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

SP-1231
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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
(