Exobiology in the Solar System & The Search for Life on Mars - ESA

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SP-1231 30 I.3.3 High-Salt Environments ‘psychrophiles’, while those capable of growth at 0ºC but with optimal growth temperatures above 15°C are ‘psychrotrophs’. Psychrotrophs usually outnumber psychrophiles <strong>in</strong> a given biotope, s<strong>in</strong>ce <strong>the</strong>y can benefit more efficiently from transient ‘warm’ conditions. For example, bacteria liv<strong>in</strong>g <strong>in</strong>side <strong>the</strong> rocks of dry valleys <strong>in</strong> Antarctica grow best at temperatures of 10-20ºC on rock faces exposed to <strong>the</strong> Sun <strong>in</strong> summer (Nienow & Friedman, 1993). Cold-lov<strong>in</strong>g organisms are ma<strong>in</strong>ly bacteria (of all types) or eucaryotic microorganisms. However, putative cold-lov<strong>in</strong>g archaea have been detected recently <strong>in</strong> glacial water from Antarctica by direct amplification of <strong>the</strong>ir genetic material (DeLong et al., 1994). Some of <strong>the</strong> most psychrophilic organisms are algae liv<strong>in</strong>g <strong>in</strong> snow-covered areas, where <strong>the</strong>y <strong>in</strong>habit <strong>the</strong> upper 1 cm layer of snow. <strong>The</strong>y often have optimal temperatures below 10ºC. <strong>The</strong> lower temperature limit <strong>for</strong> life on Earth is not so clear as <strong>the</strong> upper limit, because it is very difficult to monitor growth and/or metabolic activity at sub-zero temperatures. As a consequence, very few fundamental studies have been done regard<strong>in</strong>g this problem. In <strong>the</strong> literature, <strong>the</strong> lower limit <strong>for</strong> bacterial growth, -12ºC, comes from a 30-year-old paper. It would be valuable to resume such studies. However, -12ºC might be correct, because it could correspond to <strong>the</strong> temperature at which <strong>in</strong>tracellular ice is <strong>for</strong>med (Russell, 1992). One can speculate that, at lower temperatures, non-covalent bonds become too strong <strong>for</strong> <strong>the</strong> k<strong>in</strong>d of reversible reactions required <strong>for</strong> life to exist. A major problem faced by cold-lov<strong>in</strong>g organisms at very low temperatures is <strong>the</strong> solidification of <strong>the</strong>ir membranes, which risks los<strong>in</strong>g <strong>the</strong> fluidity required <strong>for</strong> proper function<strong>in</strong>g. To bypass this drawback, <strong>the</strong>y usually change <strong>the</strong>ir lipid composition <strong>in</strong> order to <strong>in</strong>crease membrane fluidity at low temperatures. Similarly, <strong>the</strong> structure of prote<strong>in</strong>s from psychrophilic organisms is modified <strong>in</strong> order to <strong>in</strong>crease <strong>the</strong>ir flexibility such that <strong>the</strong>ir activity is optimal at low temperatures compared to mesophilic prote<strong>in</strong>s. <strong>The</strong> type of modification is somehow a mirror of those occurr<strong>in</strong>g <strong>in</strong> hyper<strong>the</strong>rmophilic prote<strong>in</strong>s, i.e. while prote<strong>in</strong>s from hyper<strong>the</strong>rmophiles are usually more compact than <strong>the</strong>ir mesophilic counterparts, homologous prote<strong>in</strong>s from psychrophiles are often less compact with more side-cha<strong>in</strong>s loosely connected to <strong>the</strong> prote<strong>in</strong> core. Adaptation to low temperatures appears much easier than adaptation to very high temperatures <strong>for</strong> terrestrial life – whereas hyper<strong>the</strong>rmophily is restricted to certa<strong>in</strong> groups of procaryotes, psychrophily is widespread <strong>in</strong> all procaryotic and eucaryotic groups. <strong>The</strong> reason <strong>for</strong> this difference is unknown, but one can tentatively speculate that it might be due to <strong>the</strong>rmodegradation (breakage of covalent bonds), which is restricted to high temperatures. A well-studied group of extremophiles are <strong>the</strong> salt-lov<strong>in</strong>g organisms known as extreme halophiles (Gal<strong>in</strong>ski & Tyndall, 1992). Monovalent and divalent salts are essential <strong>for</strong> terrestrial life (K + , Na + , Mg ++ , Zn ++ , Mn ++ , Fe ++ , Cl - , etc) because <strong>the</strong>y are required as co-catalysts <strong>in</strong> many enzymatic activities. In that sense, all organisms are salt-dependent. However, <strong>the</strong> tolerated salt concentrations are usually quite low (
(ma<strong>in</strong>ly found <strong>in</strong> halophilic archaea) is to accumulate high concentrations of KCl and MgCl 2 <strong>in</strong> <strong>the</strong>ir cytoplasm (near-saturation). In that case, <strong>the</strong> <strong>in</strong>tracellular mach<strong>in</strong>ery is also <strong>in</strong> direct contact with <strong>the</strong> high salt content. In halophilic archaea, <strong>the</strong> <strong>in</strong>tracellular mach<strong>in</strong>ery is adapted to <strong>the</strong> high salt concentration of <strong>the</strong> cytoplasm, correspond<strong>in</strong>g to very low water activity. Prote<strong>in</strong>s from extreme halophiles are not only active at very high salt concentrations, but <strong>the</strong>y are denatured by <strong>the</strong> removal of salt. A most spectacular experience is to add water to a suspension of halophilic archaea observed under <strong>the</strong> microscope. <strong>The</strong> cells vanish as soon as <strong>the</strong> ionic strength reaches a critical po<strong>in</strong>t because membrane prote<strong>in</strong>s dissociate when <strong>the</strong> salt concentration become too low. <strong>The</strong> mechanism of stabilisation of <strong>in</strong>dividual prote<strong>in</strong>s at high salt levels is now beg<strong>in</strong>n<strong>in</strong>g to emerge from structural studies. Halophilic prote<strong>in</strong>s exhibit typical networks of negatively-charged am<strong>in</strong>o-acids at <strong>the</strong>ir surfaces, allow<strong>in</strong>g <strong>the</strong>m to reta<strong>in</strong> a stabilis<strong>in</strong>g network of salt and water molecules. However, <strong>the</strong> question as to how nucleic acid-prote<strong>in</strong> <strong>in</strong>teraction can occur <strong>in</strong> 3-4 M KCl rema<strong>in</strong>s a major challenge <strong>for</strong> future work. <strong>The</strong> chemistry of life on Earth is optimised <strong>for</strong> neutral pH. Aga<strong>in</strong>, some microorganisms have been able to adapt to extreme pH conditions, from pH 0 (extremely acidic) to pH 12.5 (extremely alkal<strong>in</strong>e), albeit ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>the</strong>ir <strong>in</strong>tracellular pH between pH 4 and 9. Many bacteria and archaea are acidophiles, liv<strong>in</strong>g at pH values below 4 (<strong>for</strong> a review see Norris & Ingledew, 1992). <strong>The</strong> record is held by <strong>the</strong> archaeon Picrophilus oshimae, which grows optimally at pH 0.7 and still grows at pH 0 (Schleper et al., 1995). Like many o<strong>the</strong>r acidophiles, this organism is also <strong>the</strong>rmophilic. Among acidophiles, <strong>the</strong> most <strong>the</strong>rmophilic ones are Sulfolobales (archaea), which can grow at pH 2 and up to 90ºC. In many cases, <strong>the</strong>se organisms are actually responsible <strong>for</strong> <strong>the</strong> acidity of <strong>the</strong> biotope <strong>in</strong> which <strong>the</strong>y live. For example, Sulfolobales are sulphur-respir<strong>in</strong>g organisms and produce sulphuric acid as a by-product of <strong>the</strong>ir metabolism. Many bacteria and a few archaea, <strong>the</strong> alkaliphiles, live at <strong>the</strong> o<strong>the</strong>r extreme of <strong>the</strong> pH range, from pH 9 up to pH 12 (<strong>for</strong> a review see Grant & Horikoshi, 1992). <strong>The</strong>y are present everywhere on Earth. Some of <strong>the</strong>m, which have been discovered <strong>in</strong> soda lakes rich <strong>in</strong> carbonates, are also halophiles (haloalkaliphiles). Most alkaliphiles are mesophiles or moderately <strong>the</strong>rmophilic, but Stetter and co-workers have recently described <strong>the</strong> first hyper<strong>the</strong>rmophilic alkaliphile, <strong>The</strong>rmococcus alkaliphilus (Keller et al., 1995). Both acidophiles and alkaliphiles rely on sophisticated transport mechanisms to ma<strong>in</strong>ta<strong>in</strong> <strong>the</strong>ir <strong>in</strong>tracellular pH near neutrality by pump<strong>in</strong>g or excret<strong>in</strong>g protons. As <strong>in</strong> <strong>the</strong> case of moderate halophiles, <strong>the</strong>y protect <strong>the</strong>ir <strong>in</strong>tracellular environment aga<strong>in</strong>st <strong>the</strong> external extreme values of this parameter. <strong>The</strong>y should be also able to ma<strong>in</strong>ta<strong>in</strong> adequate gradients of protons or Na + to susta<strong>in</strong> <strong>the</strong>ir energy-produc<strong>in</strong>g mach<strong>in</strong>ery. Among <strong>the</strong>m, <strong>the</strong> <strong>the</strong>rmophiles also have to deal with <strong>the</strong> specific problem of ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g <strong>the</strong> correct membrane permeability <strong>in</strong> an unbalanced ionic environment. This may expla<strong>in</strong> why <strong>the</strong> upper temperature limits <strong>for</strong> acidophiles and alkaliphiles is presently 90ºC and not 110ºC (it might be significant that <strong>the</strong> upper pH limit of <strong>The</strong>rmococcus alkaliphilus is only pH 9.5). As with temperature, <strong>the</strong> <strong>in</strong>tracellular mach<strong>in</strong>ery cannot escape <strong>the</strong> <strong>in</strong>fluence of pressure. However, <strong>the</strong>re are organisms <strong>in</strong> <strong>the</strong> deepest parts of <strong>the</strong> ocean (pressure 1100 bar). <strong>The</strong> extreme pressure limit <strong>for</strong> life on Earth is unknown – environments of above 1100 bar have not been explored. However, it might be quite high, because macromolecules and cellular constituents apparently only beg<strong>in</strong> to denature at 4- 5000 bar. Some microorganisms liv<strong>in</strong>g <strong>in</strong> <strong>the</strong> deep ocean can be obligate barophiles, but most of <strong>the</strong>m are only barotolerant. In fact, it is not yet clear if barophiles have had limits of life under extreme conditions/I.3 I.3.4 Acidic and Alkal<strong>in</strong>e Environments I.3.5 High-Pressure Environments 31
Page 1 and 2: SP-1231 Exobiology
Page 3 and 4: Cover Fossil coccoid bacteria, 1 µ
Page 5 and 6: 4 I.5 Potential Non-Martian Sites <
Page 7 and 8: 6 6.2 Imaging of F
Page 9 and 10: The Exobio
Page 11 and 12: pyrolysis, or similar techniques, f
Page 13 and 14: Ignasi Casanova, Institute
Page 15 and 16: I AN EXOBIOLOGICAL VIEW OF THE SOLA
Page 17 and 18: SP-1231 18 surface pressure of CO 2
Page 19 and 20: SP-1231 20 10 bar befor</st
Page 21 and 22: SP-1231 22 kilometres in</s
Page 23 and 24: SP-1231 24 Laboratory Investigation
Page 25 and 26: I.3 Limits of Life
Page 27: temperatures of 95-106ºC. This sug
Page 31 and 32: cannot exclude fin
Page 33 and 34: Responses to Cosmic Radiation <stro
Page 35 and 36: acid to identify the</stron
Page 37 and 38: Horneck, G. (1993). Responses of Ba
Page 39 and 40: SP-1231 Fig. I.4.2.2/1. Size distri
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Page 43 and 44: SP-1231 Fig. I.4.2.3/1A (left). Net
Page 45 and 46: SP-1231 48 A detailed chemical anal
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Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
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Page 61 and 62: SP-1231 64 ities in</strong
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Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
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II.2 The Planet Ma
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cover the range 4.
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The depletion of c
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valley networks (Fig. II.2.5/2) are
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plate tectonic evidence has been ob
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Table II.2.7.4/1. Potential Mars <s
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II.3 The Martian M
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principal atmosphe
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Lines of evidence
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indicated by carbo
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quest for
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Anders, E. (1989). Prebiotic organi
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Urey, H.C. (1968). Origin</
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SP-1231 Fig. II.4.1.1/1. A Gilbert-
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SP-1231 Fig. II.4.1.3/1. Th
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SP-1231 114 Sinuou
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SP-1231 Fig. II.4.3/1. Ages of seve
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SP-1231 118 orbital characteristics
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SP-1231 120 II.4.5 The</str
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SP-1231 122 II.4.6 Rovers and Drill
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SP-1231 124 Site 2: Apolin<
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SP-1231 126 Site 4: Chasma area, ea
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SP-1231 128 Sharp, M., Parkes, J.,
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SP-1231 Fig. II.5.1/1. The<
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SP-1231 132 II.5.3 Isotopic Analysi
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SP-1231 134 chemical composition de
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SP-1231 Fig. II.5.7.1/1. Th
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SP-1231 138 optical microscope. Bot
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SP-1231 140 protons of discrete ene
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SP-1231 142 atures should be per<st
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SP-1231 Fig. II.5.7.7/1. Example of
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SP-1231 146 soils. In this approach
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SP-1231 148 discrimin</stro
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SP-1231 150 determin</stron
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SP-1231 Fig. II.5.10.2/2. Enantiome
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SP-1231 154 References achiral colu
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II.6 Team III: The
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in width and heigh
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Searchin</
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minerals or oxidat
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A second group of rocks, however, m
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SCIENTIFIC METHOD AND REQUIREMENTS
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The sample materia
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surprising. To ach
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The main</
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Consideration of a diamond wire-saw
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Schoonen, M.A. & Barnes, H.L. (1991
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SP-1231 180 II.7.2 The</str
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SP-1231 Fig. II.7.4/1. Look
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SP-1231 184 II.7.5 A Possible <stro
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Team IV was asked to examin
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