EXTREMOPHILES as Astrobiological Models. Группа авторов

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      Preface

      Foreword

      Life on Earth is ubiquitous, with most organisms living in so-called “normal” environments that we consider ambient habitats. Many microorganisms are known to tolerate and resist harsh and extreme external conditions; they are called extremophiles [6; 7]. Among them are Archaea, Bacteria, fungi, plants and microscopical animals, like the tardigrades. Some of these microorganisms are living at elevated pressure, such as in the depths of the oceans; some are able to tolerate extreme temperatures and pH values, desiccation or strong UV radiation. Some extremophiles may be under the stress of more than one factor, and we refer to them as polyextremophiles [8]. The severe environments most probably resemble the conditions on early Earth. Some of the extremophiles may thus be considered as “living fossils” since their environments resemble the conditions that have existed during the time when life is thought to have arisen on Earth, more than 3.8 billion years ago.

      Are there any life forms outside the terrestrial regions? Why should such extremophilic organisms not also live in extraterrestrial places? If these organisms can thrive in such harsh conditions on Earth, they should be able to exist on celestial bodies with similar conditions.

      The origin of life on Earth is still unknown. Life has been suggested to evolve from hot springs, geysers, ocean depths, volcanoes, etc. Some scientists think that life originated in cold environments, and suggest connections to bacteria in extremely cold territories like Antarctica or the North Pole.

      Who and what is in this new book?

      In 1995 the first extrasolar planet was discovered by Mayor and Queloz [4]; now that number is 4,301 [5; July 2020] and the question of whether there is life—possibly intelligent life—in space is more timely than ever.

      Research is hampered by the fact that no other life-bearing planet has been found which could serve as a comparison. Or has it? The question of whether there is—or was—life on Mars, at least in the form of simple microbes, has not yet been solved unequivocally [1]. The Jovian moon Europa and the Saturnian moons Titan and Enceladus are also considered promising candidates for worlds with life [3].

      This book deals with the description of extremophilic microorganisms which live in environments with similarities to those known from several planets and moons in the solar system. Also, on Earth, environmental conditions occur which are lethal or at least harmful to many organisms, but specialists nevertheless survive or even thrive under these conditions.

The first chapters (Part I) consider extremophiles which are found in environments with possible or very likely similarities to the conditions on extrasolar planets or other celestial bodies. For example, the pH can be very low, such as 3.0, and temperatures can be high (> 90 °C), such as in volcanic steam vents or fumaroles (Bizzoco and Kelley). Using a special collector for the steam, the authors discovered a great diversity of thermophiles, which mostly belong to the Archaea.

      Acidophiles are of special interest because their chemolithotropic metabolism obtains energy from reduced minerals, thus creating the extreme acidic conditions in which they thrive. An extensive geomicrobiological characterization of the Rio Tinto basin in Spain has proven the prominent role of the iron cycle in the ecosystem (Amils and Fernández-Remolar). The identification of iron sulfates and oxides on Mars, analogous to those generated in the Tinto basin by microbial metabolism, has made Rio Tinto one of the best geochemical and mineralogical terrestrial Mars analogues.

      Recent findings suggest that microbiomes which are found in brackish, marine and hypersaline modern sapropels (‘rotten mud’) include yet uncultured Archaea that may be close to the evolutionary roots of eukaryotes and life itself (Andrei et al.). The extreme geochemistry of certain sapropels, as well as their relevance in the preservation of biomarkers, might qualify them as analogs for early Earth habitats or for the exploration of habitable extraterrestrial milieus.

      Expansive evaporite mineral deposits on Mars are evidence of ancient lacustrine systems (Bayles et al.). As the surface water dried up, hypersaline lakes would have filled the ancient lake basins. Halite and gypsum contain fluid inclusions where microorganisms may be entombed over geologic time. Haloarchaea are also resistant to other extremes, such as high radiation doses. These properties make them excellent analogues for life that could have existed in the hypersaline lakes on Mars and perhaps remained preserved in the evaporitic minerals there.

      Historical observations of NASA’s activities towards Mars (Viking experiments) are presented by Oremland, including the early enthusiasm for astrobiology (although that name was not yet coined in the 1970s). His research focuses on soda lakes, which are alkaline (pH ≥ 9.5), often hypersaline (salinity > 35 g/L), mineral-rich water bodies, and their amazingly intense microbial populations, such as haloalkaliphilic arsenotrophs, which are capable of using As(V), Fe(III), or S(0) as electron acceptors. The possibility of similar environments on Mars or planetoids (Enceladus, Titan) is considered.

      Antarctica is a rich source of extremophiles, not just psychrophiles, СКАЧАТЬ