Insight into lichen physiology and biochemistry following exposure to rapid depressurization, vacuum conditions, and ionizing radiation-does the anhydrobiotic state and melanization protect lichens against cytotoxicity?

This study tested the responses of lichen Cetraria aculeata to simulated extraterrestrial conditions, including rapid depressurization and exposure to vacuum and ionizing radiation. We studied lichens in metabolically active and anhydrobiotic states, including melanized and pale thalli, to assess whether lichen desiccation tolerance and melanin presence contribute to stress resistance. The results indicate that lichens are more sensitive to stress when hydrated. Reduced cell viability and increased membrane damage indicate that membranes are the main structures affected by rapid desiccation; however, membrane disruption proved to be largely non-oxidative. We also observed profound disturbances in antioxidant systems and in lipid and osmotic metabolism causing impaired cellular homeostasis. Chlorophyll a content was significantly reduced, suggesting the destabilization of chlorophyll a-rich reaction centers. Despite extensive photobiont cell damage, we observed a gradual recovery of photosynthetic activity and chlorophyll resynthesis after rehydration, indicating largely reversible damage and the remarkable regenerative ability of photobiont cells. Algae in melanized thalli showed better photosynthetic recovery likely due to melanin-dependent changes in the architecture of the cortex layer enhancing algal protection. Since accidental decompression is a real risk in pressurized spacecraft systems, our results clarify how lichens respond to such events and delineate their consequences.