Belgium and stopped in 1998 at a depth of 3,623 m within
ice accretion, at about 120 m from the surface of the water. In
2006, additional drilling was performed by a Russian team up
to 3,651 m. Ice samples collected at different depths between
3,540 and 3,623 m in this lake have predicted that the organic
carbon concentrations would range between 86 and 160 μM,
cell densities between 150 and 460 cells.mL
À1
, and solutes
between 1.5 and 34 mM. The concentration of dissolved O 2
was estimated to be 50 times higher than at the equilibrium
water/air surface due to the pressure and the high concentration of gas present in ice in the form of clathrate (complex gas
hydrates). Samples from melted ice accretion were analyzed
and the microorganisms were identified after isolation by
sequencing of their 16S rRNA gene. Representatives of
Proteobacteria (subdivisions alpha, beta, gamma, and epsilon), Firmicutes, Actinobacteria, and Bacteroidetes were
recovered from these samples. However, to confirm the indigenous nature of the microorganisms detected, further
investigations of the underlying water and direct sampling
will need the implementation of a considerable logistics
because of the risk of external contamination of the huge
groundwater covering the Antarctic by a layer of water 1 m
thick. In addition, it appears that these habitats which have
been regarded for long time as closed are in fact connected by
extensive networks of underground channels that could be
sources of contaminations. The rapid transfer of water masses
from one place to another by underground roads would lead to
a substantial redistribution of solutes and microorganisms.
10.2.4 Molecular Adaptations
At low temperatures, bacteria have to face several
constraints such as (1) a decrease in membrane fluidity, (2)
a decrease in the rate of enzymatic reactions, (3) the stabilization of secondary structures of nucleic acids resulting in a
decrease in the efficiency of transcription and translation,
and (4) the slowing down of the process of protein folding.
In response to these constraints, microorganisms may adapt
as described below (cf. Sect. 9.6.1).
10.2.4.1 Membranes
Low temperatures induce a decrease in membrane fluidity.
They favor synthesis of unsaturated fatty acids, polyunsaturated fatty acids, and branched fatty acids. These changes
aim to introduce a steric constraint and reduce the number of
interactions within the membrane, thus increasing its fluidity
(D’Amico et al. 2006). A decrease in the length of the
hydrocarbon chains is also observed.
For anaerobic bacteria, the fatty acid synthase is able to
synthesize saturated and unsaturated fatty acids. This is due to
an elongation system resulting in an intermediate that can be
dehydrated to synthesize unsaturated fatty acids. In aerobic
bacteria, the fatty acid synthase does not produce unsaturated
fatty acids. They therefore have an additional enzyme: a
desaturase able to desaturate fatty acids. This enzyme, with
other proteins and cofactors, form complexes in the membranes
and act as a small respiratory chain by transferring electrons
from the fatty acids to an electron acceptor (e.g., dioxygen).
When the temperature increases, there is an induction of a new
enzyme (desaturase) in aerobic bacteria, whereas in anaerobic
the existing enzymes switch their function.
There are two types of branched fatty acids, the isobranched and the anteiso branches ones, resulting from the
use of a derivative of leucine and isoleucine as the first
molecule in the synthesis of the fatty acid. Temperature
controls acyl-CoA: transacylase responsible for the conversion of amino acid derivatives (keto acids) in ACP (acyl
carrier protein)-thioester. These molecules are then used by
the fatty acid synthase to produce branched fatty acids. Some
bacteria show a higher number of short-chain fatty acids.
This is the case for Micrococcus cryophilus, a psychrophilic
bacterium which has a ratio of oleic acid/palmitoleic acid
four times higher at 25
C than at 4
C. The transition from
oleic acid (18 carbons) to palmitoleic acid (16 carbons) and
vice versa is catalyzed by an elongase bonded to the membrane. In other bacteria, there is synthesis of new short-chain
fatty acids using a fatty acid synthase.
10.2.4.2 Enzymes
Most enzymatic reactions obey the Arrhenius law, a law
which reflects the variation of reaction rate as a function of
temperature: k ¼ A exp
(ÀEa/RT) where k is the rate constant,
A is a pre-exponential factor, Ea is the activation energy, R is
the perfect gas constant, and T the absolute temperature in
Kelvin. According to this law, a decrease in temperature
induces an exponential decrease of reaction rate. Enzymes
of psychrophilic microorganisms have adapted to maintain
an appropriate reaction rate at low temperatures. Many psychrophilic enzymes have been studied and the threedimensional structure of more than 20 of these enzymes is
currently known (Feller 2013). These studies show that:
1. Specific activity of psychrophilic enzymes is higher at
low and mesothermic temperatures than their mesophilic
counterparts.
2. The apparent optimal temperature of activity of psychrophilic enzymes is significantly shifted to lower
temperatures.
3. Specific activity of psychrophilic enzymes is generally
lower than their mesophilic homologues. This suggests
that adaptation to cold is not quite complete (D’Amico
et al. 2006) (Fig. 10.3).
The adaptation of these enzymes to their environment is
explained by the relation “activity-stability-flexibility.” This
suggests that psychrophilic enzymes increase their overall
358
J.-L. Cayol et al.
ice accretion, at about 120 m from the surface of the water. In
2006, additional drilling was performed by a Russian team up
to 3,651 m. Ice samples collected at different depths between
3,540 and 3,623 m in this lake have predicted that the organic
carbon concentrations would range between 86 and 160 μM,
cell densities between 150 and 460 cells.mL
À1
, and solutes
between 1.5 and 34 mM. The concentration of dissolved O 2
was estimated to be 50 times higher than at the equilibrium
water/air surface due to the pressure and the high concentration of gas present in ice in the form of clathrate (complex gas
hydrates). Samples from melted ice accretion were analyzed
and the microorganisms were identified after isolation by
sequencing of their 16S rRNA gene. Representatives of
Proteobacteria (subdivisions alpha, beta, gamma, and epsilon), Firmicutes, Actinobacteria, and Bacteroidetes were
recovered from these samples. However, to confirm the indigenous nature of the microorganisms detected, further
investigations of the underlying water and direct sampling
will need the implementation of a considerable logistics
because of the risk of external contamination of the huge
groundwater covering the Antarctic by a layer of water 1 m
thick. In addition, it appears that these habitats which have
been regarded for long time as closed are in fact connected by
extensive networks of underground channels that could be
sources of contaminations. The rapid transfer of water masses
from one place to another by underground roads would lead to
a substantial redistribution of solutes and microorganisms.
10.2.4 Molecular Adaptations
At low temperatures, bacteria have to face several
constraints such as (1) a decrease in membrane fluidity, (2)
a decrease in the rate of enzymatic reactions, (3) the stabilization of secondary structures of nucleic acids resulting in a
decrease in the efficiency of transcription and translation,
and (4) the slowing down of the process of protein folding.
In response to these constraints, microorganisms may adapt
as described below (cf. Sect. 9.6.1).
10.2.4.1 Membranes
Low temperatures induce a decrease in membrane fluidity.
They favor synthesis of unsaturated fatty acids, polyunsaturated fatty acids, and branched fatty acids. These changes
aim to introduce a steric constraint and reduce the number of
interactions within the membrane, thus increasing its fluidity
(D’Amico et al. 2006). A decrease in the length of the
hydrocarbon chains is also observed.
For anaerobic bacteria, the fatty acid synthase is able to
synthesize saturated and unsaturated fatty acids. This is due to
an elongation system resulting in an intermediate that can be
dehydrated to synthesize unsaturated fatty acids. In aerobic
bacteria, the fatty acid synthase does not produce unsaturated
fatty acids. They therefore have an additional enzyme: a
desaturase able to desaturate fatty acids. This enzyme, with
other proteins and cofactors, form complexes in the membranes
and act as a small respiratory chain by transferring electrons
from the fatty acids to an electron acceptor (e.g., dioxygen).
When the temperature increases, there is an induction of a new
enzyme (desaturase) in aerobic bacteria, whereas in anaerobic
the existing enzymes switch their function.
There are two types of branched fatty acids, the isobranched and the anteiso branches ones, resulting from the
use of a derivative of leucine and isoleucine as the first
molecule in the synthesis of the fatty acid. Temperature
controls acyl-CoA: transacylase responsible for the conversion of amino acid derivatives (keto acids) in ACP (acyl
carrier protein)-thioester. These molecules are then used by
the fatty acid synthase to produce branched fatty acids. Some
bacteria show a higher number of short-chain fatty acids.
This is the case for Micrococcus cryophilus, a psychrophilic
bacterium which has a ratio of oleic acid/palmitoleic acid
four times higher at 25
C than at 4
C. The transition from
oleic acid (18 carbons) to palmitoleic acid (16 carbons) and
vice versa is catalyzed by an elongase bonded to the membrane. In other bacteria, there is synthesis of new short-chain
fatty acids using a fatty acid synthase.
10.2.4.2 Enzymes
Most enzymatic reactions obey the Arrhenius law, a law
which reflects the variation of reaction rate as a function of
temperature: k ¼ A exp
(ÀEa/RT) where k is the rate constant,
A is a pre-exponential factor, Ea is the activation energy, R is
the perfect gas constant, and T the absolute temperature in
Kelvin. According to this law, a decrease in temperature
induces an exponential decrease of reaction rate. Enzymes
of psychrophilic microorganisms have adapted to maintain
an appropriate reaction rate at low temperatures. Many psychrophilic enzymes have been studied and the threedimensional structure of more than 20 of these enzymes is
currently known (Feller 2013). These studies show that:
1. Specific activity of psychrophilic enzymes is higher at
low and mesothermic temperatures than their mesophilic
counterparts.
2. The apparent optimal temperature of activity of psychrophilic enzymes is significantly shifted to lower
temperatures.
3. Specific activity of psychrophilic enzymes is generally
lower than their mesophilic homologues. This suggests
that adaptation to cold is not quite complete (D’Amico
et al. 2006) (Fig. 10.3).
The adaptation of these enzymes to their environment is
explained by the relation “activity-stability-flexibility.” This
suggests that psychrophilic enzymes increase their overall
358
J.-L. Cayol et al.