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Astrobiological significance of the sabkha life and environments of southern Tunisia

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Abstract

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.

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... Hyper-saline lakes are widespread around the world and occur mainly in warm and arid areas where rainwater has been collected, evaporated, and left large deposits of salts (halite, gypsum) and water with a high salt concentration [1]. They are devoid of any kinds of vegetation except some clumps of grass. ...
... Southern Tunisia includes numerous ephemeral inland and costal Saharan salt lakes, known as Sebkhas and Chotts, lying south of the Atlas Mountains at the northern edge of the Sahara [1,2]. These salt ecosystems have been the subject of microbiological studies carried on saline water, sediments, and soils [3][4][5][6]. ...
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A thorough assessment of the phylogenetic diversity and community structure of halophilic archaea from three halite-crystal salts, processed from two separated saline systems of Southern Tunisia has been performed using culture dependent and independent methods targeting different regions of 16S rRNA gene sequences including DGGE, 16S rRNA clone libraries and Illumina Miseq sequencing. Two samples, CDR (red halite-crystal salts) and CDW (white halite-crystal salts), were collected from Chott-Eljerid and one sample CDZ (white halite-crystal salts) from Chott Douz. Fourteen isolates were identified as Halorubrum, Haloferax, Haloarcula, and Halogeometricum genera members. Culture-independent approach revealed a high diversity of archaeal members present in all samples, represented by the Euryarchaeal phylum and the dominance of the Halobacteria class. Nanohaloarchaea were also identified only in white halite samples based on metagenomic analysis. In fact, a total of 61 genera were identified with members of the Halorhabdus, Halonotius, Halorubrum, Haloarcula, and unclassified. Halobacteriaceae were shared among all samples. Unexpected diversity profiles between samples was observed where the red halite crust sample was considered as the most diverse one. The highest diversity was observed with Miseq approach, nevertheless, some genera were detected only with 16S rRNA clone libraries and cultured approaches
... Due to the unavailability of samples from Mars, analog studies in astrobiology are of great importance. On Earth, terrestrial hypersaline environments of arid regions are the most relevant analogues to these martian regions with regard to the search for preserved biosignatures (Stivaletta et al., 2009;Parro et al., 2011;Fernández-Remolar et al., 2013). Comprehensive studies on microbial communities and microbial biomarkers in association with evaporites may have implications for the search for life and its putative traces on martian salt environments (Marlow et al., 2011;Davila et al., 2016). ...
... By using a 900 cm hypersaline sediment core from the Qaidam Basin, it was reported that archaeal cells were predominant in the hypersaline sediments at the top, whereas the bacterial community was mostly in the low-salinity detrital sediments at the bottom ( Jiang et al., 2007). Although this phenomenon has been reported previously (Stivaletta et al., 2009), our study further confirms this with lipid analysis and offers an alternative approach in those cases where the DNA method has proven to be extremely difficult. ...
Article
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Qaidam Basin (Tibetan Plateau) is considered an applicable analogue to Mars with regard to sustained extreme aridity and abundant evaporites. To investigate the possibility of the preservation of microbial lipids under these Mars analog conditions, we conducted a mineralogical and organic geochemistry study on samples collected from two Quaternary sections in Dalangtan Playa, northwestern Qaidam Basin, which will enhance our understanding of the potential preservation of molecular biomarkers on Mars. Two sedimentary units were identified along two profiles: one salt unit characterized by a predominance of gypsum and halite, and one detrital unit with a decrease of gypsum and halite and enrichment in siliciclastic minerals. Bacterial fatty acids and archaeal acyclic diether and tetraether membrane lipids were detected, and they varied throughout the sections in concentration and abundance. Bacterial and archaeal biomolecules indicate a dominance of Gram-positive bacteria and halophilic archaea in this hypersaline ecosystem that is similar to those in other hypersaline environments. Furthermore, the abundance of bacterial lipids decreases with the increase of salinity, whereas archaeal lipids showed a reverse trend. The detection of microbial lipids in hypersaline environments would indicate, for example on Mars, a high potential for the detection of microbial biomarkers in evaporites over geological timescales. Key Words: Dalangtan playa-The Qaidam Basin-Subsurface evaporites-Lipid biomarkers-Mars. Astrobiology 17, xxx-xxx.
... Terrestrial analog environments are defined as sites on Earth that show one or more sets of geological or environmental conditions similar to those found on current or past extraterrestrial bodies (Osinski et al., 2006), such as Mars (e.g., Baker and Nummedal, 1978;Schaber et al., 1986;Golombek et al., 1995;Schaber et al., 1997;Bourke and Zimbelman, 2001;Ori et al., 2001;Wentworth Contents Amils et al., 2007;Stivaletta et al., 2009;Fairén et al., 2010), Titan (e.g., Lorenz et al., 2010), or Venus (e.g., Wei et al., 2011). They include natural environments (Doran et al., 2008;Pullan et al., 2008) as well as artificial sites or devices that mimic specific planetary conditions (Léveillé, 2009). ...
... Ori et al. (2001) and Komatsu et al. (2007) previously recognized the presence of sabkha bodies in Tunisia as potential analog sites to Mars. Similarly, Stivaletta et al. (2009) discussed the astrobiological significance of the sabkha life and environments in the case study of Chott El Gharsa. Thus, multiple terrestrial analogs are already known to be present in Tunisia, and analog sites in Tunisia hold promise to guide interpretation of martian environments. ...
Article
A multidisciplinary study of the watershed and depressions of the Mechertate–Chrita–Sidi El Hani (MCSH) system in eastern Tunisia shows that groundwater upwelling and/or seepage toward the modern surface is important in the shaping of its geomorphologic features and sediment outcrops. Along the watershed of the system, groundwater is downward enriched with evaporitic minerals. These minerals precipitate as cement and protect the sediment outcrops from aeolian erosion. The water table is the limiting control on erosion and deposition, and also influences the succession of sediment along the system. The water table further determines the local base level, which controls the deposition within depressions. With increasing humidity at the limit of the capillary fringe, the landscape of the evaporative system is organized according to three sedimentary types: (1) unconsolidated sediment of aqueous and/or aeolian origin that is eroded and transported toward depressions (away from groundwater interactions), (2) consolidated sediment that is also aqueous and/or aeolian in origin and is protected from aeolian erosion by groundwater influence, and (3) sedimentary filling of depressions located within accumulation zones. These sediments are organized along a lateral, basinward profile. Here we show that during periods of relative water table fall, sediments from the watershed prograde to cover the sabkha basin fill. The rise and fall of the water table and the connected base level result in the deposition of genetically-related progradational and retrogradational sequences. We propose that these genetic sequences can be useful to interpret the sequence stratigraphy at three locations on Mars where sedimentary formations were probably controlled by direct groundwater influence: Meridiani Planum, Terby crater, and Gale crater. At Meridiani Planum, the exposed stratigraphic sequence of the Burns formation starts with deposition of dry aeolian sediment derived from a former watershed. Then, due to the rise of the water table, wet sediments of a sabkha rest atop the dry aeolian cycle to comprise a retrogradational sequence. At Terby and Gale craters, an opposite stratigraphic sequence starts with the wet deposition of the sabkha fill. Then, due to the fall of the water table, the dry aeolian sedimentation progrades atop to the sabkha fill to comprise a progradational sequence. We conclude that the various stratigraphic sequences at the MCSH system, described here, represent different possible analog scenarios for diverse depositional sequences on Mars, in all cases involving groundwater activity.
... The detection of this biomolecule has been given a high priority on the European Space Agency's ExoMars missions as an indicator of past organic life (Vago et al., 2006). Past work within sabkha environments has highlighted the presence of organic molecules with astrobiological significance, such as carotenoids, chlorophyll, and scytonemin (Edwards et al., 2006;Stivaletta et al., 2009;Edwards et al., 2010). However, despite the detection of the organic compounds, little work has been performed on the connection between their detection and the microbial ecology responsible for producing them, as well as how those relationships pertain to extant microbial life. ...
Article
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Gypsum (CaSO4·2H2O) has been identified at the surface of Mars, by both orbiters and rovers. Because gypsum mostly forms in the presence of liquid water as an essential element for sustaining microbial life and has a low porosity, which is ideal for preserving organic material, it is a promising target to look for signs of past microbial life. In this article, we studied organic matter preservation within gypsum that precipitates in a salt flat or a so-called coastal sabkha located in Qatar. Sabkha's ecosystem is considered a modern analog to evaporitic environments that may have existed on early Mars. We collected the sediment cores in the areas where gypsum is formed and performed DNA analysis to characterize the community of extremophilic microorganisms that is present at the site of gypsum formation. Subsequently, we applied Raman spectroscopy, a technique available on several rovers that are currently exploring Mars, to evaluate which organic molecules can be detected through the translucent gypsum crystals. We showed that organic material can be encapsulated into evaporitic gypsum and detected via Raman microscopy with simple, straightforward sample preparation. The molecular biology data proved useful for assessing to what extent complex Raman spectra can be linked to the original microbial community, dominated by Halobacteria and methanogenic archaea, providing a reference for a signal that may be detected on Mars.
... Chott Djerid or Chott El Jerid, in the southwest, is the largest salt lake in Tunisia by surface area (approximately 500,000 ha). During summer, water loss by evaporation increases dissolved ions concentration, and consequently salt covers the depression [30]. Chott Djerid contains similar ions as seawater with salinity varying between 250 and 330 g/L [31]. ...
Chapter
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Saline and hypersaline environments are classified as wetlands and constitute the largest ecosystems on the planet (a half of inland aquatic ecosystems), they are widely distributed all over the continental zones (Africa, America, Europe, Australia, and Asia). African salt lakes are located in the arid northern and southern regions. Some of the African saline and hypersaline lakes are Natron, chott Melghir, chott El Jerid, Zahrez Chergui and Guerbi. Microbial diversity within salt lakes is restricted to a few extremely salt-tolerant species. Certain microorganisms can thrive in salty and extreme environmental conditions. These microorganisms can be found over the three domains (Archeae, Bacteria, and Eukarya). The microbial biodiversity of African salt lakes is of great interest not only to have an overview of halophilic microorganisms flourishing in these extreme ecosystems but also for their potential applications in different fields. Extremophiles are a group of microorganisms with high biotechnological and industrial potential, especially for their ability to produce biopolymers, pigments, antibiotics, and enzymes. Most African salt lakes are unexploited; a few studies were published on microbial diversity which is an excellent opportunity for scientists to discover new genera and/or species and also news molecules of biotechnological interest. This chapter reviews African salt lake distribution and the microbial diversity, with an emphasis on the North of Africa. An overview of microbial, diversity which can contribute to the development of African countries is also provided by presenting a prominent investigation of the bioactive molecules from salt lakes with potential application in biotechnology. Key Words: Salt lakes, Microbial diversity, African countries, Biotechnology, Metabolites.
... The microbial diversity at BSF is comparable to that of other dry saline environments. Archaeal communities are often dominated by members of Halobacteria (42)(43)(44). Bacterial communities are often dominated by Bacteroidetes (Salinibacter) and Proteobacteria (45)(46)(47). Members from Crenarchaeota, Gemmatimonadetes, Verrucomicrobia, SRB from Deltaproteobacteria, and Clostridia are also commonly reported (48)(49)(50). ...
Article
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Pleistocene Lake Bonneville, which covered a third of Utah, desiccated approximately 13,000 years ago, leaving behind the Bonneville Salt Flats (BSF) in the Utah West Desert. The potash salts that saturate BSF basin are extracted and sold as an additive for agricultural fertilizers. The salt crust is a well-known recreational and economic commodity, but the biological interactions with the salt crust have not been studied. This study is the first geospatial analysis of microbially diverse populations at this site using cultivation-independent environmental DNA sequencing methods. Identification of the microbes present within this unique, dynamic, and valued sedimentary evaporite environment is an important step toward understanding the potential consequences of perturbations to the microbial ecology on the surrounding landscape and ecosystem.
... In addition, the chlorides are ideal for biosignature preservation, thus they are of great astrobiology interest. Inclusions are common in halite and the fluid inclusions in terrestrial evaporate deposits which can provide refuge for extant and extinct life, preserving biosignatures including DNA, and viable microbial communities over geological timescales, protecting them from hash environment condition, such as highly intense sun light and ultraviolet radiation, rapid changes in salinity and water availability, high salt concentration and desiccation (Stivaletta et al., 2009;Vreeland et al., 2007;Fish et al., 2002). Schubert et al. (2009) have reported resuscitating entrained archea from halite as old as ~34 000 years, and some claims ~10 5 years (Mormile et al., 2003), while another study of Permian halite extended to ~250-300 Ma (Griffith et al., 2008). ...
Article
Over 600 chloride-bearing deposits (chlorides) have been identified on the southern highlands of Mars. These chlorides have critical implications for hydrology and astrobiology: they are indicators of an evaporating super saturated solution, and they could have provided habitat environments for halophilic microorganisms and preserved organic matter. One of the prominent geomorphology characteristics of these chloride-bearing regions is the polygonal features within them. The origin of these polygonal features is still in debate. In this study, we have surveyed 153 locations of chlorides using 441 high resolution imaging science experiment (HiRISE) images to characterize the geomorphology of polygonal features. We identified 3 types of polygonal features of distinct geomorphologic characteristics: fractures, raised ridges, and transitional polygons between fractures and raised ridges. We evaluate previously proposed hypotheses of the formation of the polygonal features, and suggest that the 3 types of polygonal features are indicators of different stages of salt crust formation. Salt crust is usually formed through multiple groundwater activities, and it often occurs in playa environment on Earth. The unique hydrological and astrobiological implications of the chlorides with polygonal features make these deposits of high priority for future landed on and/or sample return exploration missions of Mars.
... In Tunisia, Chott el-Djerid is the largest terrestrial lake covered by a large salt pan during the dry season [28]. This evaporitic environment is characterized by extreme conditions including, dryness, elevated UV irradiation and relevant salt concentration (29-37%) of chlorides and sulphates [29]. ...
Article
The production, characterization and potential application in heavy metals and dyes removal of a novel heteropolysaccharide-protein named, gpHb, produced by an haloarchaeal strain Halogeometricum borinquense strain A52 were investigated. The highest gpHb yield of 13.96 ± 0.32 g/L was produced under optimized conditions by response surface methodology. We focused on the characteristics and flocculation performance of gpHb. An important attribute of protein with 16 protein types identified that occupied a total content of 50.2% in the gpHb. Additionally, carbohydrate that occupied 30.4% of the total bioflocculant content consisted of three monosaccharides. Fourier transform infrared spectroscopy indicated the presence of carboxyl, hydroxyl, amine, amide, and sulfate groups. To further study flocculation activities, factors such as bioflocculant dosage, temperature, pH, salinity and cations addition were tested. In comparison to the chemical flocculant poly aluminium chloride, gpHb maintain high activity at large range of salinity and its flocculation activity was higher on both sides of pH 7. Addition of trivalent cation maingly Fe³⁺ enhances the flocculating rate indicating that the bioflocculant is negatively charged. Its practical applicability was established for heavy metals and dyes removal from saline aqueous solutions. The highest removal efficiency was observed with Cr³⁺ (91.4%) and Ni²⁺ (89.60%) and with Basic bleue 3 (83.8%) and Basic red (78.6%). The excellent flocculation activity of gpHb under saline condition suggests its potential industrial utility for treatment of textile and tannery wastewaters
... Microbial Cell Factories *Correspondence: noura.raddadi@unibo.it 1 Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), Alma Mater Studiorum University of Bologna, via Terracini 28,40131 Bologna, Italy Full list of author information is available at the end of the article of limited water supply and nutrient content, high temperatures, irradiation and evaporation rates and high salinity [11,12]. ...
... One aspect that, to our knowledge, has not been considered in the literature on BS/BE is their potential for improving soil water retention [8], especially in arid and desert sandy soils that have limited content of organic matter and experience extreme environmental conditions of limited water supply and nutrient content, high temperatures, irradiation and evaporation rates and high salinity [11,12]. ...
Article
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Background: Water stress is a critical issue for plant growth in arid sandy soils. Here, we aimed to select bacteria producing polyextremotolerant surface-active compounds capable of improving water retention and humidity uptake in sandy soils. Results: From Tunisian desert and saline systems, we selected eleven isolates able to highly emulsify different organic solvents. The bioemulsifying activities were stable with 30% NaCl, at 4 and 120 °C and in a pH range 4-12. Applications to a sandy soil of the partially purified surface-active compounds improved soil water retention up to 314.3% compared to untreated soil. Similarly, after 36 h of incubation, the humidity uptake rate of treated sandy soil was up to 607.7% higher than untreated controls. Conclusions: Overall, results revealed that polyextremotolerant bioemulsifiers of bacteria from arid and desert soils represent potential sources to develop new natural soil-wetting agents for improving water retention in arid soils.
... The Eastern Coast of the Misratah region includes broad areas of supratidal salt flats, termed 'dry basins' in the summer and wet with water in the winter [65]. These areas occur behind wide sandy beaches and coastal dunes in relatively low topographic areas [66] The Misratah Sabkha is situated in a coastal Sabkha plain over an approximately 1500 km 2 stretch along the Gulf of Sidra on the western coast of Misratah extending from North to South: 170 km from Gaser-Ahmed to Buerat. ...
Conference Paper
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The goal of the Libyan Environmental General Authority (EGA) and National Oil Corporation (Department of Health, Safety & Environment) during the last 5 years has been to adopt a common approach to coastal and marine spatial planning. Protection and planning of the coastal zone is a significant for Libya, due to the length of coast and, the high rate of oil export, and spills’ potential negative impacts on coastal and marine habitats. Coastal resource scenarios constitute an important tool for exploring the long-term and short-term consequences of oil spill impact and available response options that would provide an integrated perspective on mitigation. To investigate that, this paper reviews the Misratah coastal parameters to present the physical and human controls and attributes of coastal habitats as the first step in understanding how they may be damaged by an oil spill. This paper also investigates costal resources, providing a better understanding of the resources and factors that impact the integrity of the ecosystem. Therefore, the study described the potential spatial distribution of oil spill risk and the coastal resources value, and also created spatial maps of coastal resources and their vulnerability to oil spills along the coast. This study proposes an analysis of coastal resources condition at a local level in the Misratah region of the Mediterranean Sea, considering the implementation of coastal and marine spatial planning over time as an indication of the will to manage urban development. Oil spill contamination analysis and their impact on the coastal resources depend on (1) oil spill sequence, (2) oil spill location, (3) oil spill movement near the coastal area. The resulting maps show natural, socio-economic activity, environmental resources along of the coast, and oil spill location. Moreover, the study provides significant geodatabase information which is required for coastal sensitivity index mapping and coastal management studies. The outcome of study provides the information necessary to set an Environmental Sensitivity Index (ESI) for the Misratah shoreline, which can be used for management of coastal resources and setting boundaries for each coastal sensitivity sectors, as well as to help planners measure the impact of oil spills on coastal resources. Geographic Information System (GIS) tools were used in order to store and illustrate the spatial convergence of existing socio-economic activities such as fishing, tourism, and the salt industry, and ecosystem components such as sea turtle nesting area, Sabkha habitats, and migratory birds feeding sites. These geodatabases help planners investigate the vulnerability of coastal resources to an oil spill.
... Accordingly, recent studies carried out on the Southern Hemisphere since a few years ago (Cuadrado et al., , 2013Pan et al., 2013a) have recognized typical structures in an estuarine environment in Argentina, where tidal processes are the most important. Similarly, in modern coastal sabkhas, many studies identified sedimentary structures induced by microbial mats being related to marine processes (Gerdes et al., 2000a(Gerdes et al., ,b, 2008Noffke, 2010;Aref et al., 2014 among others), and several studies recognized similar structures in the rock record (Schieber, 1999;Eriksson et al., 2000;Noffke and Awramik, 2013) and also their similarity with analogs of Martian rocks (Barbieri et al., 2006;Stivaletta et al., 2009;Noffke and Awramik, 2013). ...
Article
This study focuses of sedimentary structures formed by microbial consortia, in a particular coastal setting, an ancient tidal channel, separated from the ocean by a sandy spit and connected by a blind tidal channel at the opposite end. Most studies in modern and ancient environments consider water movement as the triggering mechanism acting in the formation and deformation of sedimentary structures. As such, the paper documents the presence of several microbial structures such as shrinkage cracks, flip-over mats, microbial chips and multidirectional ripples which are related to tidal processes, while bulges and gas domes structures are formed after occasional inundation events. However, the more conspicuous structures covering a great area at the study site are folds and roll-ups, the product of deformation of microbially-induced structures by the action of sporadic spring-tidal currents due to strong winds. Therefore, the objective of this research is to document modern sedimentary structures in a coastal area and to provide a mechanistic explanation for their formation, based on the interplaying effects of the moisture variation and high shear-stress. Also, several microbial sedimentary structures are distinguished throughout vertical sediment cores, such as microbial chips, detached mat, sponge fabrics, tears and concentric structures, which are identified in a sedimentary profile. Through the recognition and interpretation of modern sedimentary deformation structures, this study contributes empirical tools for the reconstruction of analogous paleoenvironments in fossil studies.
... Wet and hypersaline NaCl-based extreme environments used as model systems for the study of life on Mars include continental saline environments such as evaporites of gypsum, halite and other materials in the alkaline Wadi Natrun, Egypt, the Salar de Atacama and other evaporite sites in the Atacama Desert, and Tunisian sabkhas [12,13]. The study of endoevaporitic cyanobacteria and other microorganisms within gypsum and halite evaporites in the lagoons of Baja California, Mexico [14] also belongs to this category. ...
Article
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Salts are abundant on Mars, and any liquid water that is present or may have been present on the planet is expected to be hypersaline. Halophilic archaea (family Halobacteriaceae) are the microorganisms best adapted to life at extremes of salinity on Earth. This paper reviews the properties of the Halobacteriaceae that may make the group good candidates for life also on Mars. Many species resist high UV and gamma radiation levels; one species has survived exposure to vacuum and radiation during a space flight; and there is at least one psychrotolerant species. Halophilic archaea may survive for millions of years within brine inclusions in salt crystals. Many species have different modes of anaerobic metabolism, and some can use light as an energy source using the light-driven proton pump bacteriorhodopsin. They are also highly tolerant to perchlorate, recently shown to be present in Martian soils, and some species can even use perchlorate as an electron acceptor to support anaerobic growth. The presence of characteristic carotenoid pigments (α-bacterioruberin and derivatives) makes the Halobacteriaceae easy to identify by Raman spectroscopy. Thus, if present on Mars, such organisms may be detected by Raman instrumentation planned to explore Mars during the upcoming ExoMars mission.
... To name but a few, the Channeled Scablands (e.g., [9,10]), central Australia (e.g., [11]), Sahara Desert (e.g., [12,13]), Tunisia (e.g., [14][15][16][17]), the Dead Sea (e.g., [18]), and the Arctic and Antarctica [2,19,20] are sites sharing similarities with Mars. Analogies of these terrestrial lands to Mars have been proposed from several viewpoints, including sedimentological (e.g., [21][22][23][24][25][26][27]), geochemical [22,23,28], mineralogical (e.g., [29,30]), hydrogeological (e.g., [25,[30][31][32][33][34][35]) and biological (e.g., [29,31,36,37]). ...
Article
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Spring mounds on Earth and on Mars could represent optimal niches of life development. If life ever occurred on Mars, ancient spring deposits would be excellent localities to search for morphological or chemical remnants of an ancient biosphere. In this work, we investigate models of formation and activity of well-exposed spring mounds in the Mechertate-Chrita-Sidi El Hani (MCSH) system, eastern Tunisia. We then use these models to explore possible spring mound formation on Mars. In the MCSH system, the genesis of the spring mounds is a direct consequence of groundwater upwelling, triggered by tectonics and/or hydraulics. As they are oriented preferentially along faults, they can be considered as fault spring mounds, implying a tectonic influence in their formation process. However, the hydraulic pressure generated by the convergence of aquifers towards the surface of the system also allows consideration of an origin as artesian spring mounds. In the case of the MCSH system, our geologic data presented here show that both models are valid, and we propose a combined hydro-tectonic model as the likely formation mechanism of artesian-fault spring mounds. During their evolution from the embryonic (early) to the islet ("island") stages, spring mounds are also shaped by eolian accumulations and induration processes. Similarly, spring mounds have been suggested to be relatively common in certain provinces on the Martian surface, but their mode of formation is still a matter of debate. We propose that the tectonic, hydraulic, and combined hydro-tectonic models describing the spring mounds at MCSH could be relevant as Martian analogs because: (i) the Martian subsurface may be over pressured, potentially expelling mineral-enriched waters as spring mounds on the surface; (ii) the Martian subsurface may be fractured, causing alignment of the spring mounds in preferential orientations; and (iii) indurated eolian sedimentation and erosional remnants are common features on Mars. The spring mounds further bear diagnostic mineralogic and magnetic properties, in comparison with their immediate surroundings. Consequently, remote sensing techniques can be very useful to identify similar spring mounds on Mars. The mechanisms (tectonic and/or hydraulic) of formation and evolution of spring mounds at the MCSH system are suitable for the proliferation and protection of life respectively. Similarly, life or its resulting biomarkers on Mars may have been protected or preserved under the spring mounds.
... These enzymes include proteases, lipases, amylases, and DNase, viewed as important candidates for various industries such as food, detergent, chemical, pharmaceutical, paper, and pulp or waste treatment [4]. Southern Tunisia features numerous ecosystems including coastal and inland salt lakes, respectively, named Sabkha or Sabkhet, and Chotts [24]. These environments are characterized by unstable climatic conditions, due to the periodic flooding by the subsurface ground water associated with high salt conditions during dry phases, making them fascinating ecosystems to study the diversity and the ecological adaptations of microorganisms thriving in saline systems. ...
Article
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Haloalkaliphiles are polyextremophiles adapted to grow at high salt concentrations and alkaline pH values. In this work, we isolated 122 haloalkaliphilic bacteria upon enrichments of 23 samples from 5 distinct saline systems of southern Tunisia, growing optimally in media with 10% salt and at pH 10. The collection was classified into 44 groups based on the amplification of the 16S-23S rRNA internal transcribed spacers (ITS-PCR). Phylogenetic analysis and sequencing of the 16S rRNA genes allowed the identification of 13 genera and 20 distinct species. Three gram-positive isolates showing between 95 and 96% of 16S rRNA sequence homology with Bacillus saliphilus could represent new species or genus. Beside the difference in bacterial diversity between the studied sites, several species ecological niches correlations were demonstrated such as Oceanobacillus in salt crust, Nesterenkonia in sand, and Salinicoccus in the rhizosphere of the desert plant Salicornia. The collection was further evaluated for the production of extracellular enzymes. Activity tests showed that gram-positive bacteria were mostly active, particularly for protease, lipase, DNase, and amylase production. Our overall results demonstrate the huge phenotypic and phylogenetic diversity of haloalkaliphiles in saline systems of southern Tunisia which represent a valuable source of new lineages and metabolites.
... As such, even if the chlorides were precipitated at extreme salinity, the materials could have entrapped and preserved halophilic microorganisms present in a precursory lower-salinity system. Finally, although microorganisms may not have been able to originate in such a high-salinity system, they may have been able to adapt to survive in such a system [Stivaletta et al., 2009;Rothschild and Mancinelli, 2001]. ...
Article
We use Thermal Emission Imaging System (THEMIS) data to identify and characterize the global distribution of distinct materials interpreted to contain chloride salts on the Martian surface. Previously mapped global geochemical and physical properties are used in concert with thermophysical and morphological observations to assess the materials' local and regional characteristics. The results of our survey have expanded the characterization of the materials from ∼200 to ∼640 distinct sites dispersed throughout low-albedo Noachian- and Hesperian-aged terrains. Our survey also shows that the materials are detected in locally thermophysically distinct terrains and display a range of morphologies. Topography indicates that the majority of the materials occur in local lows, although crosscutting relationships indicate that some sites are located in “geologic windows” implying that the materials may be older than the terrains in which they are situated. Once exposed, the materials appear to undergo erosion, which may be the reason we do not observe large laterally extensive materials at the surface. The materials are predominantly local in nature, yet their prevalence across the southern highlands suggests that they represent one or more globally ubiquitous processes. We consider a number of formation hypotheses but find that most observations are consistent with formation via ponding of surface runoff or groundwater upwelling, although efflorescence and hydrothermal activity may also be possible in some locales. The materials' inferred ages suggest that the conditions that enabled the deposition of the materials persisted for up to 1 billion years.
... Their basins capture the record of geological and environmental fluctuations over a wide range of temporal and spatial scales (Fig. 1). Terrestrial lakes host a diversity of habitats where life's adaptability can be pushed to the edge in often unstable environments (e.g., Bronmark and Hansson 1998;Price 2000;Wynn-Williams and Edwards 2000;Herbst 2001;Hollibaugh et al., 2001;Oren 2001;Warwick and Laybourn-Parry 2008;Cabrol et al., 2009;Stivaletta et al., 2009, see also chapter 13). They preserve the evidence of ancient life as sedimentation rapidly entombs dead organisms and generates anoxic conditions favoring the formation of fossils (e.g., Des Marais and Farmer 1995;Farmer 1999Farmer , 2003Des Marais et al., 2003Beaty et al., 2005;Hoffman et al., 2008). ...
Article
Chott El Jerid is the largest hypersaline ephemeral lake in southern Tunisian Sahara desert and is one of the biggest depressions at the North of Africa. This study aimed to investigate the diversity and abundance of microbial communities inhabiting Chott El Jerid during wet season (when it was flooded), using molecular methods [Illumina Miseq sequencing, DGGE and qPCR (qPCR)]. 16S rRNA gene analyses revealed that bacterial community was dominated by Proteobacteria (especially Ralstonia species), followed by Firmicutes, Bacteroidetes, Cyanobacteria, Actinobacteria and Verrucomicrobia. The results obtained using prokaryotic universal primers showed low relative abundance of Archaea dominated by few OTUs related to Methanosarcinaceae and Methanomassiliicoccaceae families and the presence of sulfate-reducing Archaea affiliated with Archaeoglobus. However, the results obtained using Archaea-specific primers showed that archaeal community was mainly composed of aerobic Halobacteria (especially Halorubrum species) and anaerobic members of Methanomicrobia. These results also provided evidence for the presence of members of the genus Halohasta in this environment. qPCR results revealed that Archaea were more abundant in studied samples than Bacteria. The sulfate-reducing Bacteria were also found abundant (~ one-third of the bacterial community) and outnumbered methanogens, suggesting their potential important role in this sulfate-rich and hypersaline ecosystem.
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Being a first notice in Tunisia, this work opens the door for forthcoming studies shedding light on potential analogies between Tunisian terrestrial analogues and extraterrestrial bodies. In the first chapter, we investigate models of formation and activity of spring mounds occurring in the Mechertate-Chrita-Sidi El Hani (MCSH) and Boujmal systems. These terrestrial analogues are the mirror of subsurface past terrestrial analogues dating back to the Trias and Messinian. We then use these models to further understand possible spring mounds formation on Mars. In the MCSH system, the genesis of the spring mounds is a direct consequence of groundwater upwelling, triggered by tectonics and/or hydraulics. Considered as fault spring mounds, they are organized along preferential orientations, probably inherited from current or past tectonic activity. This observation supports a tectonic influence in the formation of the spring mounds. On the one hand, the hydraulic pressure generated by the convergence of aquifers towards the surface of the system allows their consideration as artesian spring mounds. In the case of the MCSH system, our geologic investigations presented here show that both models are worth to be analyzed, and we propose a hydro-tectonic combined model as the adequate formation mechanism of artesian-fault spring mounds. During their evolution from the embryonic to the islet stages, spring mounds behaving as obstacles are also shaped by an accumulation of a wet aeolian sedimentation, which is enhanced by the induration process. On the other hand, the tectonic model is more candidate to control the formation of spring mounds at the terrestrial analogue of Boujmal. The wet aeolian sedimentology also advocates the accumulation of tephras layers allowing the application of tephrostratigraphy. Similarly, spring mounds may be relatively common in certain provinces on the martian surface, but their mode of formation is still a matter of debate. We propose here that the tectonic, hydraulic, and hydro-tectonic models describing the spring mounds at MCSH should be considered as relevant martian analogues because: (i) the martian subsurface may be over pressured and this overpressure would originate spring mounds on the surface, (ii) the subsurface may be fractured in such a way that explains the lining up of the spring mounds along preferential orientations, (iii) and the wet aeolian sedimentation through the induration process is a common feature on Mars. In the second chapter, a multidisciplinary study of the watershed and depressions of the MCSH sabkha system shows that groundwater upwelling and/or seepage toward the modern surface is important in the shaping of its geomorphologic features and sediment outcrops. Along the watershed of the system, groundwater enriched sediment with evaporitic minerals. These minerals precipitate as cement to protect the outcropping sediment from aeolian erosion. The water table is the limiting control on erosion and deposition, and also influences the succession of sediment along the system. It determines the local base level, which controls the deposition within depressions. With increasing humidity at the limit of capillary fringe, the landscape of the evaporative system is organized according three sedimentary types: (1) unconsolidated sediment of aqueous or aeolian origin that is eroded and transported toward depressions (out of reach from groundwater involvement); (2) consolidated sediment is also aqueous or aeolian in origin and is consolidated due groundwater influence (protected from aeolian erosion); and (3) sedimentary filling of depressions is located within accumulation zones. These sediments are organized along a lateral, basinward profile. Our study shows that during periods of relative water table fall, sediment from the watershed progrades to cover the sabkha basin fill. Applying key concepts of wet aeolian sequence stratigraphy, the rise and fall of the water table and the connected base level results in the deposition of genetically related progradational and retrogradational sequences. These genetic sabkha sequences are useful to interpret the sequence stratigraphy at three locations on Mars, which was controlled by direct groundwater influence. At Meridiani Planum, the deposition of Burns formation starts with a deposition of dry aeolian sediment derived from a former watershed. Then, due to the rise of the water table, wet sediment of a sabkha rests atop the dry aeolian cycle to comprise a retrogradational sequence. At Terby and Gale craters, an opposite stratigraphic sequence starts with the wet deposition of the sabkha fill. Then, due to the fall of the water table, the dry aeolian sedimentation progrades atop to the sabkha fill to comprise a progradational sequence. We conclude that the various stratigraphic sequences at the MCSH system, described here, represent different possible analogue scenarios for diverse depositional sequences on Mars, in all cases involving groundwater activity. In the third chapter, we discussed the chaology within the Sidi El Hani terrestrial analogue and Gale crater on Mars. Within natural systems, biologic activities and geologic conditions are linked by hierarchical cause-effect relations gener g the organic matter within the discharge playa of Sidi El Hani, high percentages of different fractions seem abnormal in such a saline context. As it has been interpreted in previous works about Sahelarea, this maturated organic matter may be due to a human polluting activity. But this hypothesis seems less convincing because this region is outside any polluting activity. Thus, this maturated organic matter should be viewed in a widest context of a multidisciplinary study taking into account of the presence of petroleum potentials in the subsurface, the converging hydrogeology and the tectonized region. For instance, the high percentage of Aromatic Polycyclic Hydrocarbon (APH) may be hence the result of a hydrocarbons migration rather than anthropogenic pollution. The hydrocarbon migration towards the surface of Sidi El Hani discharge playa may be argued based on the model of fluid migration towards Mars surface. That is to say, based on what happens on Mars, we explain the case on terrestrial analogues. Key Words: Terrestrial analogue, Mechertate-Chrita-Sidi El Hani and Boujmal systems, groundwater upwelling, spring mounds, wet aeolian sedimentation, tephrostratigraphy, wet aeolian sequence stratigraphy, Meridiani Planum, Terby crater, Gale crater, chaology, cyclostratigraphy, hydrocarbon migration, fluid migration on Mars.
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Suggested solutions to some problems of Tunisian novice researchers with academic writing in English: case studies of a Research Article (Gallela et al., 2009) and a Master Thesis (Essefi, 2009) THESIS presented to the University of Sfax in the partial fulfilment of the thesis requirement for the Professional Master's degree in Specialized English
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This work aims to study the relation between the geological history of Sidi El Hani saline environment and its response to the climatic variability. To achieve this goal, a multidisciplinary work compiled in the individual chapters' compilation thesis format was done. Added to chapter zero, this work is made up of seven chapters. Each of them has its own local purpose serving the global purpose of the work. In chapter zero, the aim is to show scientific arguments for compiling this work in English and to give the scientific name of Sidi El Hani saline environment according to the Anglophone terminology. This terminology gives a name to a saline environment according to its geologic features and the speciality of study. For instance, hydrogeologically, Sidi El Hani saline environment is a discharge playa; hydrologically, it is playa; and sedimentologically, it is a clay pan. Chapter one aims, by introducing the specific context of Sidi El Hani saline environment, to show the further increasing need for a multidisciplinary study, to give a bird's eyes sight on the coming chapters and to comment the thesis format. In chapter two, the focus was meant to be on the interaction between the global, the regional and the local frameworks on the one hand and the geodynamic context of Sidi El Hani saline environment on the other; such interaction proves vital in deciding whether the saline environment will be commanded by its geological history or by the climatic variability. Thus, on the basis of a cross-interpretation of previous works, results showed that the geological history is more likely to dominate by providing a salty context in the subsurface of Sahel area. This study proposed its own model of a saline environment inherited from the Messinian Salinity Crisis (MSC). By doing so, this study proposes two hypotheses as origin of salt: the Triassic intrusions and the Messinian Salinity Crisis. In chapter three, on the basis of a hydrological, hydrogeological and geochemical study, the aim was to reinforce the idea of a strong deep influence of somewhere domes of salt and/or salty water probably inherited from the Triassic domes and/or the Messinian Salinity Crisis (MSC) and to deduce that the climatic factor has been taking minor role since the opening of Sidi El Hani discharge playa. By proposing its own model about the groundwater contribution, this study found out that, as a consequence of the dominance of a salty groundwater contribution, the effect of the climate is considered as minor. In chapter four, the aim was to give some precision to previous studies and to examine the depositional environment of sediments in Sidi El Hani watershed, which may feed the saline environment after their erosion by wind or water. Accordingly, some outcropping sediments from the vicinities were sampled and studied. Three crania found also somewhere near the playa have permitted this study declaring Sidi El Hani zone as archeological site; and they may be an efficient tool for dating. In chapter five, the Visual Core Description focused on three reference sandy bands and on the concept of grey scale variability in order to infer the clay pan response to the climatic variability during the last two millennia. Actually, this study established its own concept of the major grey scale variability. This line of attack permitted the classification of Sidi El Hani cores according to their major grey scales variability. Only cores with high major grey scales variability were indeed useful to infer the climatic variability in Sidi El Hani clay pan. Correlations of these cores, on the basis of the major grey scale variability and the occurrence of the three reference sandy bands, were done with a known reference core from sebkha Mhabeul (southeast Tunisia) in order to guess the last 2000 years clay pan climatic history. In chapter six, cores were the object of descriptive classifications of the grain size distribution, which were meant to describe the grain size continuous variability within cores and to correlate between them. The statistical treatment of the crude data was done on the basis of two different methods (the method of Moments Statistics and the method of Inclusive Graphic Statistics) to compute statistical parameters of the grain size distribution such as mean and median. The correlations between cores were done on the basis of sand/silt/clay percentages. Eventhough it has given special care to test different methods of studying the grain size distribution, this study was not deviated from its primary purpose of investigating the filling of the playa; correlations between different cores were meant to infer their sedimentary dynamics. In chapter seven, the synthesis was done by giving a complete map of the saline environment of Sidi El Hani summing up the hydrological, hydrogeological and sedimentological features of its vicinities and its fillings. This Master opens new perspectives concerning the chaos theory and studying the organic matter. Another serious promise for research is in comparing Sidi El Hani saline environment with other saline environments in Tunisia and with analogous locations in planet Mars.
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We report the first census of natural microbial communities of the Bonneville Salt Flats (BSF), a perennial salt pan at the Utah–Nevada border. Environmental DNA sequencing of archaeal and bacterial 16S rRNA genes was conducted on samples from multiple evaporite sediment layers of the surface salt crust. Our results show that at the time of sampling (September 2016), BSF hosted a robust microbial community dominated by diverse Halobacteriaceae and Salinibacter species. Desulfuromonadales from GR-WP33-58 are also abundant in all samples. We identified taxonomic groups enriched in each layer of the salt crust sediment and revealed that the upper gypsum sediment layer found immediately under the uppermost surface halite contains a robust microbial community. We found an increased presence of Thermoplasmatales, Nanohaloarchaeota, Woesearchaeota, Acetothermia, Halanaerobium, Parcubacteria, Planctomycetes, Clostridia, Gemmatimonadetes, Marinilabiaceae and other Bacteroidetes in this upper gypsum layer. This study provides insight into the diversity, spatial heterogeneity, and geologic context of a surprisingly complex microbial ecosystem within this macroscopically-sterile landscape. IMPORTANCE Over the last ∼13,000 years the Pleistocene Lake Bonneville, which covered a large portion of Utah, drained and desiccated leaving behind the Bonneville Salt Flats (BSF). Today BSF is famous for its use as a speedway, which has hosted many land-speed records and a community that greatly values this salty landscape. Additionally, the salts that saturate BSF basin are extracted and sold as an additive for agricultural fertilizers. The salt crust is a well-known recreational and economic commodity, but the roles of microbes in the formation and maintenance of the salt crust are generally unknown. This study is the first geospatial analysis of microbial diversity at this site using cultivation-independent environmental DNA sequencing methods. Identification of the microbes present within this unique, dynamic, and valued sedimentary evaporite environment is an important step toward understanding the potential consequences of perturbations to the microbial ecology on the surrounding landscape and ecosystem.
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Chott El Jerid is the largest hypersaline ephemeral lake in southern Tunisian Sahara desert and is one of the biggest depressions at the North of Africa. This study aimed to investigate the diversity and abundance of microbial communities inhabiting Chott El Jerid during wet season (when it was flooded), using molecular methods [Illumina Miseq sequencing, DGGE and qPCR (qPCR)]. 16S rRNA gene analyses revealed that bacterial community was dominated by Proteobacteria (especially Ralstonia species), followed by Firmicutes, Bacteroidetes, Cyanobacteria, Actinobacteria and Verrucomicrobia. The results obtained using prokaryotic universal primers showed low relative abundance of Archaea dominated by few OTUs related to Methanosarcinaceae and Methanomassiliicoccaceae families and the presence of sulfate-reducing Archaea affiliated with Archaeoglobus. However, the results obtained using Archaea-specific primers showed that archaeal community was mainly composed of aerobic Halobacteria (especially Halorubrum species) and anaerobic members of Methanomicrobia. These results also provided evidence for the presence of members of the genus Halohasta in this environment. qPCR results revealed that Archaea were more abundant in studied samples than Bacteria. The sulfate-reducing Bacteria were also found abundant (~ one-third of the bacterial community) and outnumbered methanogens, suggesting their potential important role in this sulfate-rich and hypersaline ecosystem.
Chapter
Natural potash evaporites are a typical part of a brine evaporation series, crystallizing at the higher concentration or bittern end, either at the surface (primary salts) or in the shallow subsurface (secondary salts). Today, bedded accumulations of primary potash evaporites are a relatively rare occurrence. Extremely high solubility of most potash salts means they accumulate in highly restricted, some would say highly continental, modern depositional settings (Cendon et al. 2003). Wherever Quaternary potash does occur naturally, as in the playas of the intermontane Qaidam Basin in China and in the Danakil Depression in the Afar Rift of Africa, carnallite, not sylvite, is the dominant potash salt. This has led some to postulate that carnallite is the archetypal primary marine potash phase, while sylvite is a secondary diagenetic mineral formed by incongruent dissolution of carnallite. Others have argued that ancient sylvite was sometimes a primary precipitate, deposited by the cooling of highly saline surface or near surface brines and from seawater with ionic proportions different to those of today (Hardie 1996).
Chapter
This chapter discusses the morphological, geological, and mineralogical features of lakes on Mars. The existence of lakes on ancient Mars is now widely accepted, but that was not always the case. The history of science shows that knowledge on any scientific question is shaped by the means of exploration and those means are molded by what we think the world is. Prior to MGS of the late 1990s, the relatively low resolution of orbital imagery made it difficult to confirm Martian paleolakes by direct observations, though their existence was inferred because valley networks had already been identified on Viking and Mariner 9 images. Whether the early climate of Mars was much warmer and wetter in the Noachian compared to the later geological epochs is still the subject of ongoing debate. The existence of standing bodies of water on Mars required that at some point in its history, possibly repeatedly, physicochemical and environmental conditions allowing water to circulate and to pond were met. In addition to mineralogy and morphology, another clue about the conditions at the time of lake formation can be revealed by numbers. At Viking resolution, 179 putative impact crater lakes were identified and 210 open lake systems were cataloged from a global survey using the most recent datasets. Combined, the results from morphology, mineralogy, and estimation of lake production support the idea that favorable conditions did exist on Mars to produce a few large bodies of water, and many more, smaller lakes that formed by processes analogous to those occurring on Earth.
Chapter
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.
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The wide number of industrial processes applying cellulases highlights the importance of discovering robust enzymes able to work under harsh conditions. In this study, carboxymethyl cellulase (CMCase) activity of Paenibacillus tarimensis was characterized. A high activity was observed in pH range 3.0-10.5 and 9mM-5M NaCl. In high salt buffer at 80°C, >80% and >76% of relative activity was retained at 20% of the ionic liquids (ILs) [EMIM]Ac and [BMIM]Cl; while >40% was detected with 40% [BMIM]Cl. Five CMCases were detected by renaturing SDS-PAGE. Their activity was retained in presence of 1.7 up to 5M NaCl (for CMC1) or 4.6M KCl; 5% organic solvents or 10mM bivalent ions, EDTA and heavy metals; under neutral and halo-alkaline conditions. These cellulases stabile and highly functional under harsh conditions are promising candidates for application in detergents, textiles, paper/pulp industry; and simultaneous ILs treatment-saccharification of lignocellulose.
Article
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. Copyright © 2011 John Wiley & Sons, Ltd.
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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.
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The scarcity of liquid water in the hyperarid core of the Atacama Desert makes this region one of the most challenging environments for life on Earth. The low numbers of microbial cells in the soils suggest that within the Atacama Desert lies the dry limit for life on our planet. Here, we show that the Ca-sulfate crusts of this hyperarid core are the habitats of lithobiontic micro-organisms. This microporous, translucent substrate is colonized by epilithic lichens, as well as endolithic free-living algae, fungal hyphae, cyanobacteria and non photosynthetic bacteria. We also report a novel type of endolithic community, "hypoendoliths", colonizing the undermost layer of the crusts. The colonization of gypsum crusts within the hyperarid core appears to be controlled by the moisture regime. Our data shows that the threshold for colonization is crossed within the dry core, with abundant colonization in gypsum crusts at one study site, while crusts at a drier site are virtually devoid of life. We show that the cumulative time in 1 year of relative humidity (RH) above 60% is the best parameter to explain the difference in colonization between both sites. This is supported by controlled humidity experiments, where we show that colonies of endolithic cyanobacteria in the Ca-sulfate crust undergo imbibition process at RH >60%. Assuming that life once arose on Mars, it is conceivable that Martian micro-organisms sought refuge in similar isolated evaporite microenvironments during their last struggle for life as their planet turned arid.
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Five brightly red-pigmented, motile, rod-shaped, extremely halophilic bacteria were isolated from saltern crystallizer ponds in Alicante (two strains) and Mallorca (three strains), Spain. They grew optimally at salt concentrations between 20 and 30% and did not grow below 15% salts. Thus, these isolates are among the most halophilic organisms known within the domain Bacteria. The temperature optimum was 37-47 degrees C. A single, yet to be identified pigment was present, with an absorption maximum at 482 nm and a shoulder at 506-510 nm. The G+C content of the DNA was 66.3-67.7 mol% and, together, they formed a homogeneous genomic group with DNA-DNA similarities above 70%. The 16S rRNA gene sequences were almost identical to sequences recovered earlier from the saltern biomass by amplification of bacterial small-subunit rRNA genes from DNA extracted from the environment. This phylotype, earlier described as 'Candidatus Salinibacter', was shown by fluorescence in situ hybridization to contribute between 5 and 25% of the prokaryote community of the saltern crystallizers. We have therefore succeeded in isolating a bacterium from the natural environment that, although being a major component of the community, was previously known by its phylotype only. Isolation of the organism now allows formal description of a novel genus and species, for which we propose the name Salinibacter ruber gen. nov., sp. nov. The type strain is strain M31T (= DSM 13855T = CECT 5946T).
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Playas may form in both hydrologically open and closed systems. Several previous classification schemes of playas rely heavily on geomorphic features rather than the groundwater hydrology of the system and the degree to which the basin is hydrologically open or closed. This paper presents a review of the literature on playa classifications and their hydrologic characteristics, which can be used to better define the hydrology of playas. A modification of the original hydrologically based classification schemes of Meinzer (1922) and Snyder (1962) illustrates the importance of groundwater in playa systems. In this paper a playa is defined as an intracontinental basin where the water balance of the lake (all sources of precipitation, surface-water flow, and groundwater flow minus evaporation and evapotranspiration) is negative for more than half the year, and the annual water balance is also negative. The playa surface must act as a local or regional discharge zone. Evidence of evaporite minerals will generally be present in parts of the basin. This includes carbonate minerals that can be demonstrated to have been formed through evaporative processes. A recharge playa is defined as above, except the playa surface acts as a means for recharging water to the aquifer. In this case, evaporite minerals are absent. Although evaporite formation is an important part of a playa sequence, significant accumulations of subaqueously deposited evaporites are only possible when saline water bodies are partially maintained by a constant inflow of groundwater. In hydrologically closed basins, the brine chemistry is influenced by the lithology of the sediments and bedrock within the playa catchment. In hydrologically open (through-flow) discharge complexes, the brine composition may be determined by the rate of groundwater flow through the basin (leakage ratio) relative to the weathering rate of the surrounding sediments and bed rock. If the groundwater inflow greatly exceeds the rate of chemical weathering, then the brine composition may be dominated by the chemical composition of regional or local precipitation. Extensive accumulations of subaqueous evaporites form in playas when there is sufficient groundwater to maintain a shallow brine pond for an extended period oftime. This has important implications for paleoclimate reconstructions of closed basin playa sequences. Extremely arid periods, when there is insufficient water to maintain a brine, will result in displacive intrasediment growth of evaporites; and extremely wet periods may be too fresh for a brine to develop. It is only in the intermediate periods when evaporation is high and water input is balanced by evaporation that extensive subaqueous evaporites will accumulate in closed basin playas.
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In January 1990, an exceptional rainfall event in southern Tunisia caused the Chott el Djerid, an ephemeral salt playa, to fill with water. Under an arid climate, the ephemeral lake on Chott el Djerid evaporated to dryness in ten months. During March, May and September 1990 we sampled the lake brines. Chemical analysis of the major solutes showed that the dilute waters that flow into the Chott el Djerid basin (groundwater, wadis and aquifer waters) have a consistent chemistry, generally saturated with respect to gypsum. This may result from the uniform basin geology, which is made up of Cretaceous, Mio-Pliocene, and Quaternary sediments, dominated by marine evaporites. Potassium was conserved throughout the evaporation sequence, suggesting the saturation of sorption surfaces within the playa. With increasing evaporation, the precipitation of gypsum and halite are predicted and observed, the final, most-concentrated, brines being saturated with respect to sylvite. XRD analyses of salt crusts from the Chott el Djerid reveal a mineral assemblage of gypsum, halite and carnallitite (carnallite with halite). The overall nature of both the predicted and observed salt phases suggests that the main control on the geochemistry of the playa is the recycling of ancient marine evaporites.
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The presence of halite, an aqueous precipitate, is indicated by MER analyses of rinds and certain rock coatings at Meridiani Planum.
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The purported oldest living organism, the spore-forming bacterium Virgibacillus sp. Permian strain 2 9-3, was recently cultured from a brine inclusion in halite of the 250 Ma Permian Salado Formation. However, the antiquity of Virgibacillus sp. 2 9-3 has been challenged; it has been argued that the halite crystal and the fluid inclusion from which the bacterial spores were extracted may be younger than the Permian Salado salts. Here we report that brine inclusions from the same layer of salt that housed Virgibacillus sp. 2 9-3 are composed of evaporated Late Permian seawater that was trapped in halite cement crystals precipitated syndepositionally from shallow groundwater brines at temperatures of 17 37 °C. These results support the 250 Ma age of the fluid inclusions, and by inference, the long-term survivability of microorganisms such as Virgibacillus sp. 2 9-3.
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Marine evaporites occurring in modern subaqueous (salina) settings and subaerial (sabkha) settings are different. Subaqueous Holocene evaporites occur as shoalingupward lacustrine sequences up to 10 m thick. They are evaporite dominated and are composed primarily of bottom-nucleated crystals that may be deposited as massive, laminated, or rippled units. Each coastal lake is dominated by laminated evaporites with subordinate carbonate sediments. In plan view, they show a well-developed bull's-eye pattern with a sulfate center and a carbonate rim. In contrast, subaerial (sabkha) evaporites occur as part of a laterally prograding, shoaling-upward, peritidal sequence in which the supratidal unit is usually no more than 1 m thick. Sabkha sequences are matrix dominated, not evaporite dominated, with the bulk of the sulfate phase occurring as diagenetic nodules, enteroliths, or diapirlike structures. These sulfates were formed during syndepositional diagenesis by replacement and displacement processes. The various facies of the sequence tend to accumulate in belts parallel with the shoreline. Relative to the sea level or the brine level, sabkhas tend to form over paleotopographic highs whereas salinas tend to occur in paleotopographic lows. Some of the characteristics that distinguish Holocene subaerial and subaqueous evaporite sequences can be used to do the same for similar ancient facies, even when gypsum has been converted to nodular anhydrite. The distinction is important for it can be used by explorationists in the oil industry to define the paleotopography of the associated underlying porous and nonporous carbonates.
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The calibrated Pathfinder alpha proton X-ray spectrometer (APXS) instrument is capable of measuring concentrations of all major and minor rock-forming elements ranging from carbon through zirconium in atomic number. Therefore it is capable of constraining the petrology of the measured samples. In particular, the Pathfinder data are the first in situ measurements of Martian rocks and may be compared with previous remote-sensing measurements as well as with the compositions of SNC meteorites. Pathfinder soil and rock sample abundances from the alpha, proton, and X-ray modes have been quantified. The abundances suggest that (1) the rocks are covered with various amounts of soil; (2) the soil-free rocks, on a volatile-free basis, have some element ratios similar to Mars meteorites, yet have different bulk chemistry indicative of more evolved rocks with higher silica abundances; (3) the carbon and nitrogen contents are below detection limits; and (4) the alpha mode oxygen reveals excess amounts of oxygen in some samples, which is indicative of sample-bound water (contained within minerals or glasses in samples).
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tudying extremophiles on Earth may provide helpful metrics in our search for life elsewhere in the universe. Spe- cies that thrive in hypersaline environ- ments are among the most fascinating models for studying mechanisms of survival away from our planet. The genomic sequences ofsixsuchextremehalophilesandpost-genomic studies of these microorganisms are providing insights about what it takes to live in environ- ments that were once thought uninhabitable. A unifying requirement for life is liquid water. However, evaporative forces dissipate liquid water in arid environments on Earth, on dry planets such as Mars, and in space. In brines on Earth, evaporating water concentrates dissolved salts and increases osmotic stress for all life forms. This process, which is common in natural and artificial solar salterns, leads calcium sulfate (gypsum) and sodium chlo- ride (halite) to precipitate from seawater, leaving mainly potassium and magnesium salts in solution. During this process, dense microbial photosynthetic mats initially form but later decline, leaving only a few extremely halophilic archaea and bacteria (Fig. 1). The saturation point of sodium chloride (5.3 M), is about nine times the salinity of seawater and marks the upper limit of resistance of cultivated biological forms.
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[1] The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) is a hyperspectral imager on the Mars Reconnaissance Orbiter (MRO) spacecraft. CRISM consists of three subassemblies, a gimbaled Optical Sensor Unit (OSU), a Data Processing Unit (DPU), and the Gimbal Motor Electronics (GME). CRISM's objectives are (1) to map the entire surface using a subset of bands to characterize crustal mineralogy, (2) to map the mineralogy of key areas at high spectral and spatial resolution, and (3) to measure spatial and seasonal variations in the atmosphere. These objectives are addressed using three major types of observations. In multispectral mapping mode, with the OSU pointed at planet nadir, data are collected at a subset of 72 wavelengths covering key mineralogic absorptions and binned to pixel footprints of 100 or 200 m/pixel. Nearly the entire planet can be mapped in this fashion. In targeted mode the OSU is scanned to remove most along-track motion, and a region of interest is mapped at full spatial and spectral resolution (15–19 m/pixel, 362–3920 nm at 6.55 nm/channel). Ten additional abbreviated, spatially binned images are taken before and after the main image, providing an emission phase function (EPF) of the site for atmospheric study and correction of surface spectra for atmospheric effects. In atmospheric mode, only the EPF is acquired. Global grids of the resulting lower data volume observations are taken repeatedly throughout the Martian year to measure seasonal variations in atmospheric properties. Raw, calibrated, and map-projected data are delivered to the community with a spectral library to aid in interpretation.
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The hyperarid core of the Atacama Desert is considered the dry limit for life on Earth. Soils in this region have very low abundance of heterotrophic bacteria and are practically barren of photosynthetic microorganisms because of the extreme dry conditions (2 mm a À1 rainfall). However, relatively abundant endolithic communities of cyanobacteria (Chroococcidiopsis) occur within halite crusts in paleolake evaporitic deposits. By means of continuous monitoring of the microclimate conditions (temperature, relative humidity, water vapor density, wetness, and photosynthetically active radiation) inside and around the halite crusts, we demonstrate here that water vapor condenses within the pore space of the halite at relative humidity (RH) levels that otherwise hinder the occurrence of liquid water in the surrounding environment. Water condensation occurs at RH >75%, which corresponds to the deliquescence point of halite. We have estimated a total of 57 deliquescence events (i.e., water condensation) within the halite crusts, as opposed to only 1 liquid water event outside. These wet events resulted in a total of 213.8 h of potential photosynthetic activity for the endolithic microorganisms versus only 6 h for organisms outside the halite crusts. Halite crusts may therefore represent the last available niche for photosynthetic activity in extreme arid environments on Earth.
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Cryptoendolithic microbial communities living within Antarctic rocks are an example of survival in an extremely cold and dry environment. The extinction of these micro-organisms formerly colonizing sandstone in the Mount Fleming area (Ross Desert), was probably provoked by the hostile environment. This is considered to be a good terrestrial analogue of the first stage of the disappearance of possible life on early Mars. To date, only macroscopically observed indirect biomarkers of the past activity of cryptoendoliths in Antarctic rocks have been described. The present paper confirms, for the first time, the existence of cryptoendolith microbial fossils within these sandstone rocks. The novel in situ application of scanning electron microscopy with backscattered electron imaging and simultaneous use of X-ray energy dispersive spectroscopy allowed the clear detection of microfossils left behind by Antarctic endoliths. Careful interpretation of the morphological features of cells, such as preserved cell walls in algae, fungi and bacteria, cytoplasm elements such as chloroplast membranes in algae and organic matter traces, mineral associations, and the spatial context of these structures all point to their identification as cryptoendolith microfossils. This type of investigation will prompt the development of research strategies aimed at locating and identifying the signs that Martian microbiota, probably only bacteria if they existed, may have been left for us to see.
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Solar radiation is the primary energy source for surface planetary life, so that pigments are fundamental components of any surface-dwelling organism. They may therefore have evolved in some form on Mars as they did on Earth. Photosynthetic microbes are major primary producers on Earth, but are concurrently vulnerable to ultraviolet (UV) damage. Using non-intrusive laser Raman spectroscopy to recognize the component parts of biomolecules, we have shown not only the abundance of microbial photosynthetic and photoprotective pigments in situ, but also their spatial distribution within their microhabitat. This essential aspect of their screening or avoidance survival strategies is lost on extraction with solvents. This precise approach is eminently suited to analysis of epilithic (surface) and endolithic (within rocks) communities in Antarctic desert habitats, which are putative analogues of early Mars. Raman spectra for key biomolecules (e.g. the UV screen parietin and the antioxidant β-carotene in epilithic lichens) enable not only the detection of organics in light-stratified habitats, but also the characterization of unknown pigments. Typical biomarkers of astrobiological relevance in our Raman spectral database include scytonemin (a UV screen), chlorophyll (primary photosynthetic pigment), phycocyanin (accessory pigment for shade adaptation) and a hopanoid extracted from 2·5 Gya microbial stromatolite from Australia. This compound dates from the same time period when a wetter Mars could have had a potentially flourishing surface microbial community of its own. Analyses with a laboratory Raman instrument have been extended to a novel miniature Raman spectrometer, operating at the same optimal excitation wavelength (1064 nm) via an In-Ga-As detector. After evaluation in Antarctica, this instrument will be space-qualified for a proposed Mars rover mission to detect biomolecules in the near-surface sediment profile of palaeolakes, using experience with Antarctic biomarkers to interpret alien spectra of fundamental components, without the need for prior knowledge of the identity of the target compounds.
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We characterized a layered community of cyanobacteria and purple bacteria that developed within a gypsum crust on the bottom of a hypersaline saltern pond (salinity 280 to 290 g l(-1)) in Eilat, Israel. The upper 1 to 2 cm of the 4 to 5 cm thick gypsum crust is inhabited by carotenoid-rich unicellular cyanobacteria (Aphanothece sp. and others), imparting an orange-brown color to the gypsum. Under the brown layer, a green layer dominated by unicellular cyanobacteria of the genus Synechococcus is found, with filamentous Phormidium-type cyanobacteria as a minor component. Below these layers of oxygenic phototrophs is a red layer of purple bacteria. We studied the optical properties of the gypsum crust, both by characterization of the pigments present in the different layers and by measuring spectral scalar irradiance at different depths in the crust, using fiber-optic microprobes. In the upper 2 mm of the crust, a maximum of scalar irradiance of up to 200% of incident light was measured. Light in the blue range of the spectrum (400 to 500 nm) was effectively absorbed by the protective carotenoids (myxoxanthophyll, echinenone, and others) in the upper brown layer. However, significant amounts of Light in the red part of the spectrum penetrated down to the green layer to enable photosynthesis: about 1% of the incident irradiance at 620 and 675 nm reached the green layer at a depth of 15 mm, and >1% of the incident light in the infrared part of the spectrum reached the purple bacteria located at a depth of 20 to 23 mm.
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In the chott El Jerid in southern Tunisia, waters are strongly concentrated by evaporation under an arid climate. The diluted waters are of the Ca/1bMg/1bSO4 type whereas the brines are of the Na/1bCl type, relatively concentrated in K and Mg. In concentrated waters, Ca and silica concentrations remain at a very low level. The use of Pitzer's equations for calculating the ion activities has permitted accurate saturation tests and a fairly good evaluation of the mineral sequences controlling the geochemistry of the basin.
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We have used fatty acid analyses to study the community structure of a layered endoevaporitic microbial community within a gypsum crust that covers the bottom of a saltern evaporation pond in Eilat, Israel. This community, living at a salinity of 218–238gl−1 total dissolved salts, consists of an upper brown layer dominated by unicellular cyanobacteria, a green layer with filamentous cyanobacteria, a red-purple layer with both Chromatium and Ectothiorhodospira/Halorhodospira type of purple sulfur bacteria, and a black layer in which dissimilatory sulfate reduction occurs. An olive-green layer is sometimes present below the red-purple layer. Analysis by gas chromatography/mass spectrometry of the fatty acid methyl esters prepared from the different fractions showed characteristic patterns in each layer, and these could be related to fatty acid composition data from the literature and to fatty acid analyses of representative halophilic microorganisms isolated from the site. The nature of the fatty acids in the green layer suggests that the cyanobacteria present there use the oxygen-independent pathway for production of unsaturated fatty acids, a pathway only occasionally encountered in filamentous cyanobacteria. The facultative anaerobic nature of the cyanobacteria in the green layer was confirmed by their ability to perform anoxygenic photosynthesis with sulfide as electron donor. Specific signature fatty acids identified for each layer corresponded well with the microscopic and functional analysis of the biota present.
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Chapter
Neogene gypsum deposits provide good examples of microbially induced structures (planar laminites, columnar buildups) whose study elucidates the intimate relations between microbial components and gypsum crystallization. This study closely links petrography and comparisons with modern settings. All the examples studied in both modern and ancient gypsum deposits show that the microbial features in gypsum result from periodically controlled phases of microbial mat development and gypsum precipitation. Although most of the traces of microbial communities proliferating in the evaporitic settings are poorly preserved or absent in the fossil record, the relations between microbial remains and gypsum seems to be controlled by three main processes: (1) interstitial precipitation of gypsum within microbial mats, (2) incorporation of organic material within crystals during gypsum precipitation, and (3) organic material deposited on the surface of gypsum crystals during interruption of crystal growth.
Chapter
Signatures of microbial life in shallow evaporite systems are discussed using examples from modern coastal hypersaline settings. Organisms contributing to microbial sediments are assigned to moderate halophiles (e.g. cyanobacteria, other phototrophic bacteria, diatoms, non-phototrophic eubacteria) and extremely halophilic taxa (e.g. green algae and halobacteria). Primary production creates the organic base upon which biogeochemical cycles are based that produce a variety of authigenic minerals found in deposits of hypersaline settings. Characteristic microbial sediments include stromatolitic laminae, biolaminoid facies and sedimentary augen structures. Communities dominated by stenotopic major taxa often contribute with less unambiguous laminated structures, e.g. flocculent organics, to the sedimentary record. Based on the criteria of brine depth and salinity, a biofacies classification of marine-derived microbial sediments is proposed.
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The paper briefly summarizes what is known about the history of water on Mars. First the author considers the long-term evolution of martian water. This discussion includes the geomorphic evidence for water on Mars, the chemical evidence concerning the total amount of water that has been outgassed, and the various ways in which water on Mars can be stored or lost. This is followed by a discussion of water cycles that operate over time scales of tens or hundreds of thousands of years, driven by climatic changes that appear to be related to variations in the orbital elements of Mars.
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Solar salterns, based on a multi-pond system, give a discontinuous gradient of salt concentrations. The heterotrophic bacterial populations of ponds containing from 10% salt to saturation have been studied. Saltern samples were spread on agar plates containing different media for halophilic bacteria and one medium made with water of the pond plus nutrients. Replica plating was done to determine the salt range for growth of the colonies. We studied 150 strains to determine the salt spectra of growth, the morphology, and nutrient requirements. The following conclusions were reached: (a) In salt concentrations above 10% (total salts), most bacteria are halophilic and few are halotolerant; (b) the two types of halophilic bacteria, moderate and extreme, show different distributions; in these ponds a narrow overlap exists between 25% and 32% salts with moderate halophiles predominating below this interval and extreme halophiles above it; (c) the populations of moderate halophiles are highly heterogeneous, and the salt concentration of their habitat affects their taxonomic composition, salt range for growth, and nutrient requirements. The population composition of extreme halophiles is less affected by the salt concentrations at which these bacteria are found.
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Photogeological studies of the Elysium volcanic province, Mars, show that its sinuous channels are part of a large deposit that was probably emplaced as a series of huge lahars. Some flows extend 1000 km from their sources. The deposits are thought to be lahars on the basis of evidence that they were (1) gravity-driven mass-flow deposits (lobate outlines, steep snouts, smooth medial channels, and rough lateral deposits; deposits narrow and widen in accord with topography, and extend downslope); (2) wet (channeled surfaces, draining fea- tures); and (3) associated with volcanism (the deposits and channels extend from a system of fractures which also fed lava flows). Heat associated with magmatism probably melted ground ice below the Elysium volcanoes and formed a muddy slurry that issued out of regional fractures and spread over the adjoining plain. The identification of these lahars adds to the evidence that Mars has a substantial volatile-element endowment.
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Phyllosilicate and sulfate deposits in Meridiani have been mapped with high resolution MRO CRISM data. The transition on Mars to an acid sulfate ground water system may be recorded in the phyllosilicate and layered deposits within Miyamoto crater.
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A ∼1 km vertical section of etched terrain and hematite-bearing plains materials and nearby cratered terrain surfaces was mapped in the northern portion of Meridiani Planum using MOC, THEMIS, MOLA, and OMEGA data. The oldest materials are the cratered plains, which have 0.4 to 2.5 μm spectral reflectance dominated by a mix of low and high calcium pyroxenes. Etched plains materials overlie this unit and are exposed within a 120 km NW-SE trending valley to the south of the cratered plains. Lower etched plains materials exhibit a kieserite signature on a plateau-forming horizon and polyhydrated sulfate signatures on the main valley floor. The upper etched plains unit exhibits signatures consistent with hydrated iron oxides and is covered by what is interpreted to be a relatively thin layer of basaltic sand and hematitic concretions (the hematite-bearing plains unit). The youngest unit consists of ejecta deposits from a cluster of six craters that mantle the eastern portion of the study area. The thick section of etched plains materials (∼900 m) with exposures of hydrated sulfates and hydrated iron oxides implies that measurements made by the Opportunity rover (located ∼390 km to the southwest of the study area) are at the top of an extensive section of layered sedimentary materials formed in and/or altered in an aqueous environment and exposed by aeolian erosion.
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The Chotts Trough of southern Tunisia is a sedimentary basin that lies immediately north of the Saharan platform. Within this basin there exist several structurally controlled depocentres, which were created by Miocene to Early Pleistocene compression associated with the Atlas orogeny. One of these depocentres, the Chott Djerid basin, has an average Quaternary subsidence rate of 0.01–0.27 mm yr1. Additional structural features within the Chotts Trough include east-west trending anticlines and synclines, at least some of which are controlled by east-west striking subsurface faults and northwest striking right-lateral strike-slip faults. Present movement along the northwest striking faults is a direct consequence of the Africa-Europe collision. The northwest striking faults and the east-west striking faults have cut the Chotts Trough into numerous smaller structural blocks, which experience different rates of subsidence. For the Quaternary, subsidence is caused by local extension associated with movement along the various strike-slip faults.
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The gas vacuoles which occur in various prokaryotic organisms can be estimated quantitatively by the change in light scattering which takes place when they are destroyed by pressure. The gradual disappearance of gas vacuoles under rising pressure is explained by the intrinsic variation in critical collapse pressure of their constituent gas vesicles. These collapse instantaneously at pressures exceeding their critical pressure, but withstand repeated and prolonged application of pressures below this value. Gas vesicle membranes are freely permeable to gases, and as a consequence the vacuole gas is at atmospheric pressure in aerated suspensions. Increasing or decreasing the pressure of gas in the gas vacuoles brings about a corresponding change in the pressure required to collapse them, indicating that the vacuole gas helps to support the structure. Pressures in excess of the vacuole gas pressure are borne by the membrane itself. The pressure required to collapse gas vacuoles present in blue-green algae is increased if the cells are suspended in a hypertonic sucrose solution, because this removes the cell turgor pressure acting on them. This observation, which confirms the classical theory on the osmotic relationships of plant cells, provides the first reliable method of estimating turgor pressures in prokaryotic organisms. Cell turgor pressure was found to be higher in a blue-green alga than in a purple sulphur bacterium investigated; no cell turgor could be detected in a halobacterium which grows in saturated brines, suggesting that the salt concentration must be the same inside and outside the cell. The gas vesicles in these organisms seemed to be adapted to withstand the pressures they were likely to encounter, those of the alga being the strongest, and those of the halobacterium the weakest. Even so, the range of turgor pressure overlapped the critical pressure range of the gas vesicles in the alga and purple sulphur bacterium so that turgor pressure alone may effect their collapse under certain circumstances. With the alga this seems to happen in conditions promoting photosynthesis, providing the organism with a means of regulating its buoyancy. It is suggested that the width of a gas vesicle is important in determining its strength, and that this explains the differences in size and shape of the gas vesicles which have evolved in the three organisms. Interfacial tension could in theory exert considerable pressures on the highly curved surface of a gas vesicle but this effect would be minimized if its outer surface were of a hydrophilic nature. Several observations have been made which support this idea. Pressures generated by centrifugation will collapse isolated gas vesicles and must be considered when using this technique to purify them. Sufficient pressure to collapse gas vesicles can also be developed in small columns by the massive negative accelerations developed in collisions. This phenomenon, which may have application in engineering fields, must also be reckoned with in handling these pressure-sensitive structures. It is concluded that even though the gas vesicle membrane must tear during its collapse, the gas it contains diffuses away rather than escaping as a bubble.
Article
Chotts Djerid and Fedjadj lie in the arid zone of southern Tunisia, and are chemically active, artesian environments which give rise not only to extensive emergent groundwater gypsum and salt deposits, but also to landforms of more complex architecture. In some geological situations belts of spring mounds up to 25 or 30 m high straddle former high-level late Pleistocene lake shorelines. Spring mounds are circular, gypsiferous or calcareous, volcano-like structures fed by point sources of artesian water rising from aquifers in the Continental Intercalaire and Complexe Terminal aquifer series. Similar mounds have been identified around playas in the USA, Australia, Egypt, Iraq and elsewhere. In some mounds a central well is occupied by a small pond with water conduits in the base, whilst others are dry and degraded. In these latter, water is only present at depth and wind erosion and redeposition are moulding the landforms. The morphology of spring mounds is determined by the balance between aquifer pressure head, output, chemical content, net evaporation, aeolian activity, and plant colonization. Their stratigraphy comprises alternations of crystalline materials, wind-blown gypsiferous sand, wash and splash deposits, and organic layers arranged in overlapping sequences which dip outwards from the water source. Such deposits offer a potentially sensitive index of environmental changes in the late Quaternary and Holocene.
Article
The Mars Exploration Rover Spirit and its Athena science payload have been used to investigate a landing site in Gusev crater. Gusev is hypothesized to be the site of a former lake, but no clear evidence for lacustrine sedimentation has been found to date. Instead, the dominant lithology is basalt, and the dominant geologic processes are impact events and eolian transport. Many rocks exhibit coatings and other characteristics that may be evidence for minor aqueous alteration. Any lacustrine sediments that may exist at this location within Gusev apparently have been buried by lavas that have undergone subsequent impact disruption.
Article
Analyses of Mars Express OMEGA hyperspectral data (0.4–2.7 μm) for Terra Meridiani and western Arabia Terra show that the northern mantled cratered terrains are covered by dust that is spectrally dominated by nanophase ferric oxides. Dark aeolian dunes inside craters and dark streaks extending from the dunes into the intercrater areas in mantled cratered terrains in western Arabia Terra have similar pyroxene-rich signatures demonstrating that the dunes supply dark basaltic material to create dark streaks. The dissected cratered terrains to the south of the mantled terrains are dominated spectrally by both low-calcium and high-calcium pyroxenes with abundances of 20–30% each retrieved from nonlinear radiative transfer modeling. Spectra over the hematite-bearing plains in Meridiani Planum are characterized by very weak but unique spectral features attributed to a mixture of a dark and featureless component (possibly gray hematite) and minor olivine in some locations. Hydrated minerals (likely hydrous ferric sulfates and/or hydrous hydroxides) associated with poorly ferric crystalline phases are found in the etched terrains to the north and east of the hematite-bearing plains where erosion has exposed ∼1 km of section of layered outcrops with high thermal inertias. These materials are also found in numerous craters in the northern Terra Meridiani and may represent outliers of the etched terrain materials. A few localized spots within the etched terrain also exhibit the spectral signature of Fe-rich phyllosilicates. The ensemble of observations show that the evidence for aqueous processes detected by the Opportunity Rover in Meridiani Planum is widespread and confirms the extended presence of surface or near-surface water over this large region of Mars. The scenarios of formation of Terra Meridiani (“dirty” acidic evaporite, impact surge or weathering of volcanic ash) cannot satisfactorily explain the mineralogy derived from the OMEGA observations. The formation of the etched terrains is consistent with leaching of iron sulfides and formation of sulfates and hydrated iron oxides, either in-place or via transport and evaporation of aqueous fluids and under aqueous conditions less acidic than inferred from rocks examined by Opportunity.
Article
Rosickyite is a rare form of sulfur (gamma-sulfur; monoclinic symmetry) that is not thermodynamically predicted to be stable at Earth's surface temperatures; instead, it reverts to the more common alpha-sulfur form (orthorhombic symmetry). Here we show, for the first time, that rosickyite exists and is stably maintained within an endoevaporitic microbial community from the salt pan of Death Valley, California. We hypothesize that this mineral is formed by a cycle of microbial dissolution of gypsum (CaSO4·2H2O) to sulfide and reoxidation of the sulfide to elemental sulfur (rosickyite) within a stable oxygen-sulfide gradient maintained by the organisms. Furthermore, we report a microstratigraphic layering of mineral types that correlates with layering of the microbial community. Knowledge of how microbial communities can affect the mineral assemblages of evaporite deposits on Earth can help us to identify potential markers of the past or present existence of life on extraterrestrial bodies bearing evidence of ancient seas or lakes.
Article
The Complexe Terminal Aquifer is a multi-layered aquifer that supports a thriving date industry in the Algerian-Tunisian Sahara. Although there are numerous uncertainties regarding the exact size of this aquifer and the amount of water in place, it is certain that more water is being removed from the aquifer than is replenished by recharge. Calculations of aquifer volume, discharge rates, and recharge rates indicate that the lifespan of the Complexe Terminal Aquifer is limited, and that depletion of the aquifer will probably occur sometime well before the year 2266.
Conference Paper
Recent discoveries and important oil shows have proven the existence of hydrocarbons in newly identified depocenters and reservoirs. In general, except for some areas around the producing fields, Tunisia is largely underdrilled. The national company ETAP has decided to release data and to publish a synthesis on the petroleum geology of Tunisia. The geology of Tunisia provides a fine example of the contrast between Alpine folding, which typifies northern Tunisia and the African craton area of the Saharan part. Eastern Tunisia corresponds to an unstable platform forming plains or low hills and extending eastwards to the shallow Pelagian Sea. There are a wide variety of basins: central and northern Tunisia represents a front basin the Saharan Ghadames basin or the Chott trough are sag basins; the Gulf of Gabes was formed as a distension margin the Gulf of Hammamet is a composite basin and several transversal grabens cut across the country, including offshore, and are rift-type basins. All these features are known to be oil prolific throughout the world. Two large fields and many modest-size pools are known in Tunisia. Oil and gas fields in the surrounding countries, namely the Saharan fields of Algeria and Libya the large Bouri field offshore Tripolitania and discoveries in the Italian part of the Straits of Sicily, suggest a corresponding potential in Tunisia. Exposed paleogeographic and structural maps, balanced sections, and examples of fields and traps will support an optimistic evaluation of the future oil exploration in Tunisia.
Article
Surface habitats in Antarctic deserts are near the limits of life on Earth and resemble those hypothesized for early Mars. Cyanobacteria dominate the transient riverbeds, stromatolitic sediments in ice-covered lakes, and endolithic communities in translucent rock. There is still no direct evidence of photosynthetic life on early Mars, but cyanobacteria are amongst the earliest microbes detectable in the fossil record for analogous habitats on Earth. Key biomolecules persist in Antarctic microbial habitats, even after extinction by excessive low temperatures, desiccation and UV-B stress within the Ozone Hole. Pigments (or their fossil residues), such as chlorophyll and the UV-protectants scytonemin, carotene and quinones, are good biomarkers. To show not only their presence but also their micro-spatial distribution in situ, we describe the use of FT–Raman spectroscopy with 1064 nm excitation to avoid autoouorescence from the pigments. We report not only the diversity of biomolecules that we have diagnosed from their unique Raman spectra of Antarctic cyanobacterial communities, but also their functional stratiÿcation in endolithic communities. Our analyses of Antarctic habitats show the potential of this remote, non-intrusive technique to probe for buried biomolecules on future Mars missions and in Antarctic Lake Vostok, ¿ 4 km beneath the Central Ice Sheet, with implications for the putative analogous sub-ice ocean on Europa.
Article
Abstract— Water-soluble ion concentrations from the martian achondrite Nakhla and three asteroidal achon drites are reported. The Nakhla sample contains significant concentrations of chloride, sulfate, Mg, Na, Ca, and K ions. The results are interpreted to indicate that this rock has been in contact with a seawater-like brine on the martian surface.
Article
Dunaliella tertiolecta (marine) and D. viridis (halophilic) were each trained by serial transfer to growth at salt concentrations previously regarded as the other's domain. D. viridis then had a salt optimum at 1.0–1.5 M sodium chloride whereas that for D. tertiolecta was less than 0–2 M. Nevertheless D. tertiolecta grew faster than the halophil at all salt concentrations up to 3.5 M, the highest at which they were compared.Both species accumulate glycerol, which is necessary for growth at elevated salinities and which responds in its content to water activity (a w ) rather than specifically to salt concentration. Variation in glycerol content is a metabolic process which occurs in the dark from accumulated starch as well as photosynthetically. Regulation of glycerol content by a w does not require protein synthesis. The NADP-specific glycerol dehydrogenase of each of the algae is likely to be directly involved in the regulation of glycerol content. Kinetic studies, together with those described in an earlier publication, show that the enzyme has regulatory properties, and that both glycerol and dihydroxyacetone act as effectors as well as reactants. A mechanism of the reaction is tentatively proposed.
Article
Marine salterns are habitats for a large variety of halophilic bacteria. In the anoxic zones, halophilic sulfur bacteria develop mainly at the sediment surface, but only a few of them have so far been isolated from such environments. Among the phototrophic sulfur bacteria that sometimes form purple layers underneath the green cyanobacterial layers, members of the generaEctothiodhodospira, Chromatium (C. salexigens), Thiocapsa (T. halophila) were isolated. They grow by using sulfide as an electron donor. In the marine salterns, sulfide originates from active sulfate reduction. Among the halophilic sulfate-reducing bacteria, onlyDesulfovibrio halophilus andDesulfohalobium retbaense have so far been isolated. The ecology and physiology of both kinds of bacteria are discussed in this paper.
Chapter
When water on a planet begins to evaporate the dissolved minerals become more concentrated, form a brine, eventually precipitating out of solution and forming evaporite deposits. The brines and evaporites form a highly saline environment that is hostile to most life forms, but provides an ideal habitat for organisms that are osmophilic, or halophilic. Halophiles are salt loving organisms that inhabit environments with salt concentrations ranging from 15 % to saturation. On Earth, representatives of halophiles occur in all three domains of life, the Archaea, Bacteria and Eukarya.