Deserts cover about one-fifth of the Earth’s surface area and present some of the most hostile living conditions on the planet and therefore are the home to only groups of microorganisms that are able to tolerate the extreme conditions. Deserts are extremely dry, and cold and always have water as a limiting factor . Nevertheless, studies have found a considerable number of microbes that have developed methods to adapt and survive beneath the sandstone rock surface in microbiotic crusts that dominates many desert environments .
Physicochemical environmental conditions
These microbes are not only subject to water deprivation but also face a number of other environmental factors such as wide temperature ranges and nutrient starvation . Hot deserts can reach temperatures up to 65 degrees Celsius and cold deserts can drop below -30 degrees Celsius . The annual rainfall for a desert is defined to be at most 50 cm annually, but is generally much less ranging from around 1-10 cm . The vegetation is usually extremely sparse and the soil is very deficient in organic matter. The aridity of the soil can be further accelerated by the occurrence of hot and dry winds and the low water retention capability of the sand . The pH of arid soils is fairly alkaline, measuring at pH values of around 9-10 . At high environmental pH’s, aerobic microbes face the challenge of proton scarcity and must be able to overcome energetic barriers to successfully use their membrane bound ATP synthase .
Culture-based methods to evaluating the diversity in a population are not necessarily representative of the sample being studied . For this reason, the predominant use of ribosomal RNA genes are used to assess microbial diversity. The different deserts around the world have given rise to different kinds of microbial communities. In the hot desert of Tataouine, for example, analysis of 16S rRNA gene libraries suggested majority of sequences corresponded to phyla of Proteobacteria, Actinobacteria and Acidobacteria . Similarly, in a separate study, the molecular diversity of the Rubrobacter, a genus of Actinobacteria, was shown in Australian dry soil . The existence of a monophyletic group called ‘Rubrobacteria’ was found by comparative analysis with other soil, sediment and water environments . Surprisingly, some microbes even function as primary producers in their highly competitive environment. Microorganisms such as hypoliths and Cyanobacteria are able to grow on the undersides of rocks and photosynthesize at irradiance levels as low as 0.1% of the incident light, minimal water conditions and freezing temperatures . This is achievable through periglacial activity, which aligns rocks into specific patterns and results in areas where the penetrating light is enough to support carbon assimilation and facilitate photosynthesis .
Mechanisms for survival
The different ways that extremophiles are able to survive the environmental stresses of the desert are considered for the most part undiscovered, however many strategies are still known. General mechanisms include the utilization of non-reducing sugars such as sucrose and trehalose to prevent damage to cellular structures . At high temperatures in the desert, the fluidity of membranes naturally increases so many organisms adjust the levels of saturated and unsaturated lipids to retain optimal fluidity . Proteins and DNA are also affected by higher temperatures and undergo denaturation and chemical modification in most organisms. Thermophiles, on the other hand, increase their ion-pair content in their proteins that allow higher-order oligomers to form that have increased rigidity . Another mechanism involves decreasing the length of surface loops in a protein in order to maximize hydrophobic interactions and enhance electrostatic interactions for increased stability of the protein .
Significance and Applications
Extremely arid environments such as the Atacama Desert provide conditions that are extreme enough to be comparable to Mars and therefore could have implications for the discovery of life on another planet . More specifically, a hypersaline subsurface microbial environment was discovered in the Atacama Desert containing halite, nitrate and perchloratecontaining salts . The presence of chlorides and perchlorates are essential as they are two key inorganic compounds that make this terrestrial habitat possibly analogous to subsurface systems on Mars . Measurements were made in this subsurface environment with a life detector chip containing 300 antibodies designed to tag biological material . The LDC picked up different bacteria, Achaea and other types of biological material whose likelihood of being present on Mars can now be individually assessed in future studies.
Chanal, A., Chapon, V., Benzerara, K., Barakat M., Christen, R., Achouak, W., Barras, F., and Heulin, T. "The desert of Tataouline: an extreme environment that hosts a wide diversity of microorganisms and radiotolerant bacteria". "Environmental Microbiology." 2006. Volume 8. p. 514-525.