Lichen from the Mojave Desert can survive, and replicate, under levels of extreme solar radiation – levels of radiation equivalent to that found on Earth-like planets in other solar systems. This is due to a microscopic “sunscreen” layer that protects the cells within the complex.
Lichens are organisms that consist of a symbiotic association of fungi and algae (or, more accurately, cyanobacteria). The complexes can be found in various environments worldwide and are known for their ability to colonise surfaces like tree bark and rocks.
Lichens play an important role in ecosystems, serving as indicators of air quality by absorbing pollutants and contributing to cleaner air. They are also often visually impressive, appearing as colourful patches on trees and rocks.
The structure of lichens includes layers of fungi and algae, with the thallus being the prominent non-reproductive body.
Mojave Desert lichen
Researchers from the U.S. Desert Research Institute have been examining the common lichen Clavascidium lacinulatum, found in the Mojave Desert. The lichen survived for three months under levels of solar radiation previously considered lethal. Remarkably, the lichen was able to recover and replicate.
C. lacinulatum is a dark brown squamulous terricolous lichen. In one of the areas it can be commonly found – Joshua Tree National Park, U.S. – it is the most common of the biological soil crust lichens.
According to lead scientist Henry Sun: “The study was motivated by a curious observation…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.”
Exoplanets and other life
In relation to life on other worlds, scientists have pondered whether many of the Earth-like planets discovered in recent years could possibly harbour life. However, several of these planets revolve around stars known as M and F stars that emit intense UVC radiation, especially during solar flares. UVC rays represent the shorter, more damaging wavelengths, proving lethal to life after a given period of exposure.
Sun collected the lichen from the Mojave Desert close to Las Vegas. He 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.
Why is the lichen resistant?
Sun investigated the lichen’s protective layer by cutting a cross-section of it and he found that the top layer was darker, analogous to a human’s suntan. When the algal cells were separated from the fungi and protective layer, exposure to the same UVC radiation killed the fungal cells in less than a minute.
The discovery that lichen has evolved this protective layer to UVC radiation was surprising, because it is not necessary for their survival. Earth’s atmosphere filters out UVC rays. Sun assumes the protection is a mere bonus, as by-product of the lichen’s protection from the type of ultraviolet radiation that reaches the surface – UVA and UVB.
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.
The scientists conclude that the lichen’s top layer – a less than one millimetre thick – assures that all the cells below are protected from radiation. This layer acts as a photo stabiliser and protects the cells from harmful chemical reactions caused by the radiation, including reactive oxygen.
The research appears in the journal Astrobiology, titled “UVC-Intense Exoplanets May Not Be Uninhabitable: Evidence from a Desert Lichen.”
