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Biodata of Joseph Seckbach, author of the chapter “The Relevance of Halophiles and other Extremophiles to Martian and Extraterrestrial Environments” and the “Foreword” of this book. Professor Joseph Seckbach is the initiator and chief editor of Cellular Origins, Life in Extreme Habitats and Astrobiology (COLE) book series, and author of several chapters in this series. He earned his Ph.D. from the University of Chicago, Chicago, IL (1965) and spent his postdoctoral years in the Division of Biology at Caltech (Pasadena, CA). Then he headed at the University of California at Los Angeles (UCLA) a team for searching for extraterrestrial life. Dr. Seckbach has been appointed to the faculty of the Hebrew University (Jerusalem, Israel), performed algal research, and taught biological courses. He spent his sabbatical periods in Tübingen (Germany), UCLA and Harvard University, and served at Louisiana State University (LSU) (1997-1998) as the first selected occupant of the John P. Laborde endowed Chair for the Louisiana Sea Grant and Technology transfer, and as a visiting Professor in the Department of Life Sciences at LSU (Baton Rouge, LA). Among his publications are books, scientific articles concerning plant ferritin (phytoferritin), cellular evolution, acidothermophilic algae, and life in extreme environments. He also edited and translated several popular books. Dr. Seckbach is the co-author (with R. Ikan) of the Chemistry Lexicon (1991, 1999) and other volumes, such as the Proceeding of Endocytobiology VII Conference (Freiburg, Germany, 1998) and the Proceedings of the Algae and Extreme Environments meeting (Trebon, Czech Republic, 2000); see: http://www.schweizerbart.de/pubs/books/bo/novahedwig051012300-desc.ht). His recent interest is in the field of enigmatic microorganisms and life in extreme environments. E-mail: [email protected] 123 N. Gunde-Cimerman et al. (eds.), Adaptation to Life at High Salt Concentrations in Archaea, Bacteria, and Eukarya, 123-136. © 2005 Springer. Printed in the Netherlands. THE RELEVANCE OF HALOPHILES AND OTHER EXTREMOPHILES TO MARTIAN AND EXTRATERRESTRIAL ENVIRONMENTS JOSEPH SECKBACH The Hebrew University of Jerusalem Home: P.O.Box 1132, Efrat 90435 Israel 1. Introduction Life in various forms appears almost all over Earth, and most organisms live in environments considered “normal” from our anthropocentric point of view. There are other organisms, however, that are exceptional, living in very harsh conditions—they are termed extremophiles. The extremophiles live in extraordinary habitats; some of them are not able to grow in more moderate conditions or even to tolerate them for short periods. Such harsh environments are lethal for most other organisms. The majority of known extremophiles are unicellular—members of the domains Archaea and the Bacteria; there are also a few members of the Eukaryotes (protists and multi-cellular forms) among these exceptional organisms. The study of extremophiles and their habitats increases our understanding of the limits of life and the nature of the first organisms on early Earth, and in addition gives us an insight into the possibilities of extraterrestrial life (which is a part of the study of astrobiology). Astrobiology, an interdisciplinary research field, is the new scientific branch of studies that includes the origin of life, various parameters of terrestrial environmental conditions and their inhabitants (extremophiles), astronomy, astrophysics and the possibilities for extraterrestrial life. In essence, astrobiology is all encompassing. From comparison of the physical and atmospheric conditions on some celestial bodies with the inhospitable niches and environments on Earth, we find that some extremophiles could serve as candidates or analogues for extraterrestrial life in the Solar System and beyond. Thus, the study and understanding of extremophiles is valuable for astrobiology. Among the current celestial places suitable as candidates for finding past and perhaps present life are Mars, the Jovian satellites such as Europa, and – for exploring initial stages of life as they may have occurred on young Earth – Saturn’s moon Titan. Thus extremophiles, living at the edge of life, may serve as models for life forms in or beneath the surface of Mars, Jupiter’s satellites (e.g. Europa), and other planetary bodies in the Solar System and beyond. 2. The Extremophiles Most extremophiles are (micro)organisms that thrive in habitats which for other terrestrial life-forms are intolerably hostile or even lethal environments. They are 126 JOSEPH SECKBACH classified (see below) according to the conditions in which they grow as thermophiles, hyperthermophiles, psychrophiles, halophiles, acidophiles, alkaliphiles, barophiles, etc. (Table 1). In addition to the various extremophilic factors one may add that microbial spores can withstand a variety of different hostile conditions in their dormant stage for long periods of time. There are some limits in long-term dormancy, namely, thermal decomposition and ionizing radiation (McKay, 2000). Extremophiles thrive in hot niches, ice, salt solutions, acid and alkaline media; some may grow in toxic waste, organic solvents, heavy metals while extremophiles are found as well in several other habitats that were previously considered inhospitable for life and toxic for organisms (see Table 1). TABLE 1. The extremophiles on Earth and possibly on extraterrestrial bodies Environmental parameter Salinity Type Location and Conditions Examples Halophiles Cyanobacteria, diatoms, flagellates, Dunaliella; Halobacteriaceae. pH Acidophiles In salt lakes (neutral, and alkaline pH). In salt solutions up to saturation. Synthesis of glycerol to balance the external osmotic pressure pH 0 - 4 some hot springs Alkaliphiles pH >9 in African soda lakes Psychrophiles -20oC - <15oC Thermophiles 50oC - 80oC Hyperthermophiles Barophiles 80oC - >100oC Weight lovers Piezophiles Pressure lovers High level of radiation Temperature Hydrostatic pressure Radiation, Ultraviolet and ionizing radiation Thriving in organic solvents and under various atmospheres Gravity Desiccation Picrophilus oshimae, Cyanidium caldarium, Dunaliella acidophila, Natronobacterium, Spirulina sp., fungi Psychrobacter, Polaromonas, cyanobacteria, diatoms and icy green algae, some insects. Cyanidium, prokaryotes as Mastigocladus Pyrolobus fumarii (113oC) Prokaryotic strain MT41 tolerates >100 MPa. Cyanobacteria, desert plants Deinococcus radiodurans CO2 gas Thriving at pure gas Organic toxic solutions solutions of damaging agents Hyper-gravity >1 g Hypo-gravity <1g Xerophiles Anhydrobiotic condition Low water activity Cyanidiaceae (grown under pure CO2) Such microorganisms are not known on Earth (this factor may be related to extraterrestrial environments) Prokaryotes, extreme halophiles or endoliths, fungi and lichens (a symbiotic system), nematodes Based on Oren and Seckbach (2001), Seckbach and Oren (2004), and other sources. EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS 127 Many of the extremophiles cope with more than one environmental parameter, and may be termed as multi- or poly-extremophiles (Rothschild and Mancinelli, 2001). For example, the unicellular rhodophytan Cyanidium caldarium (Seckbach, 1994, 1999; Seckbach and Walsh, 1999) and its cohorts are thermo-acidophiles growing in an acid media (pH 0.5 to 3.5) at elevated temperatures (maximal temperature 57oC); they tolerate salinity range from 3 to 10% (Pinto et al., 1994), 1 N H2SO4 (Allen, 1959) and thrive under pure CO2 (Seckbach, 1994). Likewise the barophiles/piezophiles, the weight/pressure lovers are also either thermophiles, living near hydrothermal vents at the bottom of the oceans, or psychrophiles (cold lovers) in the depth of the seas and oceans. Among the extremophiles are representatives of all domains of life [Prokarya (Archaea, Bacteria) and Eukarya]. Most extremophiles are prokaryotes (Archaea and Bacteria), but this group also includes eukaryotic cells (Roberts, 1999). Among the Eukaryotes are protists (e.g., algae, fungi, protozoa) and multicellular forms living in some of these harsh habitats. Some extremophiles are relegated to a particular habitat and are not able to survive milder conditions, while other are more flexible and can grow also in “normal” environments. A number of these drastic environments may represent the primeval conditions that existed for the pioneering microbes evolving during the early periods in young Earth. For example, hyperthermophiles live in very warm or hot conditions and obtain their energy by heterotrophic and/or chemotrophic (with inorganic chemicals) metabolic pathways, while thermophiles of less extreme temperatures (i.e., below 74oC) may even be photosynthetic. The ability of thermophiles and hyperthermophiles to cope with such extreme environments may agree with the idea proposed by several scientists that the first organisms evolved in warm zones. The thermophiles are also placed at the roots of the archaeal branch in the tree of life based on molecular sequences. They may have evolved in the primordial soup of little ponds, in hot springs, inside rocks or even at the hydrothermal vents at the openings found around the black smokers at the bottom of the oceans. The hydrothermal vents are located at mid-ocean ridges where plates spread apart and spew out hot water containing dissolved minerals providing material and energy for a rich diversity of extremophilic life. Other extremophiles may have evolved at great depth of beneath the surface environments. Living in such places, these microorganisms (if they existed) may have been protected from the impact of the asteroid bombardment on the planet and shielded from high doses of UV irradiation during the early stages of Earth’s evolution. Huge extraterrestrial asteroids and boulders from space hit young Earth between 4.5 Ga and 3.8 Ga (bya = billion years ago). During this period, some microbes may have found refuge by descending into underground areas where the temperature could reach ~93oC and around hydrothermal vents it could well over 100oC. There is some evidence that life appeared when this impact ceased at 3.5 to 3.8 Ga (Glausiusz, 2004). The conditions below Earth’s surface were suitable for thermophiles deriving their energy via chemoautotrophic oxidation of inorganic chemicals, e.g., Acidithiobacillus thiooxidans oxidizes sulfur or sulfide to sulfate, A. ferrooxidans oxidizes ferrous to the ferric iron, while others oxidize hydrogen to water, nitrites to nitrates. Other microorganisms may have survived by blasting into space and returning to Earth later when the conditions became milder. The fact that there is almost no limit in the habitats on Earth brings the astrobiologists to 128 JOSEPH SECKBACH propose that extremophiles may serve as models for candidates for extraterrestrial life in planets and moons of the Solar System and beyond. Some enzymes from extremophiles are tolerant of extreme parameters such as heat, salinity, acidity, alkalinity, and high or low pressures. They have been introduced recently into applied biotechnology systems and industry. We can find the extremophiles and their cellular productions in food and dairy products, health food, medical and pharmaceutical lines, antibiotics, detergents, in fermentation processes, and so on. While this chapter covers most extremophiles, it does not deal with microorganisms thriving under anaerobic conditions. The anoxic atmospheres appeared as the original natural environment, while the oxygenated atmosphere is a secondary evolution. The oxidative atmosphere evolved after the appearance of the oxygenic photosynthesis process by cyanoprokaryota (Avers, 1989; Shapiro, 1986). Additional reviews on extremophiles can be found in various chapters in the series Cellular Origins, Life in Extreme Habitats and Astrobiology; see: www.springeronline.com, and in Horikoshi and Grant (1998), Roberts (1999), Rothschild and Mancinelli (2001), and Seckbach (1999, 2000a, 2000b, 2003) and Seckbach and Walsh (1999). 2.1. HALOPHILES – THE SALT LOVERS In the Solar System our Earth is considered the sole water-endowed planet, some 71% of its area are seas and oceans, mostly salt water. Similar water bodies have not yet been proven to exist outside the Solar System. The dawn of the appearance of the first organisms is unknown. They were most probably simple cells belonging to the Prokaryotes and ruled on Earth for 2–2.5 Ga (until the evolution of the eukaryotic cell). In this “Prokaryotic world” the Archaea and Bacteria were the sole organisms on Earth and they occupied almost every niche on the surface and subsurface of Earth. The prokaryotic microbes may have evolved 3.6 bya within saline solutions in the oceans; it seems, therefore, that at present too, all organisms “remember their roots” and use salt in their cells. Brines are the medium for halophilic microorganisms, and such environments occur ubiquitously on Earth. Halophiles represent a class of extremophilic organisms that are also adapted to high salt solution. Among the hypersaline halophiles are photosynthetic green algae (such as Dunaliella species), diatoms, and others. The enigmatic Cyanidiaceae, a group of thermo-acidophilic red algae, including Cyanidioschyzon merolae, Cyanidium caldarium and Galdieria sulphuraria tolerate NaCl solutions from 3 to 10% of NaCl (Pinto et al., 1994; Seckbach, 1994, and see above). Among the halophiles at different ranges of pH are also cyanoprokaryota, green and purple Bacteria, sulfur-reducing Bacteria, and methanogenic Archaea. Most halophiles live in the marine environments (containing water of 3% salt), others thrive in inland lakes within bodies of salty water, while the hyper-halophiles grow in very high concentration salt solutions. The rod shaped halobacteria strains grow within a concentration of ~ 30% salt as salt concentration is present in the Dead Sea (Israel-Jordan) and in the Great Salt Lake (Utah, USA). Other may tolerate solutions of saturated salt solutions. As the water evaporates from salty aqueous bodies, high concentrations of salt solutions remain, then only halophiles will remain and obtain EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS 129 these areas for themselves. Vreeland et al. (2000) revived bacteria that were enclosed in salt crystals for 250 million years. Assuming this report is authentic and fully reliable, we may have a new record of halophilic life in dry condition for vast periods. The halophiles have to react against the high gradient gaps in osmotic pressure of their external solution vs. their internal cytosol medium. They have to avoid plasmolysis – the process where cell aqueous solution escapes due to the hypertonic external solution that causes the cytoplasm shrinkage. To avoid the escape (plasmolysis) of cell liquid, the halophiles balance these concentration differences of the solutes inside and outside the cell membrane by synthesizing intracellular organic compounds (e.g., glycerol, betaines, sucrose, K+ ions, etc.). These compounds are synthesized and/or accumulated in response to osmotic stress at high salinity. Halophiles may regulate the movement of solutions by other means as well (Oren, 2002). The cell membrane may also regulate the osmotic pressure on both sides of the plasmalemma. Some Antarctic lakes are salty at different concentration and some can reach saline concentration close to the Dead Sea level. Such saline lakes do not freeze during the Antarctic winter even when the temperature declines to -50oC. For further literature on the halophiles, see Oren (1999, 2002) and the chapters in this volume. 2.2. LIVING AT THE END RANGES OF TEMPERATURE AND pH 2.2.1. Thermophiles and Hyperthermophiles The organisms living in warm or hot habitats are the thermophiles and hyperthermophiles, respectively. On the tree of life these microbes are placed close to the root of the prokaryotic position. It has been reported that among organisms there are some differences in heat tolerance (Table 1). For example, the upper temperature limit for eukaryotes is 62oC, while the thermo-cyanoprokaryotes can occur at < 70oC, thermophilic bacteria may thrive at 70oC to <100oC and hyperthermophilic Archaea can be detected at a maximum of 113oC (observed in the archaeon Pyrolobus fumarii – see also Seckbach and Walsh, 1999 and tables therein). This latter temperature point is considered the highest current temperature for life and might support the source where life has originated. Under special treatments and conditions the maximum temperature of life and the edge of stability can reach higher values, such as 121-125oC (Kashefi and Lovley, 2003; Seckbach and Oren, 2004). Such archaeal anaerobes gain their energy from reducing Fe+3 into Fe+2 via formation of magnetite. The thermophiles thrive in geothermic sources, such as in hot springs, hot geysers, or at hydrothermal vents at the ocean floors. Black smokers on the sea floor expel hot water at 350oC, gases, minerals, and other materials from the inside of the Earth. One of the microbes associated with the walls of black smokers is Methanopyrus that produces methane (CH4) gas. 2.2.2. The Psychrophiles Like Cold and Chilly Environments Life cannot grow where liquid water does not exist. On the opposite scale from the spectrum of the thermophiles are the psychrophiles (cold-loving organisms) that grow at very cold ranges of temperature, even below freezing some of these may live in icy layers. In snow and ice layers one can find eukaryotic diatoms, green algae (e.g., Chlamydomonas nivalis) that “paint” snow with their pigmentation (green, red, orange). 130 JOSEPH SECKBACH Psychrophilic microorganisms are in permafrost of Siberia, in the Arctic zones, and in the icy depth close to the Vostok Lake (Antarctica). The Antarctic sea is populated with Bacteria, algae, and diatoms. In Antarctica the endolithic microbes—among them Bacteria, cyanobacteria, algae, and the symbiotic association-lichens live a few millimeters below the surface of rocks at very cold temperatures. In addition, at the ocean floor are worms dwelling in methane-enriched environments. The psychrophiles can tolerate severe cold environments at the level of –20oC (Rivkina et al., 2000; Junge et al., 2004). Furthermore, lichens survive Antarctica winters at –65oC. Many psychrophiles are also barophiles (poly-extremophiles) and such microorganisms possibly could survive on Mars when the air temperature reaches -10oC during daytime throughout the warmest part of the year. 2.2.3. Acidophiles Acidophilic members are among all three domains of life. Some may be phototrophic others heterotrophic – all require acid media. Seckbach (2000a) presented a table of the lower pH limits for different groups of organisms. Most of the thermo-acidophiles are prokaryotes (Archaea and Bacteria) with the exception of some fungi and the algal members. Their habitats are in warm, hot (such as occur at volcanic fields, solfatara fields, hot springs), ambient and cold areas. Some acidic habitats are formed by microorganisms, such as after oxidation of elemental sulfur or ferrous ions and other compounds by bacteria (e.g., Acidithiobacillus thiooxidans, A. ferrooxidans), which leads to accumulation of sulfuric acid and ferric ions that precipitate Fe(OH)3 with release of protons in various environments, according to the following equation: 2 Fe2+ + 0.5 O2 + 5 H2O Æ 2 Fe(OH)3 + 4 H+ Acid environments occur around active geothermal vents and springs, abandoned mines, or from sulfuric compounds’ contaminations and other polluted places with acid wastes turning the area to acidic. Several poly-extremophiles thrive in various ranges of pH, from milder solutions to very acidic media. Among the prokaryotes are thermo-acidic organisms (Oren and Seckbach, 2001; Seckbach and Walsh, 1999; Weiss Bizzoco, 1999; see tables therein). The archaeal hyperacidophilic Picrophilus oshimae and Picrophilus torridus grow down to pH –0.06, the lowest level of pH known to support life. Thermoplasma acidophilum, another archaeon, grows at pH 1.8-2 (min. at 0.4). The eukaryotic thermoacidophilic red alga Cyanidium caldarium thrives at pH 0-3.5 (Seckbach, 1994, 1999) and tolerates 1 N sulfuric acid (Seckbach, 1994, 2000b). Some green algae are acidophiles, such as Dunaliella acidophila at pH 0.5-3.0 (Pick, 1999), Chlamydomonas acidophila (pH 1-2) or Euglena mutabilis (pH 1-5). Among diatoms, Pinnularia braunii grows at pH 0-4. Cyanobacteria are known to thrive at neutral to alkaline pH. Yet there are some exceptions, such as the Oscillatoria and Spirulina discovered in Bavarian lakes at acidic condition measured a pH of 0.06 to 2.9 (Steinberg et al., 1998) but not shown to grow at these levels (Castenholz, personal communication). Hundreds of lakes in Sweden and Canada showed cyanobacteria present even in the most acidic lakes (Steinberg et al., 1998) which are very oligotrophic. There are three species of fungi growing at pH near 0 (Schleper et al., 1995): Acontium cylatium, Cephalosporium EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS 131 spp., and Trichosporon ceribriae, which grow at pH –0.06 while their optimum growth is at pH ~ 0.5. These acidic pH values are only external to the cells; the intracellular cyto-solution (cytosol) keeps its pH level around neutral pH (Beardall and Entwisle, 1984; Oren and Seckbach, 2001; Seckbach, 2000a). The cell membrane rejects and prevents the H+ from entering the cell by using a variety of mechanisms for also pumping the protons out. Strong cyto-proton pump or low proton membrane permeability regulates the H+ ions and stops them from entering the plasmalemma. The internal neutral pH has to protect some cyto-constituents like DNA, ATP, and chlorophyll from the acidification damage. Sasaki et al. (1999) reported about intracellular extract solutions from four species of Dictyotales to be highly acidic (at pH 0.5 to 0.9), similar values of pH were observed in some species of Desmarestiales (both groups are brown algae, Phaeophyta). Their intracellular pH measured to be 0.5-0.9, because their vacuoles contain sulfuric acid. Apparent these low values are due to the vacuolar content, but the true cytoplasmic pH is supposedly much higher. Further data on acidophiles are reported in volumes 1, 2, and 6 of Cellular Origins, Life in Extreme Habitats and Astrobiology (COLE) and elsewhere. See website: www.spingeronline.com 2.2.4. Alkaliphiles In some soils and soda lakes in east Africa and under dry climates (such as in Mojave desert), the soil is very alkaline. There are microorganisms (Archaea, Bacteria, cyanobacteria, and protists) that prefer to live on the high scale of the pH (alkaline levels). Such high pH levels were found together with high salt solutions. In highly alkaline-saline lakes, as in Mono Lake, CA, are diatoms, and green algae such as Nanochloris, Chlamydomonas and Dunaliella. The cyanobacterium Spirulina platensis is an obligate alkaliphile, and reaches a high population at pH values of 11 and above. Plectonema nostocorum has been reported to grow up to pH 13 which is considered the highest level at which life has been found. The diatoms are prominent components in the biota of many alkaline lakes. These alkaliphiles keep the highly alkaline pH outside the cell and pump ions in and out the cells (similar to the acidophiles) to keep a lower and milder intracellular milieu which might reach up pH 9 (Horikoshi, 1998; see table 6.2 there) while the external pH level is much higher. For further data on alkaliphiles see Horikoshi (1998), Oren and Seckbach (2001), and Seckbach and Oren (2004). For additional data on alkaliphiles see Horikoshi and Grant (1998). The alkaline hypersaline lakes and their inhabitants may serve as analogues for ancient microbial habitats on Mars (see below). 2.3. XEROPHILES—THE DRY LIVING AND DESICCATED ORGANISMS Some microorganisms have the ability to grow in low water activity and tolerate water stress. Low water availability can cause high concentration of salt and other solutes. Deserts and dry valleys of Antarctica or the Siberian permafrost support a variety of microorganisms in dry conditions. A survey (Davis, 1972) has shown that cyanobacteria (e.g., Nostoc sp.) survive desiccation for a record of 107 years. The diatom Nitzschia palea and the chlorophytes Pleurococcus sp. and Cystococcus sp. were shown to be viable after 98 years of dry storage in a herbarium. 132 JOSEPH SECKBACH Spores have a low if any rate of metabolism, and remain in long-term periods of dormancy before being revived. Well known are the cases claiming of viable bacterial spores resuscitated from the stomachs of insects imbedded in amber dated at 25–40 million years (Cano and Borucki, 1995). If the report (Vreeland et al., 2000) of the restoration to life of bacteria that had been encapsulated for 250 million years in a salt crystal is supported by unquestionable ground, then we have strong evidence on the long period during which life can be persevered. 2.4. ORGANISMS THRIVING AT HIGH PRESSURE Barophiles (weight loving) and piezophiles (pressure loving) microorganisms both in the subsurface of Earth and in the depths of the ocean are obligated to live heterotrophically, chemoautotrophically or chemolithotrophically, in darkness and at high pressure. This elevated hydrostatic pressure can reach over a thousand atmospheres (at the bottom of the ocean). For every 10 m depth in the water column the hydrostatic pressure increases by about 1 atmosphere (0.1 MPa). The pressure at the deepest point in the ocean (at the bottom of Mariana Trench near the Philippines) is about 110 MPa. The barophilic bacteria are poly-extremophiles being psychrophilic in addition. Among the Archaea are the baro-thermophiles observed in the hydrothermal vents; Themococcus barophilus grows at temperatures up to 100oC. Our knowledge about barophiles must be expanded for us to understand more about this fascinating group. For additional discussion on deep-sea Bacteria, see Yayanos (2000). 2.5. MORE EXTREME PARAMETERS FOR MICROORGANISMS There are other factors of extreme environments that support life under various atmospheres. Some algae thrive under pure CO2 (Seckbach, 1994), and other microorganisms grow under ammonia (Siegel, 1999) or on hydrogen gas (e.g., the methanogenic Archaea). Microorganisms can grow in environments with chemicals, heavy metals, organic solvents that are lethal for most other organisms. Another parameter is gravity, which might be relevant more to life in space or in celestial places, rather than to earthling organisms. 3. Astrobiology In recent years the new multidisciplinary field of astrobiology has been established. Currently, in this area there are numerous books, publication of a plethora of articles in various journals and proceedings of conferences, meetings, workshops and a great deal of research being conducted. The wide variety of astrobiology-related disciplines include prebiotic chemistry, astronomy, astrophysics, origin of life, microfossil records, evolution, diversity of microorganisms, extremophiles, geobiology, molecular biology, extremophiles and extraterrestrial possibilities of life. It also includes life in the Universe, discovery of new stars, planets and satellites and the search for extraterrestrial intelligent life (SETI). Astrobiology has been referred also as bioastronomy, exobiology, extraterrestrial life and the Russian term cosmobiology. EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS 133 The extremophiles that are ubiquitous may be considered as candidates for inhabiting celestial places. Furthermore, if there is life to based on silicon instead on carbon as well in solvents other than water (non-aqueous solvents), then life may perhaps exist on planets and satellites. The most popular extraterrestrial bodies within the Solar System that may carry living forms (as we know it), or their precursors are Mars and the Jovian moons such as Europa. Titan, the satellite of Saturn, may serve as a prebiotic laboratory resembling the Earth’s prebiotic periods. Europa has a very cold environment (minus 162oC) and from its image one can distinguish huge cracked shells of ice floating and moving on water. These ice sheets may cover and float over an ocean of warm water originating from Europa’s hydrothermal vents that may reach a couple of hundred degrees Celsius. The surface appearance of Europa resembles Alaska and Antarctica with their frozen seas. If Prokaryotes can survive such severe conditions on Earth, why not there in the heavenly bodies? One can ask the question: How unlikely is it that life does not exist elsewhere? Similar to Jupiter, Saturn is a gas planet surrounded by hydrogen and helium (with traces of methane and ammonia). Its largest moon Titan is covered by a reddish fog over the icy continent and contains organic molecules including hydrocarbon oceans (while no oxygen is present in its atmosphere). Similar prebiotic organic chemistry may have led to life on Earth. 3.1. MARS Our knowledge about Mars has increased but still not complete. Wherever there is free liquid water and other certain elements, there should be life. Water makes all cells “running” in their enzymatic reactions, natural processes and other functions in all cells. There are still debates among the astrobiologists whether Mars contains briny water on the upper layers of the surface (Möhlmann, 2004) or whether the Martian water is within layers present deeper in the subsurface. Such water might be accumulated in salty oceans warmed up by volcanic sources, while it could be also located as hydrated minerals. On Mars, similar to Earth, liquid water should goes together with life since water is vital for cell metabolism, enzymatic reactions, processes as photosynthesis, etc. The recently discovered methane in the Martian atmosphere provides encouragement for the scientists searching for life (Formisano et al., 2004). This detection of Martian methane may not originated from chemical reaction but rather it is believed that it may have evolved from biological sources The images of Mars indicate that in the past it was warmer (during the caldera’s age) and wetter, with plenty of running water in rivers, canyons, lakes and large oceans. The bulk of the Martian water might be tied up in the subsurface, either as permafrost in the craters in the polar latitudes, or in deep aquifers as reservoirs of water (McKay and Marinova, 2001). Possibly some forms of life may have flourished there in the past. Perhaps spores from the ancient “green Martian Planet” are still present as dormant spores and might be revived when environmental conditions will be improved by terraforming (Graham, 2004). If Martian surface conditions are at present noninhabitable for living forms, the subsurface (lithologies in subterranean areas) may offer the only dwelling place for extant life forms. Therefore, finding of liquid water on Mars is vital for the presence of contemporary life on this planet. 134 JOSEPH SECKBACH Today the red planet is dry, cold (-50oC to +12oC); the Martian salty solutions are lowering the freezing point of its water so that microbes might grow in liquid water. Mars also has a very low atmospheric pressure (~ 6 to 10 mbars) and the oxygen in the atmosphere is very low (<0.15%). Its atmosphere contains 95% CO2, 2.7% N2, 1.6 % Ar and other minute ingredients. The soil of Mars contains mineral ions such as: Na, Mg, Ca, Cl and K. Organisms require sulfur, potassium, sodium, chloride, potassium, iron, and trace elements, all thought to be present on Mars. Since it would be difficult for organisms to grow at the surface regions in the cryosphere and under UV radiation of Mars, if life does exist on Mars today, it could be located in the subsurface where warmer ground waters may be present. The thin Martian ozone layer does not significantly block UV rays, but the radiations of very short wavelengths penetrate all the way to the surface. If some species of Bacteria or Archaea may overcome Martian conditions (similar to those Bacteria existing in permafrost – see Möhlmann 2004), then this planet might perhaps harbor psychrophilic life. Also, microbial dormant spores may have life preserved there in 3.5 to 4 Ga old sediments that were left over from when the planet was warmer and had running water. Earthling Arctic and Antarctic life forms wait for the icy coverage to melt into liquid water (conditions that might be similar on Mars). Mars (and other planets’ moons) might carry life in hydrothermal lakes under the surface or in ice-rich regions similar to the way microbial life is harbored in the icy layer and in subsurface warm water which is assumed to exist in Vostok Lake (Antarctica). As the early water evaporated from Mars, a high concentration of dissolved salt was left so that only halophiles remained and had the planet for themselves. By understanding osmophily (halophily) and extremophiles on Earth we may detect life that lives or has lived on other planets. Such osmophiles are best suited for life in brines and evaporates. Halophiles may possibly occur in the Martian icy soil, as they thrive in the hypersaline ponds of Antarctica. Therefore, a preparatory search for extant and extinct life of Mars should be a focused on osmophiles or on their fossilized remnants (Mancinelli, this volume). All the answers about Martian life can be found only after samples from Mars will be brought to Earth. Recently there have been some proposals to create a greenhouse effect for warming Mars towards restoring habitable conditions on this planet. Such terraforming ideas for Mars have been published (Friedmann and Ocampo-Friedmann, 1995; McKay and Marinova, 2001). Only in time will we know if such proposals are realistic or just wishful dreams. Is the future export or transfer of microbes from Earth to Mars practical? Some scientists believe that the red planet is currently too cold and dry to allow the survival of any terrestrial organisms. Terrestrial life forms could perhaps survive there only after warming up the planet to a minimal level. Such warming of Mars can take place by using artificial greenhouse gases (Marinova et al., 2004). Mars appears to have sufficient water, if warmed, to form large bodies, and to once again be capable of harboring life. There might be a possibility that genetic engineering may “create” and convert organisms and equip them with special features to resist extraterrestrial environments such as those existing on Mars. If introduced on Mars they may assist in the terraforming there. EXTREMOPHILES AND EXTRATERRESTRIAL ENVIRONMENTS 135 Acknowledgements I thank Prof. Richard Castenholz, Prof. David J. Chapman, Dr. Rocco Mancinelli, and Dr. Christopher McKay for their efficient reviews on this chapter and for providing practical suggestions. I appreciate the kind invitation of Prof. Nina Gunde-Cimerman to take part in the Halophile-2004 conference in Ljubljana (Slovenia) and I am grateful for the generous support bestowed upon my attendance there. References Avers, C.J. 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