UVC and Planetary Habitability: Insights from Desert Lichens and Cyanobacteria

An important question in astrobiology is whether life can exist under the levels of UVC radiation found on Archean Earth, Mars, and exoplanets. We investigated this question by studying the UVC survival of colonial cyanobacterium and lichens that grow in harsh desert environments and possess natural sunscreens. We irradiated them nonstop in a desiccated state with a UVC flux closely representing the upper limit on those planets: 55-60 W/m². After irradiation, we rehydrated and assessed their survival using chlorophyll fluorometry and by counting the number of viable colonies that formed on agar plates. All three study organisms survived two to three months of UVC exposure. Additionally, two of the study organisms, colonial cyanobacterium and cyanolichen, were irradiated for nine days under metabolically active conditions (hydrated and under visible light) to UVC fluxes found on Archean Earth. The hydrated exposure resulted in a significant decline in photosynthetic activity in the colonial cyanobacterium after just one day; however, the cyanolichen showed no reduction in activity throughout the nine-day experiment. These survival results under both dry and wet irradiation, combined with further experiments to understand the source of UVC injury to lichens, suggest that surviving under high UVC fluxes may not be an issue for life. Future work studying the growth and reproduction potential of these organisms could suggest that the surface of Mars and exoplanets are not necessarily uninhabitable just because they are exposed to UVC radiation.