Survival of Xanthoria parietina in simulated space conditions: spectroscopic analysis and vitality assessment

One of the main topics of astrobiology is the study of life limits in stressful environments. This field of research has the aim to understand the physiological and biochemical effects on unprotected biological samples in extreme conditions, such as space. Moreover, these studies provide indications about organisms’ adaptive plasticity under a climate change perspective, the terrestrial geological past and future scenarios, as well as extra-terrestrial habitats as Mars surface.

The biological specimen chosen for this study was Xanthoria parietina (L.) Th. Fr. It is a widespread foliose lichen growing on bark and rocks which has a broad spectrum of tolerance to air pollutants such as NO_X and heavy metals, and resistance to UV-radiation because of the screening properties provided by the secondary metabolism product parietin. In this study we evaluated the ability of this lichen specie to survive under simulated UV space radiation in two different extreme environments i.e., in N₂ atmosphere (N₂) and in vacuum (10⁰~10^-2 Pa) (VAC).

Thalli of X. parietina were randomly collected in a remote area of Tuscany, Italy in June and July 2020. Thalli were dehydrated for 24 h at room temperature (25°C) and stored at -18°C until treatment. Three days before the treatment, thalli were allowed to recover their normal metabolic conditions in a growth chamber at 25 °C and 70 μmol m^-2 s^-1 PAR photons. Overnight, thalli were covered with a black cotton cloth and kept moistened by spraying with distilled water.

The simulated UV space radiation was produced using a Xe-enhanced UV lamp with a sun-like emission spectrum (wavelength range 185-2000 nm). The aforementioned atmospheric conditions (N₂ and VAC) were chosen to set up an extreme and dehydrating environment for the lichen. The total absorbed UV radiation dose was 1.34 MJ m^-2 for each exposed sample. During the irradiation, the IR reflectance spectrum of the lichen was monitored in situ with infrared spectroscopy to assess changes in spectral bands.

The efficiency of the photosynthetic apparatus was assessed as indicator of vitality, and was expressed in terms of chlorophyll a fluorescence (F_V/F_M) and Normalized Difference Vegetation Index (NDVI). The examination of X. parietina recovery through eco-physiological analysis revealed the capacity of this lichen species to survive in extreme conditions such as those simulated in this investigation. It has been highlighted the significant difference between treatments about the photosynthetic efficiency parameters recovery trends, finding that UV-radiation in vacuum produces more intense effects on F_V/F_M values. After 72h, UV N₂ fluorescence mean values recovered up to 93% of the starting ones, while UV VAC fluorescence recovered up to 45% of the pre-exposure values. The IR analysis revealed several spectral band changes in the fingerprint region. The most visible variation was the 5200 cm^-1 water band, disappearing in the overtone region. This analysis suggests that the disappearance of H₂O band after treatment is strictly linked to the thalli dehydration due to the atmospheric simulated conditions represented by N₂ insufflation and high vacuum application. Nevertheless, X. parietina was able to survive to UV-radiation in N₂ atmosphere and in vacuum, and for this reason it may be considered a candidate for further evaluations on its survival capacity in extreme conditions.