Mathematics – Logic
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..897b&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Mathematics
Logic
Scientific paper
The environment in space and on planets such as Mars, can be lethal to living organisms and high levels of tolerance to desiccation, cold and radiation are needed for survival: rock-inhabiting cyanobacteria belonging to the genus Chroococcidiopsis can fulfil these requirements [1]. These cyanobacteria constantly appear in the most extreme and dry habitats on Earth, including the McMurdo Dry Valleys (Antarctica) and the Atacama Desert (Chile), which are considered the closest terrestrial analogs of two Mars environmental extremes: cold and aridity. In their natural environment, these cyanobacteria occupy the last refuges for life inside porous rocks or at the stone-soil interfaces, where they survive in a dry, dormant state for prolonged periods. How desert strains of Chroococcidiopsis can dry without dying is only partially understood, even though experimental evidences support the existence of an interplay between mechanisms to avoid (or limit) DNA damage and repair it: i) desert strains of Chroococcidiopsis mend genome fragmentation induced by ionizing radiation [2]; ii) desiccation-survivors protect their genome from complete fragmentation; iii) in the dry state they show a survival to an unattenuated Martian UV flux greater than that of Bacillus subtilis spores [3], and even though they die following atmospheric entry after having orbited the Earth for 16 days [4], they survive to simulated shock pressures up to 10 GPa [5]. Recently additional experiments were carried out at the German Aerospace Center (DLR) of Cologne (Germany) in order to identify suitable biomarkers to investigate the survival of Chroococcidiopsis cells present in lichen-dominated communities, in view of their direct and long term space exposition on the International Space Station (ISS) in the framework of the LIchens and Fungi Experiments (LIFE, EXPOSEEuTEF, ESA). Multilayers of dried cells of strains CCMEE 134 (Beacon Valley, Antarctica), and CCMEE 123 (costal desert, Chile ), shielded by 3 mm of Antarctic sandstone, were exposed in EVT-E1 to vacuum conditions (10-5Pa for 1 week), 50 freezethaw cycles (-20 to 20 °C for 2 weeks), UV-C (254 nm, at 10, 100 and 1000 J/ m2) and total UV (200-400 nm, at 1.5, 1.5x103, and 1.5x105 kJ/ m2). In EVT-E2 samples were tested in CO2 Mars atmosphere in presence and absence of total UV (200-400 nm, at 1.5x105 kJ/ m2). In exposed cells subcellular damage was evaluated in situ by using membrane integrity and redox related probes along and by evaluating autofluorescence of the photosynthetic pigments. While the genome was evaluated by assessing its suitability as template in polymerase chain reaction (PCR)-based assays, e.g. random amplification of polymorphic DNA (RAPD) with primers derived from repetitive sequences present in cyanobacterial genomes and gene locus amplification. In addition, the colony forming ability of exposed cells was evaluated. Chroococcidiopsis CCMEE 123 survived to all EVTE1 and EVT-E2 as suggested by intact plasmatic membrane integrity, unbleached pigments, respiration capability upon rewetting and colony forming ability. Nevertheless, DNA damage occurred in cells exposed to vacuum, freeze-thaw cycles, 1000 J/m2 (UV-C) and 1.5x103 and 1.5x105 kJ/ m2 (total UV radiation), as revealed by the failure of PCR-based assays. Whereas virtually identical PCR profiles were yielded from controls and cells exposed to lower doses of UV-C radiation (10 and 100 J/m2 ) and total UV radiation (1.5 kJ/m2). Chroococcidiopsis CCMEE 134 exposed to EVT-E1 and EVT-E2 exhibited membrane integrity, photosynthetic pigments autofluorescence and dehydrogenase activity, but were unable to form colonies. Moreover, only cells exposed to 10 and 100 J/m2 (UV-C) were positive to PCR-based analysis, although they yielded altered RAPD profiles. Even though the synergistic action of vacuum and UV radiation was not investigated, these results highlighted an extraordinary survival potential of dried cells of Chroococcidiopsis isolated from hot desert after prolonged exposure to space and Martian conditions if shielded by few mm of rocky material. In addition, these results further corroborate evidences for the existence in Chroococcidiopsis of mechanisms to both avoid (or limit) and repair DNA damage, which must take place, not only during its prolonged dry storage - when oxidative processes continue even in absence of metabolic activity - but also when dried cells experience additional environmental stressors, including those present in space or on Mars. Indeed, unravelling the DNA repair systems in a desiccation-, radiation- tolerant desert strain of Chroococcidiopsis is the task of ongoing researches at Department of Biology, Università of Rome "Tor Vergata", carried out in the framework of the MoMa project (ASI). Hence, in order to overcome impairments due to the lack of its genome sequence, two genetic approaches were developed, which take advantage of sequenced cyanobacterial genomes. The first one aims to the screening of a prey genomic library of Chroococcidiopsis by using DNA repair baits obtained from Synechocystis PCC 6803. While the second one aims to identify DNA repair genes in the Chroococcidiopsis genome by using evolutionary PCR. Finally, in order to visualize DNA repair factories in Chroococcidiopsis, a GFP-tagging genetic system was developed (Fig. 1). These efforts will contribute to future astrobiological experiments by providing Chroococcidiopsis DNA repair mutants and by offering a real-time monitoring of DNA damaging conditions by using proper GFP fusions.
Billi Daniela
Cockell Charles S.
Ghelardini P.
Horneck Gerda
Onofri Silvano
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