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The Atacama Desert (Chile), one of the most arid places on Earth, shows hostile conditions for the development of epilithic microbial communities. In this study, we report the association of cyanobacteria (Chroococcidiopsis sp.) and bacteria belonging to Actinobacteria and Beta-Gammaproteobacteria and Firmicutes phyla inhabiting the near surface of salt (halite) deposits of the Salar Grande Basin, Atacama Desert (Chile). The halite deposits were investigated by using optical, confocal and field emission scanning electron microscopes, whereas culture-independent molecular techniques, 16S rDNA clone library, alongside RFLP analysis and 16S rRNA gene sequencing were applied to investigate the bacterial diversity. These microbial communities are an example of life that has adapted to extreme environmental conditions caused by dryness, high irradiation, and metal concentrations. Their adaptation is, therefore, important in the investigation of the environmental conditions that might be expected for life outside of Earth.

This paper investigates endolithic microorganisms in modern gypsum crusts located in arid and saline areas of southern Tunisia. In arid regions, the endolithic mode of life facilitates the survival of the microorganisms and protects them from extreme environmental conditions. The gypsum crusts are the product of evaporite precipitation atop spring mounds developed at the margins of the Chott el Jerid, a saline lake close to the Algerian border. The early mineral precipitation has prevented the complete degradation of these microorganisms and their metabolic products because of the rapid sealing provided by the gypsum crusts. The spring mound structures of the Chott el Jerid offer an opportunity to study the endolithic communities in evaporite and arid environments. An understanding of where and how life survives in such extreme conditions, and how life signal prints in the rocks may help in search for life in the geological record and for astrobiological purposes.

This study investigates the geomicrobiological potential of Upper Pleistocene evaporite deposits of the Chott el Gharsa, a wide continental sabkha in southern Tunisia. Organic and inorganic-derived biosignatures are mostly contained in microcrystalline, laminated gypsum lithofacies consisting of light/dark alternations of concordant laminae, which have precipitated from high salt concentrated waters. These biosignatures include mineralized microbial-interpreted morphologies, such as mucilage, rods, and microfibers, and dumbbell morphologies in the hollow cores of dolomite crystals that are associated with sulfates. Mineral products that are induced by microbial activity and their organic compounds lead to the formation of lenticular-shaped gypsum crystals, with a high length/width ratio, dolomite precipitation and formation of pyrite framboids. Morphological and structural aspects of these biosignatures, and their composition, in laminated, dolomite-rich sulfate deposits could be detected through microscopic investigations and micro-analyses performed by the instrumentation that is planned for ongoing Mars sample return missions.

At El Tatio hydrothermal field, located at more than 4,200 meters above mean sea level in the Andean Cordillera (Chile), massive and poorly coherent pyroclastic, rhyolitic ignimbrites make up most of the local substrate. In these rocks, light colored amorphous silica precipitated to form different types of deposits at geyser vents and, also, during the digression of braided streams departing from hot spring pools. The rapid sinter precipitation enabled the preservation of a variety of biotic and abiotic features along beds and margins of the streams in deeply weathered ignimbrites. Biofilms and filamentous communities provided a favorable substrate for silica precipitation and, together with the adhesive properties of diatom mucilage and other mineral precipitation, also played a primary role for holding together a porous and brittle ignimbrite rock. Optical and scanning electron microscopies associated with molecular methods based on the amplification and sequencing of small subunit (SSU) rRNA genes were carried out for a comparative evaluation of the microbial diversity associated with the silicified structures. The comparative study of the microbial diversity and the taphofacies preserved at El Tatio has allowed us to define some of the interactions between colloidal silica and microbes in a hydrothermally-influenced environment that, because of the high altitude, undergoes intense ultraviolet radiation and surface oxidation processes.

Sulfates, commonly found in evaporite deposits, were observed on Mars surface during orbital remote sensing and surface exploration. In terrestrial environments, evaporite precipitation creates excellent microniches for microbial colonization, especially in desert areas. Deposits comprised of gypsum, calcite, quartz and silicate deposits (phyllosilicates, feldspars) from Sahara Desert in southern Tunisia contain endolithic colonies just below the rock surface. Previous optical observations verified the presence of microbial communities and, as described in this paper, spectral visible analyses have led to identification of chlorophylls belonging to photosynthetic bacteria. Spectral analyses in the infrared region have clearly detected the presence of gypsum and phyllosilicates (mainly illite and/or smectite), as well as traces of calcite, but not quartz. X-ray diffraction (XRD) analysis has identified the dominant presence of gypsum as well as that of other secondary minerals such as quartz, feldspars and Mg–Al-rich phyllosilicates, such as chlorite, illite and smectite. The occurrence of a small quantity of calcite in all the samples was also highlighted by the loss of CO 2 by thermal analysis (TG–DTA). A normative calculation using XRD, thermal data and X-ray fluorescence (XRF) analysis has permitted to obtain the mineralogical concentration of the minerals occurring in the samples. The combination of multiple techniques provides information about the mineralogy of rocks and hence indication of environments suitable for supporting microbial life on Mars surface.

In the current decade, two Martian missions are planned by NASA and ESA having as primary target the search for possible signs of past or present life: Mars Science Laboratory (MSL), which is part of NASA’s Mars Exploration Program, and the ExoMars of ESA’s Aurora Programme. The reasons that make Mars of special interest from an astrobiological perspective include its nearness, some of the present-day physical characteristics of the planet’s surface, and its geological history. Mars seems to have experienced earthlike conditions in its geological past, with lots of liquid water (Squyres and Kasting, 1994; Hynek and Phillips, 2003; Baker, 2006) that was able to produce the depositional processes and the erosional features described in different regions. Recently formed water features have also been described (e.g., gullies on cliffs and crater walls, Malin and Edgett, 2000), and they suggest that near surface, liquid water may episodically be present currently. Other conditions suitable for life (for example: a warmer climate) likely characterized the planet during the earlier phases of its geological history. Periods with a possible robust greenhouse warming may have taken place in the early Mars, during the Late Noachian-Hesperian period, through the combined effect in the atmosphere of gases, such as CO2, NH3, and CH4, which might have maintained a surface temperature above the freezing point of water (Beaty et al., 2005). The finding – by rovers and, remotely, by orbiters and spectrometers – of salt- (especially hydrated sulfate) rich deposits in different areas of the Martian surface (Squyres et al., 2004; Vaniman et al., 2004; Gendrin et al., 2005; Langevin et al., 2005) is a further indication of past aqueous processes.

The Salar de Atacama, located in Northern Chile, is a wide salt flat that is characterized by several salt lakes, which are locally called lagunas. The Laguna de la Piedra is one of the salt lake systems that is located in the northernmost sector of the Salar de Atacama. The present paper examines some physicochemical properties of the Laguna de la Piedra as well as the microbial diversity of the evaporite deposits. Under extreme desiccation and ambient UV flux, the evaporite deposits can create favorable endolithic microniches for the development of microorganisms. In the Laguna de la Piedra these deposits host a variety of halophilic microorganisms, which were investigated by using an optical and environmental scanning electron microscope (ESEM) as well as molecular diversity studies based on the small subunit ribosomal (SSU) rRNA of Bacteria, Archaea and Eucarya.We detected a single phylotype of halophilic archaea and a oxytrichid ciliate.Within the bacteria, a variety of Cyanobacteria, Bacteroidetes, Alpha-, Beta- and Deltaproteobacteria, as well as members of the candidate division TM6, were identified.

Continental evaporites are deposits that originate from the evaporation of saline waters in the low areas of saline lakes from all continents, except Europe, and mainly consist of chloride, sulphate and potash minerals. In recent years, the discovery on the Martian surface of hydrated salt minerals, including sulphates and chlorides, interpreted as deriving from the desiccation of preexisting large bodies of water, such as lakes, has provided further convincing evidence of liquid water activity on the surface of Mars and, consequently, it has reinforced the plausibility of finding life. Because evaporites require short-term aqueous processes for their formation, they can trap and preserve over geologic times a biological record made up of halophilic extremophiles—such as microalgae, bacteria, and their remains—that recent research on Earth has shown to be characterized by unexpectedly high biodiversity. This record may consist of varying types of fossils, including morphological fossils, chemofossils and biominerals. As a consequence, continental evaporite environments and their saline deposits are now a primary target for the near future astrobiology missions devoted to the search for fossil Martian life. Lacustrine evaporite deposits and minerals have, therefore, been identified as primary targets for the NASA–ESA joint programme of the Mars sample return, planned for the end of the current decade.

Evaporite accumulations produced by artesian waters in the arid zones of southern Tunisia led to the formation of subrounded, gypsiferous mounds consisting of irregular alternations of mineral precipitates and aeolian sand. The joint occurrence of gypsum crusts and plant colonization determined the stabilization of their top surface. Careful examination of the pigmented (green-brown) crusts revealed endolithic microbial communities just below the surface. In previous optical and scanning electron microscope studies cyanobacteria were the dominant component of these communities. Molecular diversity studies based on small subunit ribosomal RNA (SSU rRNA) gene analysis revealed that Flavobacteria, Actinobacteria, Deinococcales, Alpha- and Gamma- Proteobacteria are also important components of the microbial assemblage. Their pigment analyses, determined by high performance liquid chromatography (HPLC), detected the presence of carotenoids and chlorophyll (chl) a and b. Microbial communities that produce pigme...

In high-salinity and water-scarce environments, such as in hot and dry deserts, species develop adaptive strategies that are necessary for living in such harsh conditions. Continental ephemeral salt lakes (sabkhas) with periodic flooding from subsurface groundwater followed by high salt concentrations, such as the ones of the northern Africa Chotts, rank among the geological settings wherein the combined effect of salt concentration and fluctuation of water availability make the environment unstable and can thereafter lead to extreme changes. The present study investigates the continental sabkha environments of southern Tunisia, in which ecological niches (i.e. water and salt precipitates, including halite, gypsum, and dolomite) host microbial life. Halophilic microorganisms can be trapped in the extensive saline crusts of halite and gypsum, which can be regarded as the first step of their delivery to the fossil record. The study of halophiles can provide clues for the understanding of life strategies in extreme terrestrial environments, such as sabkhas, which are potential good terrestrial analogs for evaporite-bearing Martian deposits.

Microbial life in hot and cold desert environments inhabits endolithic niches. The endolithic microorganisms include bacteria, fungi and lichens. To protect themselves from the inhospitable conditions, such as high UV radiation, dryness, and rapid temperature variations, microorganisms migrate into fractures or in pore spaces where the necessary nutrient, moisture, and light are sufficient for survival. Examples of endolithic communities are well documented from the Negev Desert, Antarctica and the Artic regions, and the Atacama Desert. The most common substrates are porous, crystalline sandstones with calcium carbonate cements and sulfate (gypsum) and other evaporite mineral crusts. The detection of sulfate on the Martian surface has sparked off considerable interest in the astrobiological potential of the evaporite deposits of continental environments, which may potentially host (or may have hosted) endolithic microorganisms.