Also, total mineralization of organic matter requires several categories of microorganisms (oligoheterotrophs,
chemolithoautotrophs); none have been detected by the cultivation techniques used. Discovery of such organisms
would allow to claim that piezophily is a unique feature for
a majority of deep-sea microorganisms. But according to the
slow growth of deep-sea heterotrophic piezopsychrophiles,
one can expect weeks or even months for the growth of deepsea chemolithoautotrophs, which makes their studies very
difficult.
10.5.3 Taxonomy and Phylogeny of
Piezopsychrophiles
Most of piezopsychrophilic bacteria that have been isolated
were also studied for their taxonomy and phylogeny. All
belong to the gamma subdivision within the Proteobacteria.
No correlation was observed between the phylogeny of these
organisms and depth, location, or original habitat. While
many other extremophiles (halophiles, hyperthermophiles)
often belong to distinct phylogenetic lineages, piezopsychrophilic organisms studied up to now are distributed
within phylogenetic groups together with piezosensitive species. Adaptations to elevated hydrostatic pressures probably
appeared rather recently during a speciation process. The
recent utilization of molecular techniques indeed revealed
more diversity than the cultivation methods (alpha and delta
subdivisions within Proteobacteria, Actinobacteria,
Planctomycetales, etc.), but no specific piezophilic lineage
was reported so far.
10.5.4 Adaptations to Elevated Hydrostatic
Pressure
First studies about adaptation to elevated pressures
concerned the fatty acid composition of cellular membranes
for the strain CNPT3. When this strain is grown under
elevated pressures, C 14:1 , C 16:0 , and C 14:0 fatty acids
decreased, but C 16:1 and C 18:1 fatty acids increased
(Simonato et al. 2006). More globally, the ratio of unsaturated fatty acids versus saturated fatty acids shifts from 1.9
to 3, when hydrostatic pressure increased from 0.1 to
68 MPa. A similar increase of unsaturated fatty acids was
also observed for Vibrio marinus, when temperature for
incubation decreased from 25 to 15
C. Similar experiments
for strains isolated from depths in a range 1,200 to 10,476 m
also showed an increase of concentration of unsaturated fatty
acids, particularly for those with carbon chains up to 22
carbons. Such synthesis of long-chain unsaturated fatty
acids, which are not very usual for prokaryotes, could contribute to maintain the fluidity of the cellular membrane.
In the case of the strain Photobacterium SS9, molecular
adaptations to hydrostatic pressures have been shown that
again took place in the outer cellular membrane. When this
strain originated from a depth of 2,500 m is cultivated under
hydrostatic pressure, several outer membrane proteins are
repressed while some novel proteins are expressed. Among
them the 37-KDa protein OmpH (outer membrane protein
high pressure) is a nonspecific porin, the transcription level
of which increases under hydrostatic pressure (Campanaro
et al. 2005). Other environmental factors, such as cellular
density or carbon availability, may also influence the expression of Omph gene: the same promotor can be activated
either by a high cellular density at 0.1 MPa or a low cellular
density at 27 MPa. Other adaptative mechanisms controlled
by genes organized in operons and regulated by hydrostatic
pressure may exist, but most of adaptations to hydrostatic
pressure reported for piezopsychrophiles concern the cellular membrane and transmembrane transports. The modification of membrane fatty acids or associated proteins aims at
maintaining functionality of exchanges between the cell and
its environment (cf. Sect. 9.6.4).
10.5.5 A Particular Case: Deep-Sea
Hydrothermal Vents
Deep-sea hydrothermal vents were discovered in 1977, at the
East Pacific Rise. They are mostly located along oceanic
ridges at depths from 800 to 4,000 m. Steep gradients exist
particularly for temperature. Within a few decimeters, temperature shifts from 2
C (deep-sea water) to 350
C or more
for the hydrothermal fluid (water remains liquid at such
elevated temperatures because of high pressure). These
very hot temperatures incited microbiologists to search for
hyperthermophilic microorganisms, and, therefore, many
novel species and genera, particularly in the Archaea
domain, have been described (cf. Sect. 10.3).
10.5.5.1 Responses to Hydrostatic Pressure
The majority of hyperthermophilic microorganisms isolated
from deep-sea hydrothermal vents were cultivated under a
pressure of 0.2 MPa, in order to avoid boiling of culture
media. However, despite their deep-sea origins, very few
were then exposed to elevated pressures to study their
responses to high pressure. However, some strains appeared
piezosensitive, other piezotolerant, and some others
piezophilic. For heterotrophic anaerobic piezophiles, an
increase of the growth rate and an increase of the maximum
temperature for growth (in average 1–4
C) were observed as
a response to increasing pressures. Piezophily was also
observed for hyperthermophilic methanogenic Archaea,
but for the more elevated temperatures, growth and methane
10 The Extreme Conditions of Life on the Planet and Exobiology
377
chemolithoautotrophs); none have been detected by the cultivation techniques used. Discovery of such organisms
would allow to claim that piezophily is a unique feature for
a majority of deep-sea microorganisms. But according to the
slow growth of deep-sea heterotrophic piezopsychrophiles,
one can expect weeks or even months for the growth of deepsea chemolithoautotrophs, which makes their studies very
difficult.
10.5.3 Taxonomy and Phylogeny of
Piezopsychrophiles
Most of piezopsychrophilic bacteria that have been isolated
were also studied for their taxonomy and phylogeny. All
belong to the gamma subdivision within the Proteobacteria.
No correlation was observed between the phylogeny of these
organisms and depth, location, or original habitat. While
many other extremophiles (halophiles, hyperthermophiles)
often belong to distinct phylogenetic lineages, piezopsychrophilic organisms studied up to now are distributed
within phylogenetic groups together with piezosensitive species. Adaptations to elevated hydrostatic pressures probably
appeared rather recently during a speciation process. The
recent utilization of molecular techniques indeed revealed
more diversity than the cultivation methods (alpha and delta
subdivisions within Proteobacteria, Actinobacteria,
Planctomycetales, etc.), but no specific piezophilic lineage
was reported so far.
10.5.4 Adaptations to Elevated Hydrostatic
Pressure
First studies about adaptation to elevated pressures
concerned the fatty acid composition of cellular membranes
for the strain CNPT3. When this strain is grown under
elevated pressures, C 14:1 , C 16:0 , and C 14:0 fatty acids
decreased, but C 16:1 and C 18:1 fatty acids increased
(Simonato et al. 2006). More globally, the ratio of unsaturated fatty acids versus saturated fatty acids shifts from 1.9
to 3, when hydrostatic pressure increased from 0.1 to
68 MPa. A similar increase of unsaturated fatty acids was
also observed for Vibrio marinus, when temperature for
incubation decreased from 25 to 15
C. Similar experiments
for strains isolated from depths in a range 1,200 to 10,476 m
also showed an increase of concentration of unsaturated fatty
acids, particularly for those with carbon chains up to 22
carbons. Such synthesis of long-chain unsaturated fatty
acids, which are not very usual for prokaryotes, could contribute to maintain the fluidity of the cellular membrane.
In the case of the strain Photobacterium SS9, molecular
adaptations to hydrostatic pressures have been shown that
again took place in the outer cellular membrane. When this
strain originated from a depth of 2,500 m is cultivated under
hydrostatic pressure, several outer membrane proteins are
repressed while some novel proteins are expressed. Among
them the 37-KDa protein OmpH (outer membrane protein
high pressure) is a nonspecific porin, the transcription level
of which increases under hydrostatic pressure (Campanaro
et al. 2005). Other environmental factors, such as cellular
density or carbon availability, may also influence the expression of Omph gene: the same promotor can be activated
either by a high cellular density at 0.1 MPa or a low cellular
density at 27 MPa. Other adaptative mechanisms controlled
by genes organized in operons and regulated by hydrostatic
pressure may exist, but most of adaptations to hydrostatic
pressure reported for piezopsychrophiles concern the cellular membrane and transmembrane transports. The modification of membrane fatty acids or associated proteins aims at
maintaining functionality of exchanges between the cell and
its environment (cf. Sect. 9.6.4).
10.5.5 A Particular Case: Deep-Sea
Hydrothermal Vents
Deep-sea hydrothermal vents were discovered in 1977, at the
East Pacific Rise. They are mostly located along oceanic
ridges at depths from 800 to 4,000 m. Steep gradients exist
particularly for temperature. Within a few decimeters, temperature shifts from 2
C (deep-sea water) to 350
C or more
for the hydrothermal fluid (water remains liquid at such
elevated temperatures because of high pressure). These
very hot temperatures incited microbiologists to search for
hyperthermophilic microorganisms, and, therefore, many
novel species and genera, particularly in the Archaea
domain, have been described (cf. Sect. 10.3).
10.5.5.1 Responses to Hydrostatic Pressure
The majority of hyperthermophilic microorganisms isolated
from deep-sea hydrothermal vents were cultivated under a
pressure of 0.2 MPa, in order to avoid boiling of culture
media. However, despite their deep-sea origins, very few
were then exposed to elevated pressures to study their
responses to high pressure. However, some strains appeared
piezosensitive, other piezotolerant, and some others
piezophilic. For heterotrophic anaerobic piezophiles, an
increase of the growth rate and an increase of the maximum
temperature for growth (in average 1–4
C) were observed as
a response to increasing pressures. Piezophily was also
observed for hyperthermophilic methanogenic Archaea,
but for the more elevated temperatures, growth and methane
10 The Extreme Conditions of Life on the Planet and Exobiology
377