Extremotolerance and Resistance of Lichens: Comparative Studies on Five Species Used in Astrobiological Research I. Morphological and Anatomical Characteristics

Abstract

Lichens are symbioses of two organisms, a fungal mycobiont and a photoautotrophic photobiont. In nature, many lichens tolerate extreme environmental conditions and thus became valuable models in astrobiological research to fathom biological resistance towards non-terrestrial conditions; including space exposure, hypervelocity impact simulations as well as space and Martian parameter simulations. All studies demonstrated the high resistance towards non-terrestrial abiotic factors of selected extremotolerant lichens. Besides other adaptations, this study focuses on the morphological and anatomical traits by comparing five lichen species-Circinaria gyrosa, Rhizocarpon geographicum, Xanthoria elegans, Buellia frigida, Pleopsidium chlorophanum-used in present-day astrobiological research. Detailed investigation of thallus organization by microscopy methods allows to study the effect of morphology on lichen resistance and forms a basis for interpreting data of recent and future experiments. All investigated lichens reveal a common heteromerous thallus structure but diverging sets of morphological-anatomical traits, as intra-/extra-thalline mucilage matrices, cortices, algal arrangements, and hyphal strands. In B. frigida, R. geographicum, and X. elegans the combination of pigmented cortex, algal arrangement, and mucilage seems to enhance resistance, while subcortex and algal clustering seem to be crucial in C. gyrosa, as well as pigmented cortices and basal thallus protrusions in P. chlorophanum. Thus, generalizations on morphologically conferred resistance have to be avoided. Such differences might reflect the diverging evolutionary histories and are advantageous by adapting lichens to prevalent abiotic stressors. The peculiar lichen morphology demonstrates its remarkable stake in resisting extreme terrestrial conditions and may explain the high resistance of lichens found in astrobiological research.

Effects of UVC254 nm on the photosynthetic activity of photobionts from the astrobiologically relevant lichens Buellia frigida and Circinaria gyrosa

“…In conclusion, the present study stresses the higher susceptibility of photobionts towards extreme environmental conditions as UVC-exposure, a stressor that does not occur on the Earth. By comparison with previous studies, the present results highlight the importance of protective mechanisms in lichens, such as morphological-anatomical traits (Meeßen et al 2013), secondary lichen compounds ) and the symbiont's pivotal ability to pass into anhydrobiosis when desiccating. …”

Space as a Tool for Astrobiology: Review and Recommendations for Experimentations in Earth Orbit and Beyond

“…SURVIVAL I-III, LICHENS and LITHOPANSPERMIA on Biopan Different methods applied to detect the vitality of the investigated organisms have shown a wide and significant level of survival after being exposed to space conditions (Meessen et al 2013;.…”

Resistance of the LichenBuellia frigidato Simulated Space Conditions during the Preflight Tests for BIOMEX—Viability Assay and Morphological Stability

“…Focusing on lichens, symbiotic organisms that consist of a fungus (mycobiont) and a photoautotrophic partner (photobiont), revealed a remarkable and consistent resistance toward nonterrestrial conditions (see references above). Such resistance toward (simulated) space conditions is attributed to a range of morphological adaptations (Meeßen et al, 2013), a set of protective secondary compounds (Meeßen et al, 2014a) and their ability to pass into anhydrobiosis, an ametabolic state when desiccated (Ertl, 1951;Crowe et al, 1992Crowe et al, , 1998Kranner et al, 2005). To substantiate these findings and assess the patterns of degradation and damage organisms may suffer during space exposure, the current BIOMEX mission again exposes two extremotolerant lichen species to space and simulated Mars conditions.…”

“…To substantiate these findings and assess the patterns of degradation and damage organisms may suffer during space exposure, the current BIOMEX mission again exposes two extremotolerant lichen species to space and simulated Mars conditions. Both lichen species, the Antarctic endemite Buellia frigida and the vagrant lichen Circinaria gyrosa, were previously subjected to several astrobiological studies (de la Torre et al, 2010;Sánchez et al, 2012Sánchez et al, , 2014Meeßen et al, 2013Meeßen et al, , 2014aMeeßen et al, , 2014bBackhaus et al, 2015).…”

“…By colonizing such niches, B. frigida displays its adaptation to extremely harsh environmental conditions as an extremotolerant organism. The peculiar morphological and anatomical traits of B. frigida as well as its secondary lichen compounds are described elsewhere (Meeßen et al, 2013(Meeßen et al, , 2014a. For the present study, B. frigida was collected by S. Ott in 2009/ 2010 in the vicinity of the German Gondwana Station at Gerlache Inlet (74°38¢S, 164°13¢E), in North Victoria Land.…”