Endurance and biosignatures of the extremophilic cyanobacterium Chroococcidiopsis sp. under space and Martian simulations in the frame of the EXPOSE-R2 space mission (BOSS and BIOMEX)

Desert strains of the cyanobacterium Chroococcidiopsis are astrobiological model systems which have been used in several ground-based simulations and space exposures onboard the BIOPAN and EXPOSE facilities. In their natural environment they thrive at the dry limit of life in extremely arid, hot and cold deserts. In the arid part of the Mojave Desert, it moreover dominates the photosynthetic hypolithic community under different rock substrate. In these extreme environments Chroococcidiopsis cells survive in a dry, ametabolic state for prolonged periods and restore active metabolism upon rewetting. In addition, desert strains of Chroococcidiopsis withstand environmental stressors not currently met in nature, like ionizing-radiations as high as 15 kGy and UVC radiation up to 13 kJ/m² even when metabolically active. In the frame of two ESA experiments, Biofilm Organisms Surfing Space (BOSS) and BIOlogy and Mars Experiment (BIOMEX), currently in space part of the EXPOSE-R2 mission, Chroococcidiopsis strains have been exposed to ground-based simulations. In BOSS the hypothesis is tested if biofilm lifestyle is better suited to support long-term survival under space and Martian conditions than planktonic growth. While in BIOMEX the endurance of selected extremophiles mixed with lunar and Martian regolith analogues is tested. Chroococcidiopsis sp. strains CCMEE 057 and CCMEE 029 were exposed to space and Martian simulations in the dried status as biofilms, multilayered planktonic samples or mixed with mineral lunar and Mars analogues before drying. Biofilms of strains CCMEE 057 and CCMEE 029 exhibited a greater endurance than planktonic counterparts under space and Martian simulations (quantified by PMA assay and qPCR analysis). In the frame of BIOMEX a protective role played by Martian and lunar analogues was revealed by the enhanced survival of planktonic dried cells of strain CCMEE 029 after polychromatic UV irradiation. Moreover, it guaranteed photosynthetic and photoprotective pigments and DNA permanence as potential biomarkers revealed by Confocal Laser Scanning Microscopy, Raman spectroscopy and PCR assays. Thus, Chroococcidiopsis might tolerate one-year exposure in space during the next EXPOSE-R2 mission, contributing to our knowledge on the survival potential of phototrophic biofilms and to the identification of biosignatures for searching life beyond Earth.