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

SP-1231
Fig. I.4.2.3/1A (left). Net photosyn<strong>the</strong>tic
response and dark respiration of a
cryptoendolythic community <strong>in</strong> Antarctica at
different levels of solar irradiance (100, 700,
1500 mol photons m -2 s -1 ). Fig. I.4.2.3/1B. Annual
gross productivity (whole bars) and net
photosyn<strong>the</strong>tic ga<strong>in</strong> (dark bars) by <strong>the</strong> same
cryptoendilitic community. (From Nienow &
Friedmann, 1993)
46
microscopy and television image analysis (Wynn-Williams, 1988). It uses <strong>the</strong>
autofluorescence of primary producers, e.g. <strong>the</strong> primary photosyn<strong>the</strong>tic pigment
chlorophyll and phycocyan<strong>in</strong>e or phycoerythr<strong>in</strong> as accessory pigments of cyanobacteria,
and detects <strong>the</strong> presence of heterotrophs by means of fluorochromes, e.g.
fluoresce<strong>in</strong>, isothiocyanate or acrid<strong>in</strong>e orange. This technique makes cells visible
relative to <strong>the</strong>ir opaque substrate. By us<strong>in</strong>g sta<strong>in</strong>s that selectively sta<strong>in</strong> viable cells
better than non-viable ones, <strong>the</strong> degree of viability of <strong>the</strong> population can be
determ<strong>in</strong>ed (Wynn-Williams, 1988). Us<strong>in</strong>g this technique, <strong>the</strong> microalgal colonisation
of <strong>the</strong> Antarctic soil surface was successfully recorded over a period of 6 years
(Wynn-Williams, 1996). To detect microorganisms that are not amenable to
cultivation, fluorescently labelled rRNA-targeted oligonucleotide probes have been
used to identify <strong>in</strong>dividual prokaryotic cells (Amann et al., 1992). Such rRNAtargeted
nucleic acid probes are ei<strong>the</strong>r highly specific at <strong>the</strong> species/subspecies level
or ra<strong>the</strong>r unspecific react<strong>in</strong>g with all liv<strong>in</strong>g organisms. <strong>The</strong>y have been widely used
to identify <strong>in</strong> situ <strong>the</strong> phylogeny of uncultured microorganisms and to analyse
complex microbial communities (Amann et al., 1992).
Direct tests of carbon and nitrogen metabolism can be per<strong>for</strong>med <strong>in</strong> situ <strong>in</strong> <strong>the</strong>
environment under <strong>in</strong>vestigation. <strong>The</strong> productivity of primary producers is generally
determ<strong>in</strong>ed by <strong>the</strong> rate of total carbon uptake, e.g. <strong>in</strong> <strong>the</strong> <strong>for</strong>m of radioactively labelled
bicarbonate, <strong>in</strong> crushed rock samples conta<strong>in</strong><strong>in</strong>g endolithic communities or <strong>in</strong> <strong>in</strong>tact
endolithic communities by <strong>the</strong> CO 2 gas exchange at <strong>the</strong> rock surface (Fig. I.4.2.3/1A).
From such data, <strong>the</strong> annual gross productivity and net photosyn<strong>the</strong>tic ga<strong>in</strong> can be
assessed, as shown <strong>in</strong> Fig. I.4.2.3/1B (Nienow & Friedmann, 1993). Similarly, carbon
fixation was determ<strong>in</strong>ed <strong>in</strong> Sun-exposed evaporites conta<strong>in</strong><strong>in</strong>g microorganisms
(Rothschild et al., 1994). In deep sea hydro<strong>the</strong>rmal vents, <strong>the</strong> presence of primary
producers was proven by <strong>in</strong> situ measurements of <strong>the</strong> uptake of 14 CO 2 over a period of
several days (Karl et al., 1980). <strong>The</strong> presence and activity of nitrogen fixation was
determ<strong>in</strong>ed <strong>in</strong> evaporites by measur<strong>in</strong>g acetylene reduction (Rothschild et al., 1994),
denitrification activity was determ<strong>in</strong>ed essentially by <strong>the</strong> same method, with acetylene
used to block nitrous oxide reductase. <strong>The</strong> activity is determ<strong>in</strong>ed by gas
chromatography from periodically collected gas samples (Rothschild et al., 1994). All
<strong>in</strong> situ analyses described above require carefully planned control experiments.
It is worth mention<strong>in</strong>g that <strong>the</strong> life-detection <strong>in</strong>strument package of <strong>the</strong> Vik<strong>in</strong>g
missions which, among o<strong>the</strong>r scientific issues, addressed <strong>the</strong> question of extant life on
Mars, comprised three experiments to detect metabolic activity of potential microbial
soil communities:
1. <strong>the</strong> pyrolitic release experiment tested carbon assimilation, i.e. photoautotrophy,
as a method of <strong>in</strong>corporat<strong>in</strong>g radioactively labelled carbon dioxide <strong>in</strong> <strong>the</strong>
presence of sunlight (Horowitz et al., 1977);