The question of whether life existed on Mars remains open to this day, and its current environment with intense radiation and a thin atmosphere does not seem at all favorable for life. But there is a theory that terrestrial lichens could survive there. And recently it was confirmed.
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Lichens are symbionts, i.e. two organisms in a mutually beneficial relationship: fungi (up to 90%) and photosynthetic components (algae or cyanobacteria). To determine the ability of some lichens to survive on Mars, a group of researchers led by Associate Professor of the Jagiellonian University Kaja Skubała, with the support of the Space Research Centre of the Polish Academy of Sciences, simulated Martian conditions for the species Diploschistes muscorum and Cetrarea aculeata.
Ionizing radiation on Mars is a threat to most life forms because it causes damage at the cellular level; it can interfere with physical, genetic, morphological, and biochemical processes depending on the organism and the level of radiation. Lichens have the advantage of having a low metabolism, little nutritional requirement, and longevity. Lichens, like tardigrades, can remain dry for long periods of time until they absorb water; they also have metabolic products that protect against ultraviolet rays and melanin pigments that protect against radiation.
The effects of UV light on lichens have been studied before, so the researchers decided to expose them to ionizing radiation when they are active – lichens need water to maintain their metabolism, so they were sprayed with water during the experiment. Each species spent five hours in a dark chamber that simulated Martian conditions: low pressure, low humidity, an atmosphere made mostly of carbon dioxide, and temperatures ranging from 18°C (64°F) during the day to -26°C (-10°F) at night. X-ray levels in the chamber were set to match those on the surface of Mars during periods of intense solar activity, although solar flares and variations in the solar wind make actual conditions on the planet unpredictable.
When the lichens were removed from the simulated Martian environment, the scientists found that both species retained some moisture despite the low moisture content of the environment, suggesting that some metabolic activity was occurring in both the fungal and photosynthetic components. Previously, ionizing radiation had only been tested on the photosynthetic component, not the fungal component. If a lichen is not dehydrated, it is more susceptible to damage from ionizing radiation. Both the fungal and algal cells of metabolically active lichens have repair mechanisms, but Diploschistes muscorum was much more resistant to radiation than Cetrarea aculeata.
Other Martian conditions, such as a carbon dioxide-dominated atmosphere, may also affect lichen metabolism, but not completely stop it. The fungal component requires oxygen to produce carbohydrates, but the processes continued even with small amounts of the latter. The researchers even suggested that the oxygen could be produced by the photosynthetic part of the lichens, and that the fungal components used it. Surprisingly, photosynthesis did not show much sensitivity to X-rays in dark conditions. Measurements of chlorophyll concentration using fluorescence imaging showed that the photosynthetic component of Diploschistes muscorum remained viable throughout the experiment, while Cetrarea aculeata showed a decrease in chlorophyll under the influence of X-rays. Both lichens froze after the experiment, but after defrosting they restored photosynthesis, with Cetrarea aculeata quickly restoring its original chlorophyll level.
To draw a conclusion about the ability of lichens to survive on Mars, Associate Professor Skubala suggests conducting additional research: to determine all the features and adaptation mechanisms that allow them to survive in conditions of intense ionizing radiation. Why Diploschistes muscorum turned out to be more effective in mitigating damage from radiation is not yet fully known. According to the results of the experiment, it was found that during its exposure, the concentration of antioxidants increased, especially glutathione – it is able to limit cellular damage in humans and other organisms. It helped the lichen survive ionizing radiation, but this does not mean that it can protect humans.