Survivability of the lichen Xanthoria parietina in simulated Martian environmental conditions

UV photo-degradation of the secondary lichen substance parietin: A multi-spectroscopic analysis in astrobiology perspective

The cortical anthraquinone yellow-orange pigment parietin is a secondary lichen substance providing UV-shielding properties that is produced by several lichen species. In our work, the secondary metabolite has been extracted from air-dried thalli of Xanthoria parietina. The aims of this study were to characterize parietin absorbance through UV-VIS spectrophotometry and with IR spectroscopy and to evaluate its photodegradability under UV radiation through in situ reflectance IR spectroscopy to understand to what extent the substance may have a photoprotective role. This allows us to relate parietin photo-degradability to the lichen UV tolerance in its natural terrestrial habitat and in extreme environments relevant for astrobiology such as Mars. Extracted crystals were UV irradiated for 5.59 h under N2 flux. After the UV irradiation, we assessed relevant degradations in the 1614, 1227, 1202, 1160 and 755 cm-1 bands. However, in light of Xanthoria parietina survivability in extreme conditions such as space- and Mars-simulated ones, we highlight parietin UV photo-resistance and its relevance for astrobiology as photo-protective substance and possible bio-hint.

Passive biomonitoring of airborne microplastics using lichens: A comparison between urban, natural and protected environments

Currently, natural and urban ecosystems are affected by different types of atmospheric deposition, which can compromise the balance of the environment. Plastic pollution represents one of the major threats for biota, including lichens. Epiphytic lichens have value as bioindicators of environmental pollution, climate change, and anthropic impacts. In this study, we aim to investigate the lichen bioaccumulation of airborne microplastics along an anthropogenic pollution gradient. We sampled lichens from the Genera Cladonia and Xanthoria to highlight the effectiveness of lichens as tools for passive biomonitoring of microplastics. We chose three sites, a "natural site" in Altipiani di Arcinazzo, a "protected site" in Castelporziano Presidential estate and an "urban site" in the centre of Rome. Overall, we sampled 90 lichens, observed for external plastic entrapment, melt in oxygen peroxide and analysed for plastic entrapment. To validate the method, we calculated recovery rates of microplastics in lichen. Particularly, 253 MPs particles were detected across the 90 lichen samples: 97 % were fibers, and 3 % were fragments. A gradient in the number of microplastic fibers across the sites emerged, with increasing accumulation of microplastics from the natural site (n = 58) to the urban site (n = 116), with a direct relationship between the length and abundance of airborne microplastic fibers. Moreover, we detected the first evidences of airborne mesoplastics entrapped by lichens. On average, the natural site experienced the shortest fibre length and the centre of Rome the longest. No differences in microplastics accumulation emerged from the two genera. Our results indicated that lichens can effectively be used for passive biomonitoring of microplastic deposition. In this scenario, the role of lichens in entrapping microplastics and protecting pristine areas must be investigated. Furthermore, considering the impact that airborne microplastics can have on human health and the effectiveness of lichens as airborne microplastic bioindicators, their use is encouraged.

Resilience of Xanthoria parietina under Mars-like conditions: photosynthesis and oxidative stress response

MAIN CONCLUSION

Xanthoria parietina survivability in Mars-like conditions was supported by water-lysis efficiency recovery and antioxidant content balancing with ROS production after 30 days of exposure. Xanthoria parietina (L.) Th. Fr. is a widespread lichen showing tolerance against air pollutants and UV-radiation. It has been tested under space-like and Mars-like conditions resulting in high recovery performances. Hereby, we aim to assess the mechanisms at the basis of the thalli resilience against multiple space stress factors. Living thalli of X. parietina were exposed to simulated Martian atmospheric conditions (Dark Mars) and UV radiation (Full Mars). Then, we monitored as vitality indicator the photosynthetic efficiency, assessed by in vivo chlorophyll emission fluorescence measurements (FM; FV/F0). The physiological defense was evaluated by analyzing the thalli antioxidant capacity. The drop of FM and FV/F0 immediately after the exposure indicated a reduction of photosynthesis. After 24 h from exposure, photosynthetic efficiency began to recover suggesting the occurrence of protective mechanisms. Antioxidant concentrations were higher during the exposure, only decreasing after 30 days. The recovery of photosynthetic efficiency in both treatments suggested a strong resilience by the photosynthetic apparatus against combined space stress factors, likely due to the boosted antioxidants at the beginning and their depletion at the end of the exposure. The overall results indicated that the production of antioxidants, along with the occurrence of photoprotection mechanisms, guarantee X. parietina survivability in Mars-like environment.