The Sahara Desert is characterized by extreme environmental conditions, which are a unique challenge for life. Cyanobacteria are key players in the colonization of bare soils and form assemblages with other microorganisms in the top millimetres, establishing biological soil crusts (biocrusts) that cover most soil surfaces in deserts, which have important roles in the functioning of drylands. However, knowledge of biocrusts from these extreme environments is limited. Therefore, to study cyanobacterial community composition in biocrusts from the Sahara Desert, we utilized a combination of methodologies in which taxonomic assignation, for next-generation sequencing of soil samples, was based on phylogenetic analysis (16S rRNA gene) in parallel with morphological identification of cyanobacteria in natural samples and isolates from certain locations. Two close locations that differed in microenvironmental conditions were analysed. One was a dry salt lake (a “chott”), and the other was an extension of sandy, slightly saline soil. Differences in cyanobacterial composition between the sites were found, with a clear dominance of Microcoleus spp. in the less saline site, while the chott presented a high abundance of heterocystous cyanobacteria as well as the filamentous non-heterocystous Pseudophormidium sp. and the unicellular cf. Acaryochloris. The cyanobacteria found in our study area, such as Microcoleus steenstrupii, Microcoleus vaginatus, Scytonema hyalinum, Tolypothrix distorta, and Calothrix sp., are also widely distributed in other geographic locations around the world, where the conditions are less severe. Our results, therefore, indicated that some cyanobacteria can cope with polyextreme conditions, as confirmed by bioassays, and can be considered extremotolerant, being able to live in a wide range of conditions.
The hyperarid Sahara Desert presents extreme and persistent dry conditions with a limited number of hours during which the moisture availability, temperature and light allow phototrophic growth. Some cyanobacteria can live in these hostile conditions by seeking refuge under (hypolithic) or inside (endolithic) rocks, by colonizing porous spaces (cryptoendoliths) or fissures in stones (chasmoendoliths). Chroococcidiopsis spp. have been reported as the dominant or even the only phototrophs in these hot desert lithic communities. However, the results of this study reveal the high diversity of and variability in cyanobacteria among the sampled habitats in the Sahara Desert. The chasmoendolithic samples presented high coccoid cyanobacteria abundances, although the dominant cyanobacteria were distinct among different locations. A high predominance of a newly described cyanobacterium, Pseudoacaryochloris sahariense, was found in hard, compact, and more opaque stones with cryptoendolithic colonization. On the other hand, the hypolithic samples were dominated by filamentous, non-heterocystous cyanobacteria. Thermophysiological bioassays confirmed desiccation and extreme temperature tolerance as drivers in the cyanobacterial community composition of these lithic niches. The results of the present study provide key factors for understanding life strategies under polyextreme environmental conditions. The isolated strains, especially the newly described cyanobacterium P. sahariense, might represent suitable microorganisms in astrobiology studies aimed at investigating the limits of life.
The hyper-arid climate of Ouargla area (Northern Sahara of Algeria) is important in the field of the paleoenvironmental reconstitution. This paper investigates the importance of palaeolake sediments as significant terrestrial archives for studying Quaternary climate changes. Through morphological and mainly micromorphological investigations of the studied sites (Sabkha of Ouargla and palaeolake of Mellala), we noted the fluctuation of climate between arid and more humid conditions during Quaternary. Some pedofeatures have recorded these environmental changes. The studied calcareous crusting has a lacustrine origin when precipitations were more important. The clay coatings around the quartz grains indicate an old origin and different geochemical conditions that led to their dissolution. Lenticular gypsum crystals were formed from groundwater saturated with Ca 2þ and SO 2À 4 ions. Calcite pseudomorphs after lenticular gypsum crystals indicate changes in local climatic parameters. Gypsum crusts can be explained by formation under wet climatic conditions in a lagoonal environment. Black and gray layers indicate evolution of organic matter at a wetter period. This paper highlights an unknown palaeolake which can be useful for the comprehension of the ecological past of the Sahara. The comparison between this palaeolake and other Saharan palaeolakes has allowed us to assign it to a probable active period between 10000 and 4000 years B.P. We are very grateful to Dr. Grillon François (center of materials Pierre-Marie Fourt, France) for the providing the SEM photomicrographs and to the Unité Environnement et Grandes Cultures (INRA-AgroPariTech, Paris-Grignon, France) for manufacturing soil thin sections. We would also like to give particular thanks to M. Belferrag Allaoua (University of Ouargla) for the constructive comments. We are also thankful to anonymous reviewers whose comments on an earlier version of this manuscript improved its quality substantially.
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