areas (solfataras and hot springs). They are acidophiles,
facultative chemolithotrophs Sulfolobus species oxidizing
sulfur compounds (H 2 S or S
) into sulfuric acid while
Acidianus species are facultative anaerobes that can oxidize
S
in H 2 SO 4 or reduce it in H 2 S.
The order Desulfurococcales gathers around ten genera
with highly diverse phenotypic characteristics. Indeed,
it includes aerobic chemoorganotrophs (Aeropyrum,
Sulfurococcus),
anaerobic
ones
(Hyperthermus,
Desulfurococcus, Staphylothermus, Pyrodictium . . .), as
well as strict chemolithotrophs (Ignicoccus, Pyrolobus),
which are frequently sulfur reducers. One of the major
characteristic of this order is that it includes some of the
most thermophilic known species. Species from the genera
Pyrodictium and Pyrolobus are able to grow at temperature
above 100
C. Pyrolobus fumarii isolated from a hydrothermal chimney of the Mid-Atlantic Ridge holds the record for
growth temperature (113
C) within the prokaryotes.
Desulfurococcus, Staphylothermus, and Hyperthermus
are
hyperthermophilic
chemoorganotrophic
sulfur
metabolizers, and Aeropyrum is one of the rare aerobic
hyperthermophiles. Species of the genus Ignicoccus were
isolated from both swallow and deep hydrothermal vents.
They are chemolithoautrophic organisms that use elemental
sulfur as terminal electron acceptor. Some Ignicoccus species live in association with a small prokaryote,
Nanoarchaeum equitans. This species form a unique lineage
at the archaeal phylogenetic tree. It is one of the smallest
described prokaryote due to either its cell volume or genome
size. The absence in its genome of most of the genes
encoding metabolic function made it being totally dependent
from its host, Ignicoccus (cf. Sect. 6.6.1).
The number of hyperthermophilic prokaryotes is considerably increasing in the last 3 years (from a few tens in the
1970s to several hundreds in 2013). Like in many other
ecosystems, cultivated species represent a small fraction of
the microbial diversity of hot ecosystems and molecular
surveys based on 16S rRNA allowed to point out an astonishing diversity. Thus, numerous lineages (both Bacteria and
Archaea) in which there is no cultivated representative were
detected in many hot environments.
Within the Archaea, many uncultivated lineages within
the Euryarchaeota were evidenced as well as the lineage
Korarchaeota that gathers uncultured organisms that are
supposed to be thermophiles due to the position of this
lineage at the basis of the phylogenetic tree (cf. Sect. 6.6.1).
The development of techniques based on the search of
gene encoding metabolic functions as well as metagenomic
and metatranscriptomic approaches would lead to enlarge
our vision of this diversity. It clearly appears that only the
combination of both molecular and innovative cultural
techniques would allow to characterize thermophilic microbial communities of hot ecosystems on Earth.
10.3.4 Life at High Temperature
Thermophilic and hyperthermophilic prokaryotes live at
temperatures much higher than those tolerated by most
other prokaryotes. Indeed, their macromolecules (proteins,
lipids, and DNA) have specificities that make them good
candidates for biotechnological applications.
10.3.4.1 Protein Stability
Thermophilic proteins have similar amino acids composition
to that of their mesophilic counterparts. Nevertheless, thermostable proteins exhibit specific characteristics that are
(1) the presence of highly hydrophobic centers that avoid
the protein to unfold, (2) a diminution of the loop size at the
surface of the protein, and (3) the presence of numerous
ionic interactions at the surface that also contribute to
maintain proteins together (Luo and Robb 2011). It was
demonstrated that it is more the protein folding that
contributes to its resistance to temperature. Thereby small
changes in the amino acids sequence will result in significant changes in the protein folding and so its behavior at
high temperature. Also, the presence of chaperons proteins
(heat-shock proteins) helps to the folding on partially
unfolded proteins.
10.3.4.2 DNA Stability
Several mechanisms that contribute to DNA molecule stability
were identified in hyperthermophilic prokaryotes. On one
hand, the presence of compatible solutes as high intracellular
concentration of cyclic potassium 2,3-diphosphoglycerate
in Methanopyrus will contribute to chemical lesion as
depurination. On the other hand, hyperthermophiles possess
a reverse DNA gyrase which produces positive supercoiling
(while DNA gyrase from mesophiles produces negative
supercoiling) of DNA that will ensure better stability to high
temperatures. Other DNA-linked proteins were identified,
sometimes similar to histones from the eukaryotes (histonelike proteins). They contribute to the compaction of the DNA
molecule and so to its stability.
10.3.4.3 Membrane Lipids
Membrane lipids of hyperthermophilic Archaea are
composed of biphytanyl tetraethers. They are resistant to
temperature due to the presence of a covalent link between
the phytanyl units that allows the formation of a one-layer
membrane by opposition to the classic bilayer phospholipids
membrane (cf. Sect. 4.1.5). This structure shored up by
covalent link is more resistant to temperature which tends
to separate the phospholipids bilayer. This type of lipids was
also found in some thermophilic members of the Bacteria
domain.
366
J.-L. Cayol et al.
facultative chemolithotrophs Sulfolobus species oxidizing
sulfur compounds (H 2 S or S
) into sulfuric acid while
Acidianus species are facultative anaerobes that can oxidize
S
in H 2 SO 4 or reduce it in H 2 S.
The order Desulfurococcales gathers around ten genera
with highly diverse phenotypic characteristics. Indeed,
it includes aerobic chemoorganotrophs (Aeropyrum,
Sulfurococcus),
anaerobic
ones
(Hyperthermus,
Desulfurococcus, Staphylothermus, Pyrodictium . . .), as
well as strict chemolithotrophs (Ignicoccus, Pyrolobus),
which are frequently sulfur reducers. One of the major
characteristic of this order is that it includes some of the
most thermophilic known species. Species from the genera
Pyrodictium and Pyrolobus are able to grow at temperature
above 100
C. Pyrolobus fumarii isolated from a hydrothermal chimney of the Mid-Atlantic Ridge holds the record for
growth temperature (113
C) within the prokaryotes.
Desulfurococcus, Staphylothermus, and Hyperthermus
are
hyperthermophilic
chemoorganotrophic
sulfur
metabolizers, and Aeropyrum is one of the rare aerobic
hyperthermophiles. Species of the genus Ignicoccus were
isolated from both swallow and deep hydrothermal vents.
They are chemolithoautrophic organisms that use elemental
sulfur as terminal electron acceptor. Some Ignicoccus species live in association with a small prokaryote,
Nanoarchaeum equitans. This species form a unique lineage
at the archaeal phylogenetic tree. It is one of the smallest
described prokaryote due to either its cell volume or genome
size. The absence in its genome of most of the genes
encoding metabolic function made it being totally dependent
from its host, Ignicoccus (cf. Sect. 6.6.1).
The number of hyperthermophilic prokaryotes is considerably increasing in the last 3 years (from a few tens in the
1970s to several hundreds in 2013). Like in many other
ecosystems, cultivated species represent a small fraction of
the microbial diversity of hot ecosystems and molecular
surveys based on 16S rRNA allowed to point out an astonishing diversity. Thus, numerous lineages (both Bacteria and
Archaea) in which there is no cultivated representative were
detected in many hot environments.
Within the Archaea, many uncultivated lineages within
the Euryarchaeota were evidenced as well as the lineage
Korarchaeota that gathers uncultured organisms that are
supposed to be thermophiles due to the position of this
lineage at the basis of the phylogenetic tree (cf. Sect. 6.6.1).
The development of techniques based on the search of
gene encoding metabolic functions as well as metagenomic
and metatranscriptomic approaches would lead to enlarge
our vision of this diversity. It clearly appears that only the
combination of both molecular and innovative cultural
techniques would allow to characterize thermophilic microbial communities of hot ecosystems on Earth.
10.3.4 Life at High Temperature
Thermophilic and hyperthermophilic prokaryotes live at
temperatures much higher than those tolerated by most
other prokaryotes. Indeed, their macromolecules (proteins,
lipids, and DNA) have specificities that make them good
candidates for biotechnological applications.
10.3.4.1 Protein Stability
Thermophilic proteins have similar amino acids composition
to that of their mesophilic counterparts. Nevertheless, thermostable proteins exhibit specific characteristics that are
(1) the presence of highly hydrophobic centers that avoid
the protein to unfold, (2) a diminution of the loop size at the
surface of the protein, and (3) the presence of numerous
ionic interactions at the surface that also contribute to
maintain proteins together (Luo and Robb 2011). It was
demonstrated that it is more the protein folding that
contributes to its resistance to temperature. Thereby small
changes in the amino acids sequence will result in significant changes in the protein folding and so its behavior at
high temperature. Also, the presence of chaperons proteins
(heat-shock proteins) helps to the folding on partially
unfolded proteins.
10.3.4.2 DNA Stability
Several mechanisms that contribute to DNA molecule stability
were identified in hyperthermophilic prokaryotes. On one
hand, the presence of compatible solutes as high intracellular
concentration of cyclic potassium 2,3-diphosphoglycerate
in Methanopyrus will contribute to chemical lesion as
depurination. On the other hand, hyperthermophiles possess
a reverse DNA gyrase which produces positive supercoiling
(while DNA gyrase from mesophiles produces negative
supercoiling) of DNA that will ensure better stability to high
temperatures. Other DNA-linked proteins were identified,
sometimes similar to histones from the eukaryotes (histonelike proteins). They contribute to the compaction of the DNA
molecule and so to its stability.
10.3.4.3 Membrane Lipids
Membrane lipids of hyperthermophilic Archaea are
composed of biphytanyl tetraethers. They are resistant to
temperature due to the presence of a covalent link between
the phytanyl units that allows the formation of a one-layer
membrane by opposition to the classic bilayer phospholipids
membrane (cf. Sect. 4.1.5). This structure shored up by
covalent link is more resistant to temperature which tends
to separate the phospholipids bilayer. This type of lipids was
also found in some thermophilic members of the Bacteria
domain.
366
J.-L. Cayol et al.