| Summary: | In cold and hot deserts, where life is pushed to its physical limits due to extreme water deficit and/or extreme temperatures, microorganisms escape life-limiting conditions by colonizing rocks. They form hypolithic biofilms at the stone-soil interface, or endolithic communities in the upper few millimeters to centimeters of rock (Golubic et al., 1981). Research of photosynthesis-based communities in deserts was pioneered by E. Imre Friedmann and Roseli Ocampo-Friedmann. They first described the Chroococcidiopsis-dominated communities in the Negev Desert, Israel (Friedmann et al., 1967). Later on, they described endolithic communities in the Dry Valleys (Friedmann and Ocampo, 1976), the largest ice-free region on the Antarctic continent, which was previously thought to be sterile (Horowitz et al., 1972). Their field campaigns continued in several deserts worldwide, leading to the establishment of the Culture Collection of Microorganisms from Extreme Environments. Today, this collection gathers about 250 desert strains from the genus Chroococcidiopsis and a few related genera. It is maintained at the University of Rome "Tor Vergata". The Atacama Desert, the driest non-polar desert in the world, is often referred to by astrobiologists as an analogue of Mars due to its environmental conditions. Studies based on this analogy suggest that if microhabitats exist (or have existed) on Mars, they will be difficult to detect as dispersed in virtually lifeless surroundings (Warren-Rhodes et al., 2006). Indeed, in the Atacama, hypolithic communities diminish along the aridity gradient. But even at the hyperarid core rare, Chroococcidiopsis-based communities exist, albeit in small spatially isolated islands amidst a microbially-impoverished soil (Warren-Rhodes et al. 2006). They colonize halite deposits (Stivaletta et al., 2012; Wierzchos et al., 2006), where they take advantage of halite deliquescence (Davila et al., 2013): inactive most of the time, microbial cells recover metabolic activity when relative humidity is high enough for halite crystals to form a saturated brine droplet with absorbed atmospheric water. The adaptability of Chroococcidiopsis spp. to extreme environments was further demonstrated in another desert used as a Mars analogue: the Mojave Desert. There, Chroococcidiopsis spp. isolated from different rock types (talc, marble, quartz, white carbonate and red-coated carbonate) were found to have shifted the photosynthetic pigment emissions, presumably as an adaptation to the rocks' different light transmission properties (Smith et al., 2014). The endurance of cyanobacteria isolated from desert lithic communities is currently tested in space and under simulated Mars-like conditions in low Earth orbit (LEO) outside the International Space Station; notably within the BOSS (Biofilm Organisms Surfing Space) and BIOMEX (BIOlogy and Mars EXperiment) experiments of the EXPOSE-R2 space mission (de Vera et al., 2012; Baqu et al. 2013b; 2013c). This endeavor is of prime importance to search for life beyond Earth Cottin et al., 2015). In parallel to those experiments, ground-based simulations of space and planetary environments are performed. Such experiments allow the astrobiology community to: i) understand the limits of life and potential habitability of the Solar System and beyond (Baglioni et al., 2007; Cockell et al., 2016); ii) identify suitable biosignatures to search for past or extant life on Mars (de Vera et al. 2012 ); iii) test the lithopanspermia theory, i.e. the possibility of interplanetary transport of life by means of material ejected by asteroid and meteorite impacts (Horneck et al., 2008; Nicholson, 2009; Stffler et al., 2007); iv) improve procedures for planetary protection, to avoid contamination of bodies of interest in our Solar System with terrestrial life via probes and rovers; and v) design life support systems for beyond-Earth settlements (Verseux et al., 2016a)
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