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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, but polyextremophiles which are exposed to unusually high level of UV radiation for Earth, hyperarid conditions, hypersaline conditions, and extremely low nutrients (Abbott and Pearce). Notable similarities exist to conditions known to occur on Mars, or to what is known of the icy moons of Jupiter and Saturn.

Today, Earth harbors vast oceanic ecosystems—most of them only barely explored— which support life (psychrophilic, barophilic and chemosynthetic) adapted to conditions that may occur in other planets in the universe (George). Climate change-induced processes may slow down major ocean currents, such as the Antarctic Bottom Water (ABW) flowing into the lower hadal or ultra-abyssal zone at a depth greater than 8,000 meters, with potential creation of hypoxic hadal zones at extreme ocean depths and its consequences.

In Part II, experiments with extremophiles in space, e.g., on the International Space Station, are presented. Bacteria as well as Archaea, lichens, fungi, algae and tiny animals (tardigrades) are now being investigated for their tolerance to extreme conditions in simulated or real space environments. Experimental results from exposure studies on the International Space Station and on space probes for up to 1.5 years are presented.

Anhydrobiotic cyanobacteria of the genus Chroococcidiopsis possess a remarkable resistance to desiccation and radiation. These cyanobacteria were exposed to laboratory simulations that mimic planetary conditions, such as dryness, UV and gamma radiation, and also to real space conditions on the International Space Station (Billi). The resistances found are discussed with respect to lithopanspermia.

Lichen species, which are composite organisms consisting of algae or cyanobacteria in a mutualistic relationship with filaments of various fungi, are well-known survivors of the most stressful environments. The first space experiments were carried out with Rhizocarpon geographicum and Xanthoria elegans in the Foton-M2 satellite in 2005 (de la Torre Noetzel and Sancho). Space simulation and space exposure experiments have focused on these and other lichen species, demonstrating their high capacities of survival and recovery.

The halophilic archaeon Halococcus morrhuae and the biofilm-forming bacterium Halomonas muralis were exposed to space conditions during the EXPOSE-R2 mission. Hlm. muralis was much less resistant to extreme conditions than Hcc. morrhuae (Leuko et al.). Exposure to outer space had a strong detrimental effect on the survival and genomic stability of Hcc. morrhuae; however, Hcc. morrhuae could be re-cultivated from samples exposed on the ISS for up to 534 days. These results add to our understanding that life may be able to survive the travel through space.

The cryptoendolithic endemic black fungus Cryomyces antarcticus was isolated from sandstone collected in the McMurdo Dry Valleys in Antarctica, a place which is considered the coldest hyperarid desert on Earth and one of the best terrestrial analogues for Mars (Onofri et al.). C. antarcticus is able to survive intense ionizing radiation, probably due to the presence of a highly melanized thick cell wall.

In Part III, several authors provide reviews on specific topics, such as Jönsson on the properties of tardigrades (also called water bears), which were used in the Foton-M3 mission and survived a combined exposure to space vacuum, cosmic radiation and UV radiation. The evidence for their metabolic arrest (cryptobiosis) and multiple resistances to environmental extremes make these tiny animals (size ca. 1 mm or less) particularly useful astrobiological models.

Nicholson reviews bacterial endospores, the very first model of cells used for astro-biological purposes, and demonstrates their connection to and impact on the history of origin-of-life studies as well as the concept of lithopanspermia. Modern experimental testing of this concept involves using ballistic devices to simulate the interplanetary exchange of rocks containing bacterial spores.

Paul and Mormile consider the very basic problem of available sources of energy for microbial life on Mars. They analyzed the utilization of several available compounds— hydrogen and methane as key molecules, but also iron, sulfur and others—by extremophilic microorganisms, providing numerous examples of a wide range of well-described biochemical reactions. A cautious look at the possible connection to the energy demands of potential human settlements is also included.

Part IV contains articles on theory and hypotheses. The search for extraterrestrial life is closely connected with the question of the origin of life on Earth and its early evolution. Newer proposals emphasize the importance of environmental oscillations and suggest experiments for testing this on a laboratory scale (Kompanichenko and Levchenko).

Several theories exist on the existence of a prebiotic world, which are outlined by Jheeta. The implications of the vast—and probably still underestimated—horizontal gene transfer and the potential role of viruses and RNA are presented. One or several transitional (T)-LUCAs (last universal common ancestor) are postulated as leading to the emergence of the first cells.

Recent evidence for an unprecedented archaeal diversity (uncultivated strains) and novel bacterial phyla, due to significantly improved sampling of the subsurface and whole genome sampling, has greatly extended our views on the LUCA of all life, including viruses and their consideration as ancient forms of life (Lineweaver).

Instrumentation for the search of sulfur isotopes as biomarkers for potential habitats for extremophiles in the Solar System is proposed by Chela-Flores, together with a discussion of the pros and cons for possible life on the celestial bodies close to Earth (Moon, Mars, Europa, Titan, and the icy moons).