Billions of years ago, Mars was a warm and wet planet. Life
could have evolved on Mars and then receded to micro-habitats as the planet
subsequently became colder and dryer. Present-day micro-habitats for life on
Mars can include subterranean aquifers and cracks or fissures in rocks. J.-P. de
Vera et al. (2014) conducted a study using the lichen Pleopsidium chlorophanum and found that this Antarctic lichen can
adapt, within a span of 34 days, to the conditions expected to be present in
the micro-habitats on Mars today.
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The sample used in the study was collected from the granites
and volcanic rocks of North Victoria Land in Antarctica during the 10th German
North Victoria Land Expedition in 2009/2010. Pleopsidium chlorophanum is an extremophile that lives in very cold
and dry places. Its native habitat in Antarctica somewhat approximates the
conditions on Mars. Pleopsidium
chlorophanum is usually found within cracks and fissures in rocks. It can
remain metabolically active down to -20°C and can absorb water directly from
snow.
To simulate Mars-like environmental conditions, the lichens
were placed in the Mars Simulation Chamber (MSC) at the Mars Simulation
Facility (MSF) of the DLR Institute of Planetary Research in Berlin. The
atmosphere in the MSC was 95 percent carbon dioxide, 4 percent nitrogen and 1
percent oxygen. The pressure was held at 800 Pa, with a diurnal relative
humidity cycling of 0.1 to 75 percent and a diurnal temperature cycling of 21°C
to -50°C (i.e. similar to the temperatures observed in the equatorial to
mid-latitude regions on Mars).
The lichens were embedded within a Mars analogue soil
material. In the experiment, 3 samples of lichens were subjected to Mars-like
niche conditions (i.e. conditions expected in the micro-habitats on Mars) and
another 3 samples of lichens were subjected to the unprotected Mars-like
surface conditions for 34 days. For lichens in the unprotected Mars-like
surface conditions, they were subjected to much more intense UV irradiation.
Results from the experiment indicated that for Mars-like
surface conditions, photosynthetic activity dropped to 18 percent of
pre-experiment levels and it was unclear if the lichens remained
photosynthetically active at the end of the 34 days. However, lichens that were
subjected to Mars-like niche conditions and experienced a much lower radiation
dose fared very differently. For these lichens, photosynthetic activity only
dropped to 55 percent of pre-experiment levels after the 34 days. In fact,
photosynthetic activity at the end of the experiment was 17 percent higher than
what was measured for the lichens in their native habitat in Antarctica.
Under simulated Mars-like niche conditions, the lichens
appeared to have experienced an initial period of shock lasting ~7 days.
Following that, the lichens rapidly adapted. Photosynthetic activity increased
over the subsequent days and the increase continued to the end of the
experiment. It appears that the lichens were much more sensitive to the
intensity of UV irradiation than to other Mars-like parameters such as high
carbon dioxide concentration, very low temperatures, extreme humidity
fluctuations and very low atmospheric pressure. This study supports the notion that Earthly life can adapt
to the present-day conditions on Mars. Furthermore, life which may have
originated during the early warm and wet Mars might still survive and thrive in
present-day micro-habitats on Mars.
Reference:
J.-P. de Vera et al., “Adaptation of an Antarctic lichen to
Martian niche conditions can occur within 34 days”, Planetary and Space Science
98 (2014) 182-190