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

SP-1231
Fig. II.6.2/1. Biofilm seen us<strong>in</strong>g a scann<strong>in</strong>g
microscope.
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produce (Westall et al., 1995). Most microorganisms, such as bacteria, live attached
to substrates and build up layers of slime conta<strong>in</strong><strong>in</strong>g cells, authigenically precipitated
m<strong>in</strong>erals (e.g. carbonates, pyrite) and trapped particles.
<strong>The</strong>se layers are known as biofilms (Fig. II.6.2/1) and <strong>the</strong>y may often be preserved
even when <strong>the</strong> microorganisms that produced <strong>the</strong>m are not preserved (Gerdes &
Krumbe<strong>in</strong>, 1987). Stromatolites are famous examples of such microbially-mediated,
lam<strong>in</strong>ar build-ups and examples are given <strong>in</strong> Part I. Thus, <strong>the</strong> presence of
macroscopically biolam<strong>in</strong>ated deposits is a valid <strong>in</strong>dicator <strong>for</strong> <strong>the</strong> presence of life,
even though <strong>the</strong> microorganisms may not be visible (ei<strong>the</strong>r because <strong>the</strong>y are below
<strong>the</strong> limit of visibilty of <strong>the</strong> method employed or because <strong>the</strong>y were not fossilised).
Apart from <strong>the</strong> experimental simulations of microbial fossilisation, <strong>the</strong>re are
numerous examples of m<strong>in</strong>eralogically replaced fossils <strong>in</strong> <strong>the</strong> rock record: <strong>the</strong> oldest
com<strong>in</strong>g from 3.3-3.5 Gyr sediments from <strong>the</strong> Early Archean of South Africa and
Australia (Westall, 1999). <strong>The</strong>se early terrestrial fossils are, however, very small
(typically 1 µm but sometmes up to 4 µm) and are close to <strong>the</strong> limit of optical
resolution. O<strong>the</strong>r examples of 1-2 µm-sized fossil bacteria are found <strong>in</strong> siderite
(Wuttke, 1983), phosphate (Liebig et al., 1996), calcite (Barbieri et al., <strong>in</strong> press) and
silica (Monty et al., 1991).
II.6.2.1 Macroscopic Scale
On a macroscopic scale, two biogenically produced features can be observed:
• biolam<strong>in</strong>ae (biofilm layers) of <strong>the</strong> order of a few hundreds of microns to about
1 mm <strong>in</strong> thickness but extend<strong>in</strong>g <strong>for</strong> tens of centimetres or more;
• fossils >1 mm <strong>in</strong> width and tens of millimetres <strong>in</strong> length (cyanobacteria, algae,
mosses, lichen etc).
Biolam<strong>in</strong>ae
– large biolam<strong>in</strong>ated structures, such as stromatolitic mounds of <strong>the</strong> order of > 0.5 m