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

SP-1231
Fig. I.4.2.2/4. Evaporite with horizontal bands
of endoevaporitic microbial communities (from
Rothshild et al., 1994).
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<strong>in</strong> cold and hot deserts (Friedmann, 1982; Nienow & Friedmann, 1993). <strong>The</strong>
colonised zone below <strong>the</strong> rock crust reaches up to 10 mm deep and is composed of
dist<strong>in</strong>ct parallel bands of black, white, green and blue-green. Throughout <strong>the</strong>
colonised zone, <strong>the</strong> brown iron oxyhydroxide sta<strong>in</strong> is leached out, probably by
biogenic oxalic acid, and <strong>the</strong> quartz appears colourless. It is <strong>in</strong>terest<strong>in</strong>g to note that
both <strong>the</strong> endoevaporitic and endolithic communities are found liv<strong>in</strong>g embedded
with<strong>in</strong> a few mm of <strong>the</strong> surface m<strong>in</strong>eral matrix. <strong>Solar</strong> irradiance is attenuated by
almost one order of magnitude by <strong>the</strong> overly<strong>in</strong>g m<strong>in</strong>eral layers. Hence, endoevaporitic
and endolithic communities obviously have conquered an ecological niche
that allows microorganisms to live <strong>in</strong> an o<strong>the</strong>rwise hostile environment.
Desert crusts, <strong>for</strong>med by microorganisms, may cover even larger areas of hundreds
of square metres with a thickness of a few cm (Campbell, 1979). <strong>The</strong>se mats,
dom<strong>in</strong>ated by cyanobacteria, show stromatolitic features, such as sediment trapp<strong>in</strong>g
and accretion, a convoluted surface and polygonal crack<strong>in</strong>g. Sand and clay particles
are trapped with<strong>in</strong> a network of filamentous cyanobacteria which secrete mucous
sheets to which <strong>the</strong> particles adhere. Such desert crusts are observed on rocks or soil
<strong>in</strong> areas of flat topography. Comparable large areas of stratified mats of microbial
communities are also observed <strong>in</strong> evaporite flats of hypersal<strong>in</strong>e lagoons, such as <strong>the</strong>
Laguna Figueroa on <strong>the</strong> Pacific coast of <strong>the</strong> Baja Cali<strong>for</strong>nia (Stolz & Margulis, 1984;
Stolz, 1985) and a host of related habits.
I.4.2.3. Evidence of Microbial Activity as a Functional Characteristic of <strong>Life</strong>
<strong>The</strong> most unequivocal <strong>in</strong>dication of microbial life is obta<strong>in</strong>ed by <strong>the</strong> isolation of
microorganisms <strong>in</strong> pure culture from a sample under <strong>in</strong>vestigation, followed by a
detailed analysis <strong>for</strong> <strong>the</strong>ir biochemical properties. Microbial isolates have been
obta<strong>in</strong>ed from diverse extreme habitats. Examples are deep crystall<strong>in</strong>e rock aquifers
several hundreds of metres below <strong>the</strong> surface (Stevens & McK<strong>in</strong>ley, 1995), <strong>the</strong>
<strong>in</strong>teriors of ice-cores from drill<strong>in</strong>gs <strong>in</strong> <strong>the</strong> Antarctic ice down to a depth of several
thousand metres (Abyzov, 1993) and cores from drill<strong>in</strong>gs <strong>in</strong> permafrost regions <strong>in</strong>