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

More documents

Recommendations

Info

SP-1231 32 I.3.6 Subterranean <strong>Life</strong> to <strong>in</strong>vent specific devices to adapt to high pressure (<strong>for</strong> review, see Prieur, 1992). High pressure seems to <strong>in</strong>crease <strong>the</strong> growth rate of some hyper<strong>the</strong>rmophiles, but this phenomenon is not general and not very spectacular. Although some prote<strong>in</strong>s from hyper<strong>the</strong>rmophiles are more active at high pressure, high pressure does not <strong>in</strong>crease <strong>the</strong> <strong>the</strong>rmal stability of macromolecules (Bernhardt et al., 1984). For a long time, it was believed that deep subterranean environments were sterile. It has now been recognised that bacteria (and probably archaea, too) actually thrive <strong>in</strong> <strong>the</strong> terrestrial crust. Subterranean microorganisms are usually detected <strong>in</strong> subterranean oil-fields or <strong>in</strong> <strong>the</strong> course of drill<strong>in</strong>g experiments (Parkes & Maxwell, 1993; Parkes et al., 1994; Stevens & McK<strong>in</strong>ley, 1995). For example, recent research conducted with<strong>in</strong> <strong>the</strong> International Ocean Drill<strong>in</strong>g Program (ODP) has demonstrated that procaryotes are present much deeper <strong>in</strong> mar<strong>in</strong>e sediments than was previously thought possible, extend<strong>in</strong>g to at least 750 m below <strong>the</strong> sea floor, and probably much deeper (Parkes et al., 1994). <strong>The</strong>se microbial populations are substantial (e.g 10 7 cells cm -3 at 500 m below sea floor) and likely to be widespread. To depths of at least 432 m, microbes have been identified as alter<strong>in</strong>g volcanic glass, which comprises a substantial volume of <strong>the</strong> volcanic component of <strong>the</strong> ocean crust (Furnes et al.,1996), and may have significance <strong>for</strong> chemical exchange between <strong>the</strong> oceanic crust and ocean water. <strong>The</strong>se data provide a prelim<strong>in</strong>ary, and probably conservative, estimate of <strong>the</strong> biomass <strong>in</strong> this important new ecosystem: about 10% of <strong>the</strong> surface biosphere. <strong>The</strong>se discoveries have radically changed our perception of mar<strong>in</strong>e sediments and <strong>in</strong>dicate <strong>the</strong> presence of a largely unexplored deep bacterial biosphere that may even rival <strong>the</strong> Earth’s surface biosphere <strong>in</strong> size and diversity! Elevated procaryotic populations and activities are also associated with gas hydrates. This, toge<strong>the</strong>r with <strong>the</strong> presence of large microbial populations <strong>in</strong> oil reservoir fluids (Stetter et al., 1993, Haridon et al., 1995, Bernard, et al., 1992, Rueter et al., 1994), suggests that deep bacterial processes may even be <strong>in</strong>volved <strong>in</strong> oil and gas <strong>for</strong>mation by comparison with <strong>the</strong> key catalytic role of near-surface procaryotic communities. Procaryotes are also likely to drive deep geochemical reactions such as m<strong>in</strong>eral <strong>for</strong>mation and dissolution, be responsible <strong>for</strong> <strong>the</strong> magnetic record via production of magnetic m<strong>in</strong>erals, produce deep methane gas essential <strong>for</strong> hydrate <strong>for</strong>mation. Procaryotes <strong>in</strong> <strong>the</strong> deep biosphere must be uniquely adapted to live <strong>in</strong> this extreme environment, and populations have even been shown to <strong>in</strong>crease at depth (>200 m) as <strong>the</strong>y exploit deep geochemical fluxes such as geo<strong>the</strong>rmal methane and br<strong>in</strong>e <strong>in</strong>cursions. Heterotrophic subterranean communities use ei<strong>the</strong>r remnant organic carbon deposited with sediments or dissolved oxygen as a metabolite term<strong>in</strong>al electron acceptor. <strong>The</strong>y are thus actually <strong>in</strong>directly dependent on photosyn<strong>the</strong>tic activity. Hence <strong>the</strong>y can become ext<strong>in</strong>ct when <strong>the</strong>ir nutrients are exhausted. However, evidence has also been obta<strong>in</strong>ed <strong>for</strong> <strong>the</strong> existence of anaerobic subsurface litoautotrophic microbial systems <strong>in</strong> basalt aquifers (Stevens & McK<strong>in</strong>ley, 1995). <strong>The</strong>se communities apparently derived <strong>the</strong>ir energy from geochemically-produced hydrogen. Because <strong>the</strong> energy sources and <strong>in</strong>organic nutrients are both supplied by geochemical means, such a subsurface lithoautotrophic microbial ecosystem (SLiME) could a priori persist <strong>in</strong>def<strong>in</strong>itely even if <strong>the</strong> conditions on <strong>the</strong> Earth’s surface became alien to life. S<strong>in</strong>ce <strong>the</strong> temperature <strong>in</strong>creases with depth, it has been suggested that hyper<strong>the</strong>rmophiles, <strong>in</strong> particular chemiolitoautotrophs, are abundant <strong>in</strong> subterranean environments, <strong>for</strong>m<strong>in</strong>g a deep hot biosphere (Gold, 1992). Indeed, procaryotic activity has even been reported up to 120ºC and possibly even higher (Cragg &Parkes, 1994), i.e. above <strong>the</strong> upper temperature limit def<strong>in</strong>ed <strong>in</strong> <strong>the</strong> laboratory. <strong>The</strong> challenge now is to cultivate <strong>the</strong>se putative novel hyper<strong>the</strong>rmophiles. Up to now, all microbes sampled from subsurface environments have turned out to be close relatives of well-known bacteria, but <strong>the</strong>se studies have been ra<strong>the</strong>r limited and one
cannot exclude f<strong>in</strong>d<strong>in</strong>g really novel microbes <strong>in</strong> <strong>the</strong>se relatively unexplored biotopes. <strong>The</strong>se deep communities clearly suggest that life may also exist deep below <strong>the</strong> surfaces of o<strong>the</strong>r planets. Hence, we need to build this <strong>in</strong>to any assessment of extraterrestrial life, as conditions <strong>for</strong> life may actually improve below <strong>the</strong> surface (e.g. Mars). <strong>The</strong> absence of surface life, <strong>the</strong>re<strong>for</strong>e, may not necessarily <strong>in</strong>dicate <strong>the</strong> absence of all life. Indeed, it seems ra<strong>the</strong>r naïve to limit <strong>the</strong> search <strong>for</strong> life to a planet’s surface when we already know that <strong>the</strong> conditions are far beyond <strong>the</strong> possible ranges <strong>for</strong> life on Earth. <strong>The</strong> revival of microorganisms from ancient rocks, salt and coal has been reported many times. <strong>The</strong>re are several claims of microorganisms be<strong>in</strong>g revived from rocks more than 100 millions years old, some even from Precambrian (650 millions years old) (<strong>for</strong> an exhaustive review see Kennedy et al., 1994). <strong>The</strong>se claims have usually been disputed on various grounds (contam<strong>in</strong>ation, <strong>the</strong>oretical impossibility) but Kennedy et al. concluded that <strong>the</strong> observations are too many to be dismissed en bloc. In particular, <strong>the</strong> US Department of Energy has established a collection of 5000 revived microorganisms (bacteria and fungi) from various subsurface sites about 200 million years old. For Kennedy et al. <strong>the</strong> best alternative to revival is <strong>in</strong> situ reproduction, which is as <strong>in</strong>terest<strong>in</strong>g as real revival from <strong>the</strong> perspective of recover<strong>in</strong>g ancient liv<strong>in</strong>g <strong>for</strong>ms on Mars or o<strong>the</strong>r planets. Apart from <strong>the</strong> availability of resources, <strong>the</strong> balance between latent life and <strong>in</strong> situ reproduction <strong>in</strong> subsurface biotopes should be dependent on temperature effects: <strong>in</strong> situ reproduction be<strong>in</strong>g favoured at high temperatures and latency at low temperatures. Microorganisms <strong>in</strong> general, and even some macroorganisms, are extremely resistant to freez<strong>in</strong>g temperatures. It is well known that microorganisms are rout<strong>in</strong>ely kept alive <strong>for</strong> years <strong>in</strong> liquid nitrogen. Indeed, at extremely low temperatures, most deleterious processes l<strong>in</strong>ked to metabolic activities that can impair survival are slowed down. Many (but not all) of <strong>the</strong> microorganisms revived from ancient material are spore<strong>for</strong>mers. Bacteria of <strong>the</strong> Gram positive k<strong>in</strong>gdom (one out of <strong>the</strong> roughly 12 bacterial k<strong>in</strong>gdoms), have <strong>in</strong>deed developed <strong>the</strong> ability to produce extraord<strong>in</strong>arily resistant spores that can survive extremely harsh conditions (high temperature, absence of nutrients, high doses of radiation). Spores are also produced by fungi and plant m<strong>in</strong>ute seeds, and protozeoan cysts can be considered as k<strong>in</strong>ds of spores. Concern<strong>in</strong>g <strong>the</strong> problem of latency, a critical po<strong>in</strong>t would be <strong>the</strong> stability of DNA molecules under very long-term storage, and <strong>the</strong> possibility <strong>for</strong> <strong>the</strong> organism to repair <strong>the</strong> lesions <strong>in</strong>troduced dur<strong>in</strong>g <strong>the</strong> dormant stage, once <strong>the</strong> conditions aga<strong>in</strong> become favourable. Indeed, even <strong>in</strong> <strong>the</strong> absence of UV or ionis<strong>in</strong>g radiation, DNA is subjected to spontaneous chemical modifications such as depur<strong>in</strong>ation, cytos<strong>in</strong>e deamidation and hydrolysis of <strong>the</strong> phosphodiester bond (L<strong>in</strong>dahl, 1993). <strong>The</strong>se reactions are very slow at low temperatures but can still produce significant damage <strong>in</strong> <strong>the</strong> very longterm. This po<strong>in</strong>t has been raised aga<strong>in</strong>st <strong>the</strong> possibility of revival <strong>for</strong> organisms several millions year old. DNA <strong>in</strong> spores is protected by specific DNA b<strong>in</strong>d<strong>in</strong>g prote<strong>in</strong>s that prevent depur<strong>in</strong>ation and probably o<strong>the</strong>r damage (see below), but it is not clear how effective this protection can be over very long periods. It is <strong>in</strong>terest<strong>in</strong>g to note that high salt concentrations protect DNA aga<strong>in</strong>st chemical modifications (Marguet & Forterre, 1994). Thus, halophilic organisms might have more chance to survive, and/or <strong>for</strong> <strong>the</strong>ir DNA to rema<strong>in</strong> <strong>in</strong>tact (<strong>for</strong> analyses), than non-halophilic ones. <strong>The</strong> best conservation effect would thus be achieved <strong>for</strong> frozen extreme halophiles. Natural radioactivity <strong>in</strong> rock might be also a problem <strong>for</strong> long-term DNA stability. However, some microorganisms are extremely radio-resistant. This can be a secondary adaptation to dessication that produce DNA lesions lead<strong>in</strong>g to doublestranded breaks when <strong>the</strong> cells are aga<strong>in</strong> <strong>in</strong> contact with water. <strong>The</strong> radio-resistant bacterion De<strong>in</strong>ococcus radiodurans exhibits a very efficient repair-recomb<strong>in</strong>ation mechanism that allows <strong>the</strong> cell to reconstruct <strong>in</strong>tact chromosomes from deficient ones (Smith et al., 1992). limits of life under extreme conditions/I.3 33
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 and 28: temperatures of 95-106ºC. This sug
Page 29: (mainly found <str
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
Page 41 and 42: SP-1231 Fig. I.4.2.2/4. Evaporite w
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
Page 47 and 48: SP-1231 Fig. I.4.3.1.1/1. Scheme of
Page 49 and 50: SP-1231 Fig. I.4.3.1.2/1. A: Compar
Page 51 and 52: SP-1231 54 I.4.3.2.1 Sedimentary Or
Page 53 and 54: SP-1231 56 The iso
Page 55 and 56: SP-1231 Fig. I.4.3.2.3/1. Cholester
Page 57 and 58: SP-1231 60 References regard to non
Page 59 and 60: SP-1231 62 Klein,
Page 61 and 62: SP-1231 64 ities in</strong
Page 63 and 64: SP-1231 Fig. I.5.1.1/1. A 34×42 km
Page 65 and 66: SP-1231 Fig. I.5.2.1/1. Titan’s a
Page 67 and 68: SP-1231 Fig. I.5.2.2/2. Modelled Ti
Page 69 and 70: SP-1231 72 Galileo: images availabl
Page 71 and 72: SP-1231 74 TABLE I.6.2/1 EVIDENCE O
Page 73 and 74: SP-1231 I.6.3 What to Searc
Page 75 and 76: SP-1231 78 I.7.3 Organic Chemistry
Page 77 and 78: II.1 Introduction The</stro
Page 79 and 80: II.2 The Planet Ma
Page 81 and 82:
cover the range 4.
Page 83 and 84:
The depletion of c
Page 85 and 86:
valley networks (Fig. II.2.5/2) are
Page 87 and 88:
plate tectonic evidence has been ob
Page 89 and 90:
Table II.2.7.4/1. Potential Mars <s
Page 91 and 92:
II.3 The Martian M
Page 93 and 94:
principal atmosphe
Page 95 and 96:
Lines of evidence
Page 97 and 98:
indicated by carbo
Page 99 and 100:
quest for
Page 101 and 102:
Anders, E. (1989). Prebiotic organi
Page 103 and 104:
Urey, H.C. (1968). Origin</
Page 105 and 106:
SP-1231 Fig. II.4.1.1/1. A Gilbert-
Page 107 and 108:
SP-1231 Fig. II.4.1.3/1. Th
Page 109 and 110:
SP-1231 114 Sinuou
Page 111 and 112:
SP-1231 Fig. II.4.3/1. Ages of seve
Page 113 and 114:
SP-1231 118 orbital characteristics
Page 115 and 116:
SP-1231 120 II.4.5 The</str
Page 117 and 118:
SP-1231 122 II.4.6 Rovers and Drill
Page 119 and 120:
SP-1231 124 Site 2: Apolin<
Page 121 and 122:
SP-1231 126 Site 4: Chasma area, ea
Page 123 and 124:
SP-1231 128 Sharp, M., Parkes, J.,
Page 125 and 126:
SP-1231 Fig. II.5.1/1. The<
Page 127 and 128:
SP-1231 132 II.5.3 Isotopic Analysi
Page 129 and 130:
SP-1231 134 chemical composition de
Page 131 and 132:
SP-1231 Fig. II.5.7.1/1. Th
Page 133 and 134:
SP-1231 138 optical microscope. Bot
Page 135 and 136:
SP-1231 140 protons of discrete ene
Page 137 and 138:
SP-1231 142 atures should be per<st
Page 139 and 140:
SP-1231 Fig. II.5.7.7/1. Example of
Page 141 and 142:
SP-1231 146 soils. In this approach
Page 143 and 144:
SP-1231 148 discrimin</stro
Page 145 and 146:
SP-1231 150 determin</stron
Page 147 and 148:
SP-1231 Fig. II.5.10.2/2. Enantiome
Page 149 and 150:
SP-1231 154 References achiral colu
Page 151 and 152:
II.6 Team III: The
Page 153 and 154:
in width and heigh
Page 155 and 156:
Searchin</
Page 157 and 158:
minerals or oxidat
Page 159 and 160:
A second group of rocks, however, m
Page 161 and 162:
SCIENTIFIC METHOD AND REQUIREMENTS
Page 163 and 164:
The sample materia
Page 165 and 166:
surprising. To ach
Page 167 and 168:
The main</
Page 169 and 170:
Consideration of a diamond wire-saw
Page 171 and 172:
Schoonen, M.A. & Barnes, H.L. (1991
Page 173 and 174:
SP-1231 180 II.7.2 The</str
Page 175 and 176:
SP-1231 Fig. II.7.4/1. Look
Page 177 and 178:
SP-1231 184 II.7.5 A Possible <stro
Page 179 and 180:
Team IV was asked to examin
show all

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

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?