Simulated Space Radiation: Impact of Four Different Types of High-Dose Ionizing Radiation on the Lichen Xanthoria elegans

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.

Survivability of the lichen Xanthoria parietina in simulated Martian environmental conditions

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.

Insights into the Survival Capabilities of Cryomyces antarcticus Hydrated Colonies after Exposure to Fe Particle Radiation

The modern concept of the evolution of Mars assumes that life could potentially have originated on the planet Mars, possibly during the end of the late heavy bombardment, and could then be transferred to other planets. Since then, physical and chemical conditions on Mars changed and now strongly limit the presence of terrestrial-like life forms. These adverse conditions include scarcity of liquid water (although brine solutions may exist), low temperature and atmospheric pressure, and cosmic radiation. Ionizing radiation is very important among these life-constraining factors because it damages DNA and other cellular components, particularly in liquid conditions where radiation-induced reactive oxidants diffuse freely. Here, we investigated the impact of high doses (up to 2 kGy) of densely-ionizing (197.6 keV/µm), space-relevant iron ions (corresponding on the irradiation that reach the uppermost layer of the Mars subsurface) on the survival of an extremophilic terrestrial organism—Cryomyces antarcticus—in liquid medium and under atmospheric conditions, through different techniques. Results showed that it survived in a metabolically active state when subjected to high doses of Fe ions and was able to repair eventual DNA damages. It implies that some terrestrial life forms can withstand prolonged exposure to space-relevant ion radiation.

Potential survival of the lichen Caloplaca flavovirescens under high helium-beam doses

Testing the limits of survivability in space is the primary focus in astrobiological research. Although a number of previous studies have examined terrestrial life survival in an extraterrestrial environment, only a few have investigated how life systems respond to high doses of alpha cosmic ray, the main component of cosmic rays. We used respiration and photosynthetic rates as indicators of the vital signs of the lichen Caloplaca flavovirescens, which is a symbiotic life form including fungi and algae. Our experiment demonstrated that the photosynthetic rate decreased with increased helium-beam doses, whereas the respiration rate was relatively unaffected. Specifically, under a helium-beam dose greater than 10 Gy, the respiration rate remained nearly constant regardless of further increases in the radiation rate. Our results indicate that the different metabolic systems of terrestrial life forms might exhibit different survival characteristics when they are in space.

Mosses in Low Earth Orbit: Implications for the Limits of Life and the Habitability of Mars

As a part of the European Space Agency mission "EXPOSE-R2" on the International Space Station (ISS), the BIOMEX (Biology and Mars Experiment) experiment investigates the habitability of Mars and the limits of life. In preparation for the mission, experimental verification tests and scientific verification tests simulating different combinations of abiotic space- and Mars-like conditions were performed to analyze the resistance of a range of model organisms. The simulated abiotic space- and Mars-stressors were extreme temperatures, vacuum, and Mars-like surface ultraviolet (UV) irradiation in different atmospheres. We present for the first time simulated space exposure data of mosses using plantlets of the bryophyte genus Grimmia, which is adapted to high altitudinal extreme abiotic conditions at the Swiss Alps. Our preflight tests showed that severe UVR_200-400nm irradiation with the maximal dose of 5 and 6.8 × 10⁵ kJ·m^-2, respectively, was the only stressor with a negative impact on the vitality with a 37% (terrestrial atmosphere) or 36% reduction (space- and Mars-like atmospheres) in photosynthetic activity. With every exposure to UVR_200-400nm 10⁵ kJ·m^-2, the vitality of the bryophytes dropped by 6%. No effect was found, however, by any other stressor. As the mosses were still vital after doses of ultraviolet radiation (UVR) expected during the EXPOSE-R2 mission on ISS, we show that this earliest extant lineage of land plants is highly resistant to extreme abiotic conditions.

The Effect of High-Dose Ionizing Radiation on the Astrobiological Model Lichen Circinaria gyrosa

The lichen Circinaria gyrosa is an astrobiological model defined by its high capacity of resistance to space conditions and to a simulated martian environment. Therefore, it became part of the currently operated BIOMEX experiment on board the International Space Station and the recent STARLIFE campaign to study the effects of four types of space-relevant ionizing radiation. The samples were irradiated with helium and iron ions at doses up to 2 kGy, with X-rays at doses up to 5 kGy and with γ rays at doses from 6 to 113 kGy. Results on C. gyrosa's resistance to simulated space ionizing radiation and its post-irradiation viability were obtained by (i) chlorophyll a fluorescence of photosystem II (PSII), (ii) epifluorescence microscopy, (iii) confocal laser scanning microscopy (CLSM), and (iv) field emission scanning electron microscopy (FESEM). Results of photosynthetic activity and epifluorescence show no significant changes up to a dose of 1 kGy (helium ions), 2 kGy (iron ions), 5 kGy (X-rays)-the maximum doses applied for those radiation qualities-as well as a dose of 6 kGy of γ irradiation, which was the lowest dose applied for this low linear energy transfer (LET) radiation. Significant damage in a dose-related manner was observed only at much higher doses of γ irradiation (up to 113 kGy). These data corroborate the findings of the parallel STARLIFE studies on the effects of ionizing radiation on the lichen Circinaria gyrosa, its isolated photobiont, and the lichen Xanthoria elegans. Key Words: Simulated space ionizing radiation-Gamma rays-Extremotolerance-Lichens-Circinaria gyrosa-Photosynthetic activity. Astrobiology 17, 145-153.

The Effect of High-Dose Ionizing Radiation on the Isolated Photobiont of the Astrobiological Model Lichen Circinaria gyrosa

Lichen symbioses between fungi and algae represent successful life strategies to colonize the most extreme terrestrial habitats. Consequently, space exposure and simulation experiments have demonstrated lichens' high capacity for survival, and thus, they have become models in astrobiological research with which to discern the limits and limitations of terrestrial life. In a series of ground-based irradiation experiments, the STARLIFE campaign investigated the resistance of astrobiological model organisms to galactic cosmic radiation, which is one of the lethal stressors of extraterrestrial environments. Since previous studies have identified that the alga is the more sensitive lichen symbiont, we chose the isolated photobiont Trebouxia sp. of the astrobiological model Circinaria gyrosa as a subject in the campaign. Therein, γ radiation was used to exemplify the deleterious effects of low linear energy transfer (LET) ionizing radiation at extremely high doses up to 113 kGy in the context of astrobiology. The effects were analyzed by chlorophyll a fluorescence of photosystem II (PSII), cultivation assays, live/dead staining and confocal laser scanning microscopy (CLSM), and Raman laser spectroscopy (RLS). The results demonstrate dose-dependent impairment of photosynthesis, the cessation of cell proliferation, cellular damage, a decrease in metabolic activity, and degradation of photosynthetic pigments. While previous investigations on other extraterrestrial stressors have demonstrated a high potential of resistance, results of this study reveal the limits of photobiont resistance to ionizing radiation and characterize γ radiation-induced damages. This study also supports parallel STARLIFE studies on the lichens Circinaria gyrosa and Xanthoria elegans, both of which harbor a Trebouxia sp. photobiont. Key Words: Astrobiology-Gamma rays-Extremotolerance-Ionizing radiation-Lichens-Photobiont. Astrobiology 17, 154-162.

STARLIFE-An International Campaign to Study the Role of Galactic Cosmic Radiation in Astrobiological Model Systems

In-depth knowledge regarding the biological effects of the radiation field in space is required for assessing the radiation risks in space. To obtain this knowledge, a set of different astrobiological model systems has been studied within the STARLIFE radiation campaign during six irradiation campaigns (2013-2015). The STARLIFE group is an international consortium with the aim to investigate the responses of different astrobiological model systems to the different types of ionizing radiation (X-rays, γ rays, heavy ions) representing major parts of the galactic cosmic radiation spectrum. Low- and high-energy charged particle radiation experiments have been conducted at the Heavy Ion Medical Accelerator in Chiba (HIMAC) facility at the National Institute of Radiological Sciences (NIRS) in Chiba, Japan. X-rays or γ rays were used as reference radiation at the German Aerospace Center (DLR, Cologne, Germany) or Beta-Gamma-Service GmbH (BGS, Wiehl, Germany) to derive the biological efficiency of different radiation qualities. All samples were exposed under identical conditions to the same dose and qualities of ionizing radiation (i) allowing a direct comparison between the tested specimens and (ii) providing information on the impact of the space radiation environment on currently used astrobiological model organisms. Key Words: Space radiation environment-Sparsely ionizing radiation-Densely ionizing radiation-Heavy ions-Gamma radiation-Astrobiological model systems. Astrobiology 17, 101-109.