High-intensity ultraviolet radiation has been found to pose little trouble for a soil-dwelling lichen in the Mojave Desert. After three months of exposure to powerful UVC, the lichen still retained its photosynthetic function and cellular viability thanks to chemical shielding by its secondary metabolites. Compounds derived from these lichens could find UV-blocking applications in cosmetics, materials and agriculture and, according to researchers, provide evidence that exoplanets bombarded by intense UV light are not necessarily hostile to photosynthetic organisms.
Despite resembling plants, lichens are actually colonies of fungi and either algae or cyanobacteria living symbiotically. Many lichens can thrive in harsh environments, including the arctic tundra, alpine regions and deserts. While organisms on Earth are protected from most UVB and all UVC radiation by the ozone layer, life on exoplanets orbiting M and F-type stars would need tremendous UVC protection to survive.
When hiking in the Mojave Desert, Henry Sun, a researcher at the Desert Research Institute in Las Vegas, US, wondered why, despite being photosynthetic organisms, the lichens there were not green, but instead a range of other colours, including black. His hypothesis was sunscreen. ‘It’s just like when we go out to the desert, we wear shirts. If you don’t have sun block, you’ll wear shirts … maybe [the lichens] just didn’t coordinate what colour shirts to wear.’ This led him to ask whether ‘this shirt, or this sun blocking pigment, on the surface of all desert lichens, could it also block UVC?’
To investigate, his team continually irradiated Clavascidium lacinulatum, a black desert lichen, for 90 days with 254nm UVC light at 55W/m². Despite receiving a cumulative dose of over 400,000kJ/m², the lichen showed only modest declines in photosynthetic quantum yield and retained approximately 65% viable algal photobiont cells. In comparison, when Deinococcus radiodurans, the most radiation-resistant bacteria ever found, was exposed to the same conditions, it died in under a minute.
The chemical basis of the lichen’s resistance lies in its UV-opaque cortex, a structural layer enriched with phenolic lichen acids. These complex aromatic metabolites serve as natural photostabilisers. The researchers identified one such compound with a tentative molecular formula of C₁₀H₁₄N₂O₅.
‘We came up with this hypothesis, that you can stop UVC photons, but it’s impossible to stop the secondary chemistry effects of UVC radiation. That generates reactive oxygen species, which can only be mitigated,’ Sun explains. To dig deeper, the researchers compared radiation damage levels in lichens in ambient air versus in an anaerobic glovebox and found that those irradiated in the absence of oxygen suffered significantly less injury.
‘The evolution of lichen acids was not originally driven by the need for UVC protection. Rather, their primary function lies in combating oxidative stress, particularly under desiccating conditions,’ explains Mehmet Varol of Muğla Sıtkı Koçman University in Turkey. ‘These metabolites likely evolved in desiccation-tolerant organisms to control [reactive oxygen species] formation during dehydration–rehydration cycles. Their ability to absorb UVC radiation is likely a secondary, fortuitous property.’
The radiation resistance of lichens may have implications for exoplanet habitability. ‘From an astrobiological perspective, this work challenges the assumption that atmospheric ozone shielding is a prerequisite for surface-based life, instead proposing that biological adaptation to UVC stress may itself be viable in the absence of atmospheric protection,’ Varol says.
On Earth, lichen-derived or inspired chemicals could find applications in producing UV-resistant cosmetics and materials. ‘Due to their aromatic structures, these compounds could be integrated into polymers to enhance UVC stability in materials, particularly in aerospace applications such as robotics, surface coatings or smart paints for high-UV environments,’ Varol notes. ‘However, there are translational challenges: some lichen metabolites exhibit cytotoxicity, and rigorous photostability testing, biocompatibility assessments and biodegradability analyses would be essential before commercial deployment.’
Sun additionally wonders if such compounds could find use in agriculture. ‘Some plants don’t do very well in strong sunlight because of UV damage. So could you spray something on their leaves to help?’