This is the first report on resistance of fungal isolates and
cryptoendolithic communities from terrestrial extreme environments, to
simulated space and Martian conditions, which were applied individually or in
different combinations. Selected fungal isolates were previously demonstrated
to survive some extreme terrestrial factors, such as repeated freezing and
thawing cycles, high salt concentrations and UV-B irradiation
(Onofri et al.
). This study demonstrates high resistance of all isolates to
simulated space or Martian conditions, despite wide standard deviations.
Thirteen verification tests were carried out in both EVT-E1 and E2. Reporting
results as survival (+) or non-survival (-)
(), 13 positive
responses were recorded for C. minteri
CCFEE 5187, 10 for C.
CCFEE 515 and 6 for C. antarcticus
Moreover, results obtained by staining methods in C. minteri
() showed no significant
reductions of living cells both in control and treated cultures, with the
exception of the UV-C treatment (254 nm, 1000 Jm–2
higher resistance of C. minteri
CCFEE 5187 to both EVTs and the good
survival shown to heat shocks suggested that this isolate might be selected as
a good candidate to withstand space flight and long-term permanence in space.
Under the most selective combined condition of vacuum and maximum dose of
polychromatic UV used, this strain was unable to grow. Its positive response
in staining techniques but absence of growth might be due to the transition to
a state of “viable but non-culturable cells” (VBNC), as described
for bacteria (Weichart 1999
Apparently cells can maintain their integrity and viability but nevertheless
may lose reproductive ability. Growth of C. minteri
detection because of the transition to a specific survival state characterised
by deceleration of vital activity (DVA)
cells were not stainable, we did not have the
opportunity to verify the occurrence of viable but not culturable cells after
exposure to stressing conditions. Particularly surprising were the results
concerning single and combined irradiation of C. minteri
maximum UV polychromatic dose, since it corresponds to the irradiation
expected during the whole planned space exposure of 1.5 yr without
without neutral density filter.
Summarisation of responses of the tested isolates to the EVTs, reporded as
survival (+) or non-survival (-).
Cryomyces antarcticus CCFEE 515, which gave better results
compared with CCFEE 514 of the same species, was also selected to investigate
short and long term resistance to space conditions on the ISS.
The high resistance to space conditions shown by the three isolates tested
could be ascribed to the peculiar morpho- and physiological features of black
meristematic fungi. These microorganisms produce slowly expanding,
cauliflower-like colonies, barely differentiated structures, and thick and
heavily pigmented cell walls (Selbmann
et al. 2005
et al. 2007a
). These characteristics convey to high
tolerance to extreme terrestrial environments and, by coincidence, spacial
conditions. Melanin, for instance, is a biological macromolecule, ubiquitous
in nature, mainly known for its protective role against UV and ionising
radiation, extreme temperatures, and desiccation
). The high
tolerance to the UV-B exposure of single cells of the tested fungi has been
reported recently (Onofri et al.
). The thickness of the colony itself may represent an
additional protection for the cells in the inner layers.
The high resistance of tested isolates to temperatures up to 90 °C is
in agreement with literature on survival of dehydrated colonies of other
meristematic fungi subjected to high temperatures
(Sterflinger & Krumbein
, Sterflinger et
). The ability to enter a cryptobiotic state under
poikilohydric conditions could be aided by the presence of abundant
extracellular polymeric substances (EPS) in many species. EPS production may
be abundant (Selbmann et al.
) and may appear as a gelatinous matrix in lichen thalli
(de Vera et al. 2004
which may serve as a water reservoir to survive long dry periods (de los
Ríos et al.
). Not surprisingly, many
meristematic black fungi are commonly recorded from Mediterranean areas and
hot deserts, where substrate surface temperatures can reach very high values.
The ability to survive long-term desiccation makes these isolates pre-adapted
to the extreme conditions of space, since high-vacuum conditions produce an
extreme dehydrating effect.
Recent studies also show that lichens, as well as their isolated
photobionts and mycobionts, cope with the extreme conditions of outer space in
ground-based experiments (de Vera et al.
was able to photosynthesise under simulated Martian conditions
with light in visible wave-lengths and in the presence of water
(de Vera et al.
). Finally, samples of the lichens Rhizocarpon
and X. elegans
survived 16 d of exposure to space
in the BIOPAN-5 facility of the European Space Agency located on the outer
shell of the Earth-orbiting FOTON-M2 Russian satellite
(Sancho et al.
By means of the NASA Space Shuttle Atlantis flight launched on Feb 7, 2008,
these fungi are now exposed to actual space in the EXPOSE facility (created by
Kaiser-Threde - DE) on the outside platform EuTEF of the International Space
Station, orbiting round the Earth at a height of ~300 km, where space
conditions include pressures of 10–5 pa, temperatures ranges
between –20 and +20 °C, and full solar (including UV-A, UV-B, and
UV-C) and cosmic radiation. These conditions are normally prohibitive for
Lithopanspermia postulates the feasibility of interplanetary transfer of
living material, protected against extraterrestrial solar UV and possibly heat
within asteroids, comets and meteorites
(Nicholson et al.
). Our knowledge on the limits of life has largely expanded in
the last decades. The discovery of extremophiles, the high survival of
spores over six yr in space (Horneck et
. 1994), the survival of lichens after ground-based experiments (de
Vera et al. 2003
), as well as in space for
2 wk during the Biopan experiments (Sancho
et al. 2007
), and our results with the Experiment
Verification Tests on meristematic fungi and cryptoendolithic communities give
additional support to the idea of lithopanspermia. Considering that 1 yr is
the minimum flight time estimated for Martian meteorites landing on Earth
(Mileikowsky et al.
), the eventual survival after 1.5 yr permanence in space
planned in the LIFE experiment represents a further contribution in the
scenario of interplanetary transfer of life.