Survivability of the lichen Xanthoria parietina in simulated Martian environmental conditions

Abstract

Xanthoria parietina (L.) Th. Fr. is a widely spread foliose lichen showing high tolerance against UV-radiation thanks to parietin, a secondary lichen substance. We exposed samples of X. parietina under simulated Martian conditions for 30 days to explore its survivability. The lichen’s vitality was monitored via chlorophyll a fluorescence that gives an indication for active light reaction of photosynthesis, performing in situ and after-treatment analyses. Raman spectroscopy and TEM were used to evaluate carotenoid preservation and possible variations in the photobiont’s ultrastructure respectively. Significant differences in the photo-efficiency between UV irradiated samples and dark-kept samples were observed. Fluorescence values correlated with temperature and humidity day-night cycles. The photo-efficiency recovery showed that UV irradiation caused significant effects on the photosynthetic light reaction. Raman spectroscopy showed that the carotenoid signal from UV exposed samples decreased significantly after the exposure. TEM observations confirmed that UV exposed samples were the most affected by the treatment, showing chloroplastidial disorganization in photobionts’ cells. Overall, X. parietina was able to survive the simulated Mars conditions, and for this reason it may be considered as a candidate for space long-term space exposure and evaluations of the parietin photodegradability.

Freeze Substitution Accelerated via Agitation: New Prospects for Ultrastructural Studies of Lichen Symbionts and Their Extracellular Matrix

“…Accordingly, we did not observe any electron-lucent gaps between the cell wall and cytoplasm of the algae in the three more rapidly freezesubstituted lichen species (Figures S3 and S6). Therefore, we suggest verifying previous observations of "secretion zones" [11,17] or "secretory space" [1,12,13] by using FS to exclude the possibility that they are the result of structural damage.…”

“…Until now, the overwhelming majority of TEM data originate from lichen samples processed at ambient temperatures. Conventional chemical fixation and embedding protocols were adapted for a wide variety of lichen species, such as Parmelia sulcata [6]; Ramalina farinacea [8]; Pseudevernia furfuracea [9]; Usnea longissima and Ramalina menziesi [10]; Parmotrema pseudotinctorum [11,12]; Cladonia convoluta-C. foliacea [13]; Gyalectidium-paolae [14]; Ramalina fraxinea [2]; Xanthoria parietina [15][16][17]; Xanthoria elegans [18]; Sticta canariensis, Leptogium cyanescens and Endocarpon pusillum [19]; Circinaria gyrosa [20]; and Lobaria pulmonaria [21]. All of these publications share a common use of aldehydes as primary fixatives, sometimes alongside prolonged dehydration and infiltration, used frequently with low-viscosity resin to deal with the constraints of algal and fungal cell walls.…”