metabolic flux without apparent limit temperature (Price
2007). Conservation of species would be therefore possible
in a very old ice. In this respect, surviving species were
isolated from a Tibetan glacier from samples of
750,000 years old ice. Analysis of populations is very difficult due to the low average density (10
2 –10
4 cells mL
À1 ),
which indeed complicates the DNA extraction. In addition,
cultivation methods are performed in poor culture media
with incubation time of several months at low temperatures.
It appears that due to the narrowness of the liquid jets and the
change in diameter of them as a function of temperature, a
selective distribution of microorganisms is observed
depending on the cell size. Cells whose size is greater than
2 μm are fixed in ice crystals and those whose size is less
than 1 μm are free in the liquid streams. The dominant
species belong to Proteobacteria followed by Bacteroidetes.
Thermus, Bacteroides, Eubacterium, and Clostridium genera
have also been highlighted. Gram-positive species dominate
in this type ecosystems and this is certainly related to the
ability of some of these bacteria to form spores.
10.2.3.5 Permafrost
Permafrost areas occupy more than 20 % of the land surface
and are defined as soils whose temperature remains at or below
0
C for at least two consecutive years. Despite regarded as a
reservoir of ancient microbial life, recent experiments have
provided evidence of an active microbial life at temperatures
between 0 and À20
C in these environments.
At this place, the ice-water interfaces are of primary
importance and the “eutectophiles” term was proposed to
describe psychrophilic microorganisms existing at these critical interfaces (D’Amico et al. 2006). Oxidative activities of
organic compounds and
14 CO 2 fixation have been
demonstrated in Alaska within permafrost areas with
temperatures up to À40
C. Special techniques to enrich
microorganisms in solid phase from permafrost samples
have been described; they mainly use microcrystalline cellulose, mineral-based nutrients, and ethanol as substrates.
Measurements of CO 2 production to estimate the growth
at different temperatures were made.
Enrichments in the liquid phase conducted to the identification of bacteria at temperatures ranging from À1
C
(Polaromonas) to À17
C (Pseudomonas and Arthrobacter)
with close relationships to microorganisms isolated from
glaciers or polar sea ice. The solid-phase enrichment enabled
the preferential isolation of eukaryotes such as unicellular
yeasts and mycelial ascomycetes. To emphasize, once again,
the inconsistency of the distinction between psychrophilic and
psychrotolerant ascomycetes strains still show growth at
À17
C! To fix ideas, over temperatures between À18 and
À20
C, microorganisms retain their characteristics; they
develop exponentially, consume ethanol, and convert it into
CO 2 by half while the other half is used for biosynthesis. The
doubling time of less efficient microorganisms is about
14–35 days at À8
C. Below À20
C, stress is greater with a
transitory activation phase that probably prepares
microorganisms for prolonged survival (dormancy), thanks to
the biosynthesis of intra- and extracellular components. Bacterial population densities of 10
2
–10
8 viable cells per gram of
1–3-million-year-old permafrost have been observed in the
Arctic and Antarctic, and the relationship between aerobic
and anaerobic bacteria varies with the geological history of
soils. Studies performed in Siberian samples with a temperature of À10
C showed that the proportion of unfrozen water
was Æ 3 % and that permafrost contained 10
8 cells g
À1
. The
incorporation of lipid
14
C-acetate was used to measure doubling times at different temperatures. They were of 8 days at
À10
C and 200 days at À20
C. This suggests that functional
ecosystems exist in the permafrost and that melting might
cause a very important greenhouse effect since at the planetary
level, methane represents 30 % of the total carbon content in
soils. Moreover, these ecosystems are of primary interest for
scientists because of their similarity with glacial environments
that have been discovered on Mars.
Regarding the existing biodiversity, recent studies
showed that within permafrost samples of the Canadian
Arctic (80
N), collected at 9 m depth and at temperature
about À15
C, the majority of microorganisms were
psychrotolerant according to the definition of Morita
(1975); 50 % of them were halotolerant. Cultivated bacteria
belonged to the phyla Firmicutes, Actinobacteria, and
Proteobacteria, this latter phylum being less represented in
Siberian soils. A total of 42 bacterial and 11 archaeal
phylotypes were identified.
10.2.3.6 Antarctic Subglacial Lakes
In the 1950s, geothermal flow modeling has predicted that
beyond a certain thickness of ice, the interface between the
ice and the underlying rock could be in a liquid state. In
1973, radio soundings demonstrated the presence of 17
subglacial lakes in eastern Antarctic and nearly 150 of
these lakes in Antarctica were detected later on using different techniques. The most famous of these lakes is Lake
Vostok which name originates from the Russian Antarctic
station below which this lake was discovered. Its surface is
14,000 km
2 with depth ranging from 500 m north to 800 m
south. The thickness of ice above is about 4 km and because
of the difference in ice thickness between north and south,
Æ 300 m, the northern part is warmer (+0.3
C). This creates
a flow of water vapor from north to south which refreezes at
the basal ice of south, forming what is called ice accretion
between À3,540 and À3,743 m. The formation of this lake
dated from the Miocene period with age of the water being
estimated between 1 and 15 million years. The drilling of ice
lake was undertaken in 1989 by an international team of
researchers from Russia, United Kingdom, France, and
10 The Extreme Conditions of Life on the Planet and Exobiology
357
2007). Conservation of species would be therefore possible
in a very old ice. In this respect, surviving species were
isolated from a Tibetan glacier from samples of
750,000 years old ice. Analysis of populations is very difficult due to the low average density (10
2 –10
4 cells mL
À1 ),
which indeed complicates the DNA extraction. In addition,
cultivation methods are performed in poor culture media
with incubation time of several months at low temperatures.
It appears that due to the narrowness of the liquid jets and the
change in diameter of them as a function of temperature, a
selective distribution of microorganisms is observed
depending on the cell size. Cells whose size is greater than
2 μm are fixed in ice crystals and those whose size is less
than 1 μm are free in the liquid streams. The dominant
species belong to Proteobacteria followed by Bacteroidetes.
Thermus, Bacteroides, Eubacterium, and Clostridium genera
have also been highlighted. Gram-positive species dominate
in this type ecosystems and this is certainly related to the
ability of some of these bacteria to form spores.
10.2.3.5 Permafrost
Permafrost areas occupy more than 20 % of the land surface
and are defined as soils whose temperature remains at or below
0
C for at least two consecutive years. Despite regarded as a
reservoir of ancient microbial life, recent experiments have
provided evidence of an active microbial life at temperatures
between 0 and À20
C in these environments.
At this place, the ice-water interfaces are of primary
importance and the “eutectophiles” term was proposed to
describe psychrophilic microorganisms existing at these critical interfaces (D’Amico et al. 2006). Oxidative activities of
organic compounds and
14 CO 2 fixation have been
demonstrated in Alaska within permafrost areas with
temperatures up to À40
C. Special techniques to enrich
microorganisms in solid phase from permafrost samples
have been described; they mainly use microcrystalline cellulose, mineral-based nutrients, and ethanol as substrates.
Measurements of CO 2 production to estimate the growth
at different temperatures were made.
Enrichments in the liquid phase conducted to the identification of bacteria at temperatures ranging from À1
C
(Polaromonas) to À17
C (Pseudomonas and Arthrobacter)
with close relationships to microorganisms isolated from
glaciers or polar sea ice. The solid-phase enrichment enabled
the preferential isolation of eukaryotes such as unicellular
yeasts and mycelial ascomycetes. To emphasize, once again,
the inconsistency of the distinction between psychrophilic and
psychrotolerant ascomycetes strains still show growth at
À17
C! To fix ideas, over temperatures between À18 and
À20
C, microorganisms retain their characteristics; they
develop exponentially, consume ethanol, and convert it into
CO 2 by half while the other half is used for biosynthesis. The
doubling time of less efficient microorganisms is about
14–35 days at À8
C. Below À20
C, stress is greater with a
transitory activation phase that probably prepares
microorganisms for prolonged survival (dormancy), thanks to
the biosynthesis of intra- and extracellular components. Bacterial population densities of 10
2
–10
8 viable cells per gram of
1–3-million-year-old permafrost have been observed in the
Arctic and Antarctic, and the relationship between aerobic
and anaerobic bacteria varies with the geological history of
soils. Studies performed in Siberian samples with a temperature of À10
C showed that the proportion of unfrozen water
was Æ 3 % and that permafrost contained 10
8 cells g
À1
. The
incorporation of lipid
14
C-acetate was used to measure doubling times at different temperatures. They were of 8 days at
À10
C and 200 days at À20
C. This suggests that functional
ecosystems exist in the permafrost and that melting might
cause a very important greenhouse effect since at the planetary
level, methane represents 30 % of the total carbon content in
soils. Moreover, these ecosystems are of primary interest for
scientists because of their similarity with glacial environments
that have been discovered on Mars.
Regarding the existing biodiversity, recent studies
showed that within permafrost samples of the Canadian
Arctic (80
N), collected at 9 m depth and at temperature
about À15
C, the majority of microorganisms were
psychrotolerant according to the definition of Morita
(1975); 50 % of them were halotolerant. Cultivated bacteria
belonged to the phyla Firmicutes, Actinobacteria, and
Proteobacteria, this latter phylum being less represented in
Siberian soils. A total of 42 bacterial and 11 archaeal
phylotypes were identified.
10.2.3.6 Antarctic Subglacial Lakes
In the 1950s, geothermal flow modeling has predicted that
beyond a certain thickness of ice, the interface between the
ice and the underlying rock could be in a liquid state. In
1973, radio soundings demonstrated the presence of 17
subglacial lakes in eastern Antarctic and nearly 150 of
these lakes in Antarctica were detected later on using different techniques. The most famous of these lakes is Lake
Vostok which name originates from the Russian Antarctic
station below which this lake was discovered. Its surface is
14,000 km
2 with depth ranging from 500 m north to 800 m
south. The thickness of ice above is about 4 km and because
of the difference in ice thickness between north and south,
Æ 300 m, the northern part is warmer (+0.3
C). This creates
a flow of water vapor from north to south which refreezes at
the basal ice of south, forming what is called ice accretion
between À3,540 and À3,743 m. The formation of this lake
dated from the Miocene period with age of the water being
estimated between 1 and 15 million years. The drilling of ice
lake was undertaken in 1989 by an international team of
researchers from Russia, United Kingdom, France, and
10 The Extreme Conditions of Life on the Planet and Exobiology
357