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.

Figure 1

On the left (a), samples of X. parietina used in this experiment before the 30 days exposure. First row: Full Mars (FM) samples; second row: Dark Mars (DM) samples; third row: External Control (EC) samples. On the right, PASLAB at Berlin DLR. (b) Opened climate chamber and opened experiment chamber. (c) Detail of the opened experiment chamber with the sample holders on the turntable. (d) Detail on the sample position 1 with the above Mini PAM optical fiber.

Figure 2

(a) Experiment configuration of the PASLAB facility. (b) On the left, experiment arrangement inside the experiment chamber, showing UV Xe-lamp optical fibers and Mini PAM light fiber connections. Gas connections are shown too. On the right, above vision on the turntable/rotating platform with the eight sample holders.

Figure 3

An instance over three days of the Mars-like diurnal cycle profile of temperature (red line) and relative humidity (blue line). The cyan line indicates the frost point temperature.

Figure 4

Above, temperature (red line) and humidity (blue line) day-night cycles as performed in the PASLAB at DLR Berlin. The light-blue/yellow stripe represents the day and night photoperiod related to the UV lamp on/off. Below, the cyan line represents the gas mixture humidity frost-point. The magenta stars represent the Yield values of Full Mars FM samples and the green dots represent the Yield values of Dark Mars DM samples. See Table S1 for ANOVA results.

Figure 5

(a) Variation of the PSII (Y = F_V/F_M) efficiency before (pre_exp), after (post_exp) and 24 h, 48 h, 72 h, 96 h, 168 h and 192 h after the treatment. Blue line = external control, EC; magenta line = full Mars, FM; green line = dark Mars, DM. Error bars stands for confidence intervals. See Table S3 for ANOVA results. (b) Fluorescence (F_V/F_M) imaging of three samples’ treatments (Full Mars, Dark Mars and External Controls) before (pre-exp.), after (post-exp.) and 24 h, 48 h, 72 h, 96 h, 168 h and 192 h after the treatment performed with Imaging PAM instrument. After simulation, small segments for TEM observations and Raman spectroscopy were cut from Full Mars and Dark Mars samples. Lichen material size and color bar values’ legend is reported on the bottom right of the image.

Figure 6

Above comparison between mean FM pre-exp. Raman spectra (black line) and mean FM post-exp. Raman spectra (red line). Below) comparison between mean DM pre-exp. Raman spectra (black line) and mean DM post-exp. Raman spectra (blue line).

Figure 7

TEM ultrastructural analysis of photobiont Trebouxia sp.; (a, b): Full Mars (FM); (c, d): Dark Mars (DM) and (e, f): External Controls (EC). Each image is reported with a reference size bar: (a) 1 µm; (b) 1 µm; (c) 2 µm; (d) 1 µm; (e) 2 µm; (f) 2 µm. Abbreviation symbol stand for Chl, chloroplast; CW, cell wall; Hy, hyphae; Pg, pyrenoglobuli; Ph, photobionts; PV, peripheral vesicles; Py, pyrenoid; S, starch granules; SZ, secretion zone; T, tubules and Th, thylakoid. Black arrows indicate electron-dense lipid droplets.