Abstract |
The purpose of this work is to demonstrate the resistance of lichen under prolonged drought
stress, extremely low temperature (-196oC) and oxygen depleted conditions, using a series of
biochemical and physicochemical analyses and physiological/ functional approaches. The lichen
Pleurosticta acetabulum is revived after exposure to a long period of drought. In fact, the transition to
the dry state, not only did not stress the lichen, but also contributed to its structural and functional
maintenance, when incubated at -196oC. Fluorescence induction measurements (JIP test) and HPLC
analyses of ergosterol levels confirmed the maintenance of both symbionts. On the contrary, the
incubation of metabolically active lichen at extremely low temperature stressed the photobiont
Trebouxia, without leading to collapse and the mycobiont partner remained intact. The isolated green
algae, Trebouxia, in a non-symbiotic state, was not stressed at all when exposed at drought stress and
the extremely low temperature of -196oC, proving the fact that the isolated algae is as resistant as the
whole lichen thalli at extreme conditions. The above results confirm that the resistance of the lichen
is not directly an effect of the symbiosis between the photobiont and the mycobiont, in contrast to the
hitherto predominant acceptance, but they are two equal resistant organisms. The incubation of the
lichen in absolute dry conditions and extremely low temperature was followed by incubation in
oxygen depleted conditions. In this state, the lichen remained functional and its metabolism, and
especially its photosynthetic activity, was reorganized, leading to the production of high amount of
molecular hydrogen (H2) through the dark fermentation pathway, which combines the lipid
biosynthesis pathway with the electron transport chain of photosynthesis. Neither the drought nor the
extremely low temperature affected the ability of lichen to produce high amounts of hydrogen in
oxygen depleted conditions, which can be used as fuel to meet energy demands and has high
efficiency. The thorough analysis of differentiation in the level of fats and their fatty acids
composition, as well as the level of sugar in all treatments contributed to the understanding of the
extremophilic behavior of lichen. The unprecedented resistance and functional plasticity of this
organism in three extreme conditions that simulate space conditions, makes it the ideal organism for
space missions and astrobiological/astrobiotechnological applications. Based on the results of this
study, the theory of panspermia can be further analyzed and lichens are suggested as the organisms
that could both accompany a space mission by providing energy at zero cost and also be used in a
Reverse Assisted Panspermia, carrying life in other star systems.
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