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

I.3 Limits of <strong>Life</strong> Under Extreme Conditions
From <strong>the</strong> view po<strong>in</strong>t of exobiology, <strong>the</strong> problems raised by life under ‘extreme
conditions’ (extreme from our human po<strong>in</strong>t of view) can be considered from two
perspectives: what are <strong>the</strong> most extreme conditions <strong>for</strong> life to proliferate, and what
can life survive (and <strong>for</strong> how long)? In both cases, we should consider <strong>the</strong> most
extreme terrestrial organisms, and if <strong>the</strong>re might be similar organisms elsewhere <strong>in</strong>
<strong>the</strong> Universe. <strong>The</strong>se two aspects are often confused, but <strong>the</strong>y are not necessarily
related. <strong>The</strong> first concerns terrestrial organisms liv<strong>in</strong>g optimally under extreme
conditions, <strong>the</strong> ‘extremophiles’. <strong>The</strong> second concerns <strong>the</strong> problem of survival, which
is of utmost importance <strong>in</strong> <strong>the</strong> search <strong>for</strong> fossilised life on o<strong>the</strong>r planets or <strong>for</strong> test<strong>in</strong>g
<strong>the</strong> hypo<strong>the</strong>sis of panspermia.
<strong>Life</strong> on Earth is based on <strong>the</strong> chemistry of carbon <strong>in</strong> water. <strong>The</strong> temperature limits
compatible with <strong>the</strong> existence of life are thus imposed by <strong>the</strong> <strong>in</strong>tr<strong>in</strong>sic properties of
chemical bonds <strong>in</strong>volved <strong>in</strong> this type of chemistry at different temperatures. Two
requirements are mandatory. Firstly, <strong>the</strong> covalent bonds between carbon and o<strong>the</strong>r
atoms <strong>in</strong>volved <strong>in</strong> <strong>the</strong> structure of biological molecules should be sufficiently stable
to permit <strong>the</strong> assembly of large macromolecules with catalytic, <strong>in</strong><strong>for</strong>mational properties
or both. Secondly, non-covalent l<strong>in</strong>ks (hydrogen and ionic bonds, Van der Waals
<strong>in</strong>teractions) should be labile. This is a very important po<strong>in</strong>t s<strong>in</strong>ce only weak bonds
can allow fast, specific and reversible <strong>in</strong>teractions of biological molecules and
macromolecules. <strong>The</strong>se chemical constra<strong>in</strong>ts will ma<strong>in</strong>ly def<strong>in</strong>e <strong>the</strong> upper and lower
temperatures <strong>for</strong> life, respectively. As we will see, it is known that terrestrial
organisms can live <strong>in</strong> <strong>the</strong> temperature range from -12ºC to 113ºC (Fig. I.3.2/1).
I.3.2.1 High Temperatures
Presently, <strong>the</strong> maximum temperature limit known <strong>for</strong> terrestrial organisms is around
113ºC. For a long time, <strong>the</strong> record was 110ºC, follow<strong>in</strong>g <strong>the</strong> discovery <strong>in</strong> 1982 of <strong>the</strong>
0 60 110 oC
EUCARYOTES
PROCARYOTES
0 10 2 10 4 10 6 10 8 10 10
T OK
I.3.1 Introduction
I.3.2 Extreme Temperature
Regimes
Fig. I.3.2/1. Liv<strong>in</strong>g organisms thrive with<strong>in</strong> only
a small w<strong>in</strong>dow of temperatures found <strong>in</strong> <strong>the</strong>
Universe. Eucaryotes, organisms with a
nucleus, thrive from around 0ºC up to 60ºC,
while some procaryotes (archaea or bacteria),
organisms without a nucleus, can grow at
temperatures up to 113ºC.
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