Nunzia Stivaletta

Microbial colonization of the salt deposits in the driest place of the Atacama Desert (Chile).

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.

Biomarkers of endolithic communities within gypsum crusts (Southern Tunisia).

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 pigmentation and display an endolithic lifestyle typify the extreme environments as those found in arid/semiarid and hot desert regions.

Physicochemical Conditions and Microbial Diversity Associated with the Evaporite Deposits in the Laguna de la Piedra (Salar de Atacama, Chile)

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.

Silicified Biota in High-Altitude, Geothermally Influenced Ignimbrites at El Tatio Geyser Field, Andean Cordillera (Chile)

Biofilms and filamentous communities provided favorable sites for silica precipitation on deeply weathered ignimbrites that make up the substrate at the hydrothermal field of El Tatio (Andean Cordillera, Chile). The amorphous silica encrustation enabled the preservation of a variety of biotic and abiotic features. An integrated study based on optical/scanning electron microscopy and molecular methods of totally to partially silicified microbial communities and biofilms allowed a comparative evaluation of the microfacies and the microbial diversity in the siliceous sinters produced by the digression of a little braided stream departing from a hot spring pool. This study showed useful convergent identifications of certain groups of microbes, such as filamentous cyanobacteria attributed to the genera Phormidium and Rivularia. Together with these microbes, other presumably initial colonizers, such as the halophilic and thermophilic pennate diatoms Nitzschia and Synedra, were widely present and could have contributed to the formation of biofilms and mucus that, as potential home to early silicification, could have contributed to the preservation of microbiologically derived morphologies.

Endoliths in Terrestrial Arid Environments: Implications for Astrobiology

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.

Halophiles, Continental Evaporites and the Search for Biosignatures in Environmental Analogues for Mars

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.