UVC rays by the time lichen appeared, so the protection is a mere bonus thanks to their remarkable UVA and UVB protection.

Some of the damage that occurs from exposure to intense solar radiation is the result of chemical reactions with the atmosphere, particularly the production of ozone when oxygen, nitric oxide, and UV radiation interact. To test the lichen's protection under different atmospheric conditions, the researchers placed it in an oxygen-free box with the UVC light and found that the radiation damage was further reduced.

"We came to the conclusion that the lichen's top layer—a less than millimeter thick skin, if you will—assures that all the cells below are protected from radiation. This layer acts as a photostabilizer and even protects the cells from harmful chemical reactions caused by the radiation, including reactive oxygen."

The study offers evidence that planets beyond Earth, and beyond our solar system, may be inhabitable. In fact, "they may be teeming with colonial microorganisms that, like the lichens in the Mojave Desert, are 'tanned' and virtually immune to UVC stress," the researchers write.

"This work reveals the extraordinary tenacity of life even under the harshest conditions, a reminder that life, once sparked, strives to endure," said Tejinder Singh, who led the study with Sun and is now at NASA Goddard Space Flight Center.

"In exploring these limits, we inch closer to understanding where life might be possible beyond this planet we call home."