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

acid to identify <strong>the</strong> <strong>the</strong>oretical limits of pressure <strong>for</strong> life are thus an important l<strong>in</strong>e <strong>for</strong>
future research.
F<strong>in</strong>ally, <strong>the</strong> study of subterranean biotopes may be relevant to <strong>the</strong> problem of <strong>the</strong>
orig<strong>in</strong> of life <strong>in</strong> general. It may have been that life orig<strong>in</strong>ally developed on Earth
below <strong>the</strong> surface and <strong>the</strong>n grew upwards when conditions became less extreme <strong>in</strong> <strong>the</strong>
early Archean period. If this is correct, life might have appeared on several o<strong>the</strong>r
<strong>Solar</strong> <strong>System</strong> bodies <strong>in</strong> <strong>the</strong> appropriate temperature zone at <strong>the</strong> bondary between
external cold and <strong>in</strong>ternal heat (see Chapter I.3.2).
What can we learn from <strong>the</strong> biology of terrestrial extremophiles? An important
observation is that most terrestrial extremophiles are procaryotes. This is especially
strik<strong>in</strong>g <strong>in</strong> <strong>the</strong> case of <strong>the</strong>rmophiles and hyper<strong>the</strong>rmophiles, s<strong>in</strong>ce all microorganisms
liv<strong>in</strong>g at 60-110ºC are procaryotes. <strong>The</strong> reason <strong>for</strong> this discrim<strong>in</strong>ation is unknown. It
could be related to <strong>the</strong> requirement <strong>for</strong> long-lived messenger RNA <strong>in</strong> eucaryotes
(Forterre, 1995), but this is a hypo<strong>the</strong>sis. Why only archaea thrive above 95ºC is also
still a mystery. More work is needed to understand <strong>the</strong>se different frontiers. On <strong>the</strong>
o<strong>the</strong>r hand, we have seen that all terrestrial present-day extremophiles, although
procaryotes, are complex microorganisms, <strong>the</strong> products of long evolution, which
exhibit elaborate mechanisms <strong>for</strong> cop<strong>in</strong>g with <strong>the</strong>ir extreme environments. For
example, terrestrial hyper<strong>the</strong>rmophiles have developed strategies to protect <strong>the</strong>ir
macromolecules (prote<strong>in</strong>s, DNA and RNA) aga<strong>in</strong>st <strong>the</strong> damag<strong>in</strong>g effects of very high
temperatures (Forterre, 1996). Similarly, <strong>the</strong> two strategies used by halophiles to cope
with salty environments (production of compatible solutes and accumulation of high
<strong>in</strong>tracellular salt concentrations) require <strong>the</strong> presence of very effective transport
systems across membranes and/or complex metabolic pathways to syn<strong>the</strong>sise
compatible solutes, two hallmarks of highly evolved organisms. This aga<strong>in</strong> suggests
a priori that life, as we know it, probably evolved first <strong>in</strong> a mild environment be<strong>for</strong>e
<strong>in</strong>vad<strong>in</strong>g extreme ones. However, would it be possible to imag<strong>in</strong>e simpler and even
more robust extremophiles?
<strong>The</strong> terrestrial dichotomy between procaryotes and eucaryotes raises o<strong>the</strong>r
<strong>in</strong>terest<strong>in</strong>g questions. For example, do all life <strong>for</strong>ms on o<strong>the</strong>r planets automatically
extend at least up to 110ºC or is this expansion dependent on <strong>the</strong> appearance of a
procaryotic lifestyle? <strong>The</strong> answer to this question requires us to def<strong>in</strong>e a priori what
is <strong>the</strong> nature of procaryotes. Traditionally, procaryotes are considered to be ancient
and primitive <strong>for</strong>ms of life on Earth. In such a scenario, <strong>the</strong> procaryotic type of
cellular organisation is often viewed as an early stage of life evolution, which should
have been common to all life, consequently mak<strong>in</strong>g it <strong>the</strong> most widespread life <strong>for</strong>m
<strong>in</strong> <strong>the</strong> Universe (hence <strong>the</strong> search <strong>for</strong> martian bacteria). However, some authors
consider that procaryotes are very evolved organisms that orig<strong>in</strong>ated by reduction
from more complex <strong>for</strong>ms of life (Reanney, 1974; Forterre, 1995). In that case, <strong>the</strong>
presence of life <strong>in</strong> many extremophilic environments could be dependent on <strong>the</strong>
historical appearance of ‘procaryotes’. It might be important to determ<strong>in</strong>e which is
correct <strong>in</strong> order to def<strong>in</strong>e more precisely what we are look<strong>in</strong>g <strong>for</strong> <strong>in</strong> o<strong>the</strong>r planets. In
any case, we should not consider <strong>the</strong> procaryote/eucaryote dichotomy as valid <strong>for</strong> all
possible life <strong>in</strong> <strong>the</strong> Universe. Simpler, different or more complex <strong>for</strong>ms could exist.
F<strong>in</strong>ally, we are probably ignor<strong>in</strong>g some of <strong>the</strong> capabilities of terrestrial extremophiles.
It might be worth challeng<strong>in</strong>g <strong>the</strong>m with some environmental conditions deduced
from our knowledge of planetary atmospheres, hydrospheres and lithospheres.
Surprises could emerge from such studies.
We have suggested that life might have appeared on planets only with<strong>in</strong> a narrow
spectrum of physical parameters and that, once life existed, <strong>the</strong>se requirements could
be relaxed (but not too much) <strong>for</strong> evolved life <strong>for</strong>ms to proliferate. However, if <strong>the</strong>se
conditions changed drastically <strong>in</strong> some direction (freez<strong>in</strong>g, desiccation), liv<strong>in</strong>g
organisms could survive (wait<strong>in</strong>g <strong>for</strong> better times) <strong>for</strong> very long periods, possibly on
an astronomical scale. Question marks predom<strong>in</strong>ate <strong>in</strong> this area of research. How long
can extremophiles or resistant <strong>for</strong>ms survive <strong>in</strong> hostile conditions? Many aspects of
this problem need fur<strong>the</strong>r <strong>in</strong>vestigations. Some of <strong>the</strong>m have been reviewed by
Kennedy et al. (1994).What is <strong>the</strong> migration rate of microorganisms through a rock
limits of life under extreme conditions/I.3
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