10.3.5 Specific Constraints for the Cultivation
of Thermophilic Prokaryotes
The cultivation of thermophilic microorganisms needs the
adaptation of classic cultural techniques. As an example,
working at high temperature above 60
C prevents to use
agar for solid media. Isolation on Petri dishes or using roll
tubes is generally performed using gellan gum, a thermostable polymer that resists to temperature above 80
C (Erauso
et al. 1995). Numerous hyperthermophilic microorganisms
are anaerobes, and so cultivation techniques require strict
precautions with respect to dioxygen by using anaerobic
chambers for example (cf. Sect. 17.8.2). At high temperature, solubility of gases decreases and so their availability for
microorganisms is lower (Henry law). It is therefore necessary to incubate aerobes under agitation and under pressure
(0.1 or 0.2 MPa above atmospheric pressure) for the cultivation of chemolithotrophic autotrophs such as methanogens
that utilize H 2 and CO 2 for methane production.
Not only for the interest in evolution process and ecology,
hyperthermophilic prokaryotes were very early identified as a
potential sources of thermostable biomolecules (cf.
Sect. 10.3.5). Thus, the study of their physiological and metabolic characteristics appeared necessary to concretize their
biotechnological potential. One of the main limits was the
difficulty to obtain large amount of biomass. The cultivation
of anaerobic marine hyperthermophilic microorganisms as sulfur reducers belonging to the order Thermococcales raises a
number of technical problems linked to the use of seawaterbased culture medium and the production of hydrogen sulfide
(as a result of sulfur reduction). Combined with high temperature these conditions promote corrosion. One of the key factors
for the study of these hyperthermophiles was the implementation of cultures in specially designed bioreactors that had
allowed the study of their physiology and metabolism and the
production of enough quantities biomass (Godfroy et al. 2006).
10.3.6 Biotechnological Applications
As previously mentioned, the interest for hyperthermophiles,
which resulted in the isolation, during the 30 past years, of
numerous new species, was highly boosted by the search for
biotechnological applications and more specifically in the
biocatalyst area: enzymes from hyperthermophiles are in
most cases more resistant than their mesophilic homologues.
We will present here only a few examples.
The more known is the use for PCR (polymerase chain
reaction) of the Taq polymerase from Thermus aquaticus.
Many other polymerases from hyperthermophilic prokaryotes
were described and are marketed: Vent pol
TM from Pyrococcus
furiosus that was isolated from a swallow vent (Vulcano, Italy)
and the Deep Vent pol
TM and the polymerase Isis
TM from
Pyrococcus GBD and Pyrococcus abyssi, respectively, that
were isolated from deep-sea hydrothermal vents.
Other applications deal with hydrolase that can be used
in the food industry for starch treatment, for example, or
biomass transformation for hydrogen or biogas production
(Basen et al. 2012).
10.3.7 Hyperthermophiles and Evolution
Due to their position at the basis of the phylogenetic tree,
hyperthermophiles whatever there are Archaea or Bacteria
could be the sign of a slow evolution process, probably due to
the habitats in which extremophilic prokaryotes thrive with
hard physical and chemical constraints. These ecosystems as
they exist today existed on the early Earth and could be
those wherein life could have appeared. Thereby extreme
microorganisms (both Archaea and Bacteria) are perhaps
the closest relatives of a primitive life form. It is also interesting to notice that hydrogen metabolism, widely widespread in
thermophilic microorganisms, could be one ancient metabolism adapted to primitive life conditions (cf. Sect. 4.1).
10.4 Halophilic and Extreme Halophilic
Microorganisms
10.4.1 Presentation
Halophiles can be found in all three domains of life. The term
“halophilic” is used to qualify microorganisms that require the
presence of salts (usually sodium chloride) for their growth.
A distinction must be made between halophiles and
microorganisms that can tolerate salt even if present in large
quantities. They are called as salt-tolerant microorganisms.
In 1962, Helge Larsen defined three categories of halophilic microorganisms based on the salt concentration that
allows optimal growth (Fig. 10.8). The slightly halophiles
have optimum growth between 2 and 5 % NaCl, while
moderate halophiles organisms exhibit optimum growth
between 5 and 20 % (cf. Sect. 9.6.3).
The term “extreme halophiles” is applied to microorganisms
which have better growth at concentrations above 20 % salts.
These microorganisms can generally grow in a saturated salt
environment (about 35 %), although some species grow very
slowly at this salinity. By definition, extreme halophiles require
at least 9 % NaCl for growth.
10.4.2 Habitats of Halophilic Microorganisms
Salty habitats are common throughout the world (especially
marine ones), but hypersaline environments are relatively
infrequent, most of them being located in the hot and dry
areas on the planet (Fig. 10.9), but some are also found in
temperate and even polar regions. Microbiology of hypersaline
10 The Extreme Conditions of Life on the Planet and Exobiology
367
of Thermophilic Prokaryotes
The cultivation of thermophilic microorganisms needs the
adaptation of classic cultural techniques. As an example,
working at high temperature above 60
C prevents to use
agar for solid media. Isolation on Petri dishes or using roll
tubes is generally performed using gellan gum, a thermostable polymer that resists to temperature above 80
C (Erauso
et al. 1995). Numerous hyperthermophilic microorganisms
are anaerobes, and so cultivation techniques require strict
precautions with respect to dioxygen by using anaerobic
chambers for example (cf. Sect. 17.8.2). At high temperature, solubility of gases decreases and so their availability for
microorganisms is lower (Henry law). It is therefore necessary to incubate aerobes under agitation and under pressure
(0.1 or 0.2 MPa above atmospheric pressure) for the cultivation of chemolithotrophic autotrophs such as methanogens
that utilize H 2 and CO 2 for methane production.
Not only for the interest in evolution process and ecology,
hyperthermophilic prokaryotes were very early identified as a
potential sources of thermostable biomolecules (cf.
Sect. 10.3.5). Thus, the study of their physiological and metabolic characteristics appeared necessary to concretize their
biotechnological potential. One of the main limits was the
difficulty to obtain large amount of biomass. The cultivation
of anaerobic marine hyperthermophilic microorganisms as sulfur reducers belonging to the order Thermococcales raises a
number of technical problems linked to the use of seawaterbased culture medium and the production of hydrogen sulfide
(as a result of sulfur reduction). Combined with high temperature these conditions promote corrosion. One of the key factors
for the study of these hyperthermophiles was the implementation of cultures in specially designed bioreactors that had
allowed the study of their physiology and metabolism and the
production of enough quantities biomass (Godfroy et al. 2006).
10.3.6 Biotechnological Applications
As previously mentioned, the interest for hyperthermophiles,
which resulted in the isolation, during the 30 past years, of
numerous new species, was highly boosted by the search for
biotechnological applications and more specifically in the
biocatalyst area: enzymes from hyperthermophiles are in
most cases more resistant than their mesophilic homologues.
We will present here only a few examples.
The more known is the use for PCR (polymerase chain
reaction) of the Taq polymerase from Thermus aquaticus.
Many other polymerases from hyperthermophilic prokaryotes
were described and are marketed: Vent pol
TM from Pyrococcus
furiosus that was isolated from a swallow vent (Vulcano, Italy)
and the Deep Vent pol
TM and the polymerase Isis
TM from
Pyrococcus GBD and Pyrococcus abyssi, respectively, that
were isolated from deep-sea hydrothermal vents.
Other applications deal with hydrolase that can be used
in the food industry for starch treatment, for example, or
biomass transformation for hydrogen or biogas production
(Basen et al. 2012).
10.3.7 Hyperthermophiles and Evolution
Due to their position at the basis of the phylogenetic tree,
hyperthermophiles whatever there are Archaea or Bacteria
could be the sign of a slow evolution process, probably due to
the habitats in which extremophilic prokaryotes thrive with
hard physical and chemical constraints. These ecosystems as
they exist today existed on the early Earth and could be
those wherein life could have appeared. Thereby extreme
microorganisms (both Archaea and Bacteria) are perhaps
the closest relatives of a primitive life form. It is also interesting to notice that hydrogen metabolism, widely widespread in
thermophilic microorganisms, could be one ancient metabolism adapted to primitive life conditions (cf. Sect. 4.1).
10.4 Halophilic and Extreme Halophilic
Microorganisms
10.4.1 Presentation
Halophiles can be found in all three domains of life. The term
“halophilic” is used to qualify microorganisms that require the
presence of salts (usually sodium chloride) for their growth.
A distinction must be made between halophiles and
microorganisms that can tolerate salt even if present in large
quantities. They are called as salt-tolerant microorganisms.
In 1962, Helge Larsen defined three categories of halophilic microorganisms based on the salt concentration that
allows optimal growth (Fig. 10.8). The slightly halophiles
have optimum growth between 2 and 5 % NaCl, while
moderate halophiles organisms exhibit optimum growth
between 5 and 20 % (cf. Sect. 9.6.3).
The term “extreme halophiles” is applied to microorganisms
which have better growth at concentrations above 20 % salts.
These microorganisms can generally grow in a saturated salt
environment (about 35 %), although some species grow very
slowly at this salinity. By definition, extreme halophiles require
at least 9 % NaCl for growth.
10.4.2 Habitats of Halophilic Microorganisms
Salty habitats are common throughout the world (especially
marine ones), but hypersaline environments are relatively
infrequent, most of them being located in the hot and dry
areas on the planet (Fig. 10.9), but some are also found in
temperate and even polar regions. Microbiology of hypersaline
10 The Extreme Conditions of Life on the Planet and Exobiology
367