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

morphological and biochemical signatures of extraterrestrial life: utility of terrestrial analogues/I.4
material sealed <strong>in</strong> a m<strong>in</strong>eral matrix and hosted by a carbonate-free lithology had
obviously escaped high-temperature isotope exchange, <strong>the</strong>reby preserv<strong>in</strong>g its orig<strong>in</strong>al
biogenic δ 13 C org spread with probably little alteration.
I.4.3.2.3 Molecular Biomarkers (‘Chemical Fossils’) <strong>in</strong> Sediments
As mentioned earlier, biological markers, or ‘biomarkers’, are terms used <strong>for</strong> <strong>the</strong> wide
variety of organic compounds that are derived from liv<strong>in</strong>g organims and can be found
<strong>in</strong> sediments. Such compounds have also been termed ‘chemical fossils’ but <strong>the</strong> most
commonly-used term is that of biomarker (Peters & Moldowan, 1993; Engel &
Macko, 1983; Killops & Killops, 1993). Those biomarkers show<strong>in</strong>g little or no
change <strong>in</strong> structure from those of <strong>the</strong> orig<strong>in</strong>al biochemical compounds are easily
recognised <strong>in</strong> extracts of sediments, but compounds with strongly diagenetically or
catagenetically altered structures, where<strong>in</strong> <strong>the</strong> carbon skeletons differ from those of
<strong>the</strong> orig<strong>in</strong>al biological compounds yet carry sufficient residual portions of <strong>the</strong> orig<strong>in</strong>al
structures, may still be useful biomarkers.
Biomarkers are extractable from sediments and fossils by organic solvents, <strong>the</strong>
most studied be<strong>in</strong>g of <strong>the</strong> lipid type, although certa<strong>in</strong> pigments, e.g. metal porphyr<strong>in</strong>s,
are also referred to as biomarkers. <strong>The</strong> key to <strong>the</strong> use of biomarkers is frequently <strong>the</strong>ir
complexity of structure, s<strong>in</strong>ce this represents a major <strong>in</strong>heritance of orig<strong>in</strong>al
biologically-derived <strong>in</strong><strong>for</strong>mation. In addition, <strong>the</strong> molecular distributions <strong>in</strong> terms of
<strong>the</strong>ir relative amounts can be highly <strong>in</strong><strong>for</strong>mative, as <strong>the</strong> orig<strong>in</strong>al biosyn<strong>the</strong>sis imparts
characteristic distributions of <strong>the</strong> different compounds. However, subsequent
diagenetic alterations and m<strong>in</strong>eralisation modify those patterns. Hence, <strong>the</strong>
biochemist generally prefers to work with unaltered, orig<strong>in</strong>al molecular biomarkers,
because <strong>the</strong>se compounds carry <strong>the</strong> full orig<strong>in</strong>al biochemical <strong>in</strong><strong>for</strong>mation and exhibit
an extremely high degree of order both <strong>in</strong> structure and <strong>in</strong> relative abundances.
Molecular Characteristics of <strong>the</strong> Biosphere
What is <strong>the</strong> recognisable record of <strong>the</strong> past biosphere <strong>in</strong> <strong>the</strong> surface of <strong>the</strong> Earth? In
general terms, <strong>the</strong> biosyn<strong>the</strong>tic impr<strong>in</strong>t of a liv<strong>in</strong>g organism is seen <strong>in</strong> <strong>the</strong> carbon
skeletons, <strong>the</strong> functionalities and <strong>the</strong> stereochemistries of <strong>the</strong> compounds it
manufactures under enzymic, molecular mach<strong>in</strong>ery. <strong>The</strong> fundamental scheme of life’s
universal, but extremely precise, molecular mach<strong>in</strong>ery is simply:
DNA ====> RNA ====> Prote<strong>in</strong>s ====> Metabolites
DNA and RNA consist of genetically determ<strong>in</strong>ed sequences of five different
nucleotide bases, with <strong>the</strong> DNA sequence determ<strong>in</strong><strong>in</strong>g <strong>the</strong> RNA sequence. It, <strong>in</strong> turn,
controls <strong>the</strong> syn<strong>the</strong>sis of prote<strong>in</strong>s from some 20 homochiral alpha am<strong>in</strong>o acids. <strong>The</strong>
prote<strong>in</strong>s consist of short (10-10 3 am<strong>in</strong>o acids) polypeptide cha<strong>in</strong>s and normally
generate <strong>the</strong>ir own specific <strong>in</strong>tramolecular crossl<strong>in</strong>ks, <strong>the</strong>reby def<strong>in</strong><strong>in</strong>g <strong>the</strong> precise
shape and location of reactive functionalities required <strong>for</strong> enzymatic activity. Prote<strong>in</strong>s
<strong>in</strong> turn catalyse <strong>the</strong> syn<strong>the</strong>sis of specific metabolites and homologous metabolic
series. Enzymatic syn<strong>the</strong>sis (metabolism) and breakdown (catabolism) of molecules
provides <strong>for</strong> all <strong>the</strong> needs of <strong>the</strong> organisms. <strong>The</strong> metabolites comprise a large range of
molecules that <strong>in</strong>clude, <strong>in</strong>ter alia, <strong>the</strong> build<strong>in</strong>g blocks to syn<strong>the</strong>size a variety of
cellular biochemicals, such as lipids, polysaccharides, and co-enzymes.
Chiral centres such as those at a s<strong>in</strong>glem tetrahedrally substituted carbon atom are
almost always syn<strong>the</strong>sised with a s<strong>in</strong>gle chirality. Very unusually, an organism may
make both, while, <strong>in</strong> o<strong>the</strong>r rare <strong>in</strong>stances, one enantiometer may be made by one
species and <strong>the</strong> o<strong>the</strong>r by a different species. Cholesterol, a sterol widely distributed <strong>in</strong>
eukaryotes, is an example of a highly ordered carbon skeleton <strong>in</strong> which <strong>the</strong> overall
shape is dependent upon <strong>the</strong> stereochemistry of <strong>the</strong> r<strong>in</strong>g fusions and of <strong>the</strong> methyl and
hydroxyl substituents of <strong>the</strong> r<strong>in</strong>gs. It has eight chiral centres, as <strong>in</strong>dicated <strong>in</strong>
Fig. I.4.3.2.3/1. <strong>The</strong> structure is highly specific, both <strong>in</strong> <strong>the</strong> gross skeleton and <strong>in</strong> <strong>the</strong>
stereochemistry, so that it makes an excellent biomarker, one which has been traced
back hundreds of millions of years <strong>in</strong> immature sediments. It is also a carrier of
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