increase of 1,000 hPa or 1 atm for every 10 m depth)
(cf. Sect. 10.5).
9.6.4.1 Piezo-Tolerant and Piezophilic
Prokaryotes
The majority of piezo-tolerant and piezophilic or barophilic
microorganisms were isolated from oceanic depths. They are
confronted with three unusual parameters: low temperatures
(psychrophilic microorganisms), high pressures (piezophilic
or barophilic microorganisms), and low nutrients (oligotrophic microorganisms). In the marine environment such
as in lakes, pressure increases by approximately 1,000 hPa
or 1 atm every 10 m depth. The majority of piezo-tolerant
or piezophilic prokaryotes isolated from environments
between 4,000 and 5,000 m deep have to withstand pressures
of 400–500 atm. Piezo-tolerant prokaryotes are those that
are able to live at normal atmospheric pressure and can
tolerate high pressures ranging up to 500 atm for some.
They predominate in deep environments down to 5,000 m
depth. Below, in environments between 5,000 and 6,000 m
depth, moderately piezophilic prokaryotes are found mainly
those with an optimum growth occurring at pressure of
around 400 atm. In deeper environments (10,000 m or
more), extreme piezophiles are found that require high
pressures for growth of up to 800 or 1,000 atm, such as the
bacterium Moritella that may develop at pressures greater
than 400 atm with an optimum at 700–800 atm. These
extreme piezophilic bacteria are capable of supporting
decompression and resist for short periods at normal atmospheric pressure, but it is necessary to reapply pressure
quickly to maintain their cellular integrity, which is
destroyed during prolonged exposure to 1 atm.
The majority of piezophiles are psychrophilic and live
optimally at temperatures of 2–4
C.
However, around deep hydrothermal vents, thermophilic
and piezophilic bacteria and archaea have been isolated.
These microorganisms in most cases use chemoorganotrophic aerobic or anaerobic respiration (nitrate, iron, or
thiosulfate), or are capable of fermentation.
9.6.4.2 Adaptation of Microorganisms to Pressure
Enzymes of piezophilic bacteria have modified their folding
in order to bind to their substrates. Similarly enzymes
associated with membrane transport are actively synthesized
when pressure is higher. Microorganisms grown under
pressure accumulate unsaturated fatty acids in their membrane; this adaptive response has the effect of allowing a
better fluidity of the membrane and an increased transport of
substances at high pressures. In piezophiles, growth at high
pressure is possible through the synthesis of a specific protein of the outer membrane OmpH (“outer membrane protein
H”), which is part of the family of porins. This specific
protein that forms channels allows diffusion of small
molecules necessary to cell metabolism through the outer
membrane. It replaces the usual porins that are not functional
at high pressure. Therefore, when the cell is subjected to
high pressure (300 atm), this new protein is synthesized and
allows the transport of substances efficiently.
9.6.5 Adaptation to Desiccation (cf. Box 9.2)
Liquid water is the main factor affecting life. Thus, life is
possible at high temperature if liquid water is still present,
e.g., hydrothermal vents at more than 100
C, or at lower
temperature. Thus, scientists working in the field of exobiology have tried to detect the presence of water on other
planets, Mars in particular; this would be considered as a
strong indication for the possibility of life (cf. Sect. 10.8).
The lack of water in general occurs in a gradual way, the
first consequence for prokaryotic cells being an increase in
osmotic pressure. To cope, the cells have implemented
response systems via mechanisms of homeostasis of osmotic
pressure discussed above, followed by the appearance of
reactive species of oxygen causing oxidative stress also
discussed above.
Finally, in the situation of extreme lack of water,
microorganisms have adapted mainly through the synthesis
of structures such as dormancy spores or cysts, a phenomenon triggered by a nutritional deficiency. The spores are cells
with thickened walls that contain little free water and in
which respiratory activity is low (Actinobacteria) or absent
(Firmicutes). When they return to open water, the spores
germinate and transform into “vegetative” cells which
metabolism is back to the level anterior to the transformation
in spore. Spores can be classified as “endospores” in
Firmicutes when they are formed inside a mother cell or as
“exospores” formed by septation at one end of a sporophore
as in Actinobacteria. In cyanobacteria and many taxa of
Proteobacteria, there are forms of survival called cysts,
which are cells with thickened walls, tolerant to desiccation.
This term should not be confused with the term heterocyst
that refers to non-photosynthetic cells of cyanobacteria
(heterocysts) specialized in nitrogen fixation that also have
a thickened wall to prevent the diffusion of oxygen. The
term cyst refers to a sack containing cells that are generally
dormant, with a less intimate contact between the wall of the
cyst and the cells as in the case of spores; nevertheless the
two terms are considered by some as synonymous mainly
differentiated by habit. In the case of eukaryotes, the distinction between the two terms is clearer with the term spore
linked to a sexual differentiation while cysts are dormant
vegetative structures.
336
P. Normand et al.
(cf. Sect. 10.5).
9.6.4.1 Piezo-Tolerant and Piezophilic
Prokaryotes
The majority of piezo-tolerant and piezophilic or barophilic
microorganisms were isolated from oceanic depths. They are
confronted with three unusual parameters: low temperatures
(psychrophilic microorganisms), high pressures (piezophilic
or barophilic microorganisms), and low nutrients (oligotrophic microorganisms). In the marine environment such
as in lakes, pressure increases by approximately 1,000 hPa
or 1 atm every 10 m depth. The majority of piezo-tolerant
or piezophilic prokaryotes isolated from environments
between 4,000 and 5,000 m deep have to withstand pressures
of 400–500 atm. Piezo-tolerant prokaryotes are those that
are able to live at normal atmospheric pressure and can
tolerate high pressures ranging up to 500 atm for some.
They predominate in deep environments down to 5,000 m
depth. Below, in environments between 5,000 and 6,000 m
depth, moderately piezophilic prokaryotes are found mainly
those with an optimum growth occurring at pressure of
around 400 atm. In deeper environments (10,000 m or
more), extreme piezophiles are found that require high
pressures for growth of up to 800 or 1,000 atm, such as the
bacterium Moritella that may develop at pressures greater
than 400 atm with an optimum at 700–800 atm. These
extreme piezophilic bacteria are capable of supporting
decompression and resist for short periods at normal atmospheric pressure, but it is necessary to reapply pressure
quickly to maintain their cellular integrity, which is
destroyed during prolonged exposure to 1 atm.
The majority of piezophiles are psychrophilic and live
optimally at temperatures of 2–4
C.
However, around deep hydrothermal vents, thermophilic
and piezophilic bacteria and archaea have been isolated.
These microorganisms in most cases use chemoorganotrophic aerobic or anaerobic respiration (nitrate, iron, or
thiosulfate), or are capable of fermentation.
9.6.4.2 Adaptation of Microorganisms to Pressure
Enzymes of piezophilic bacteria have modified their folding
in order to bind to their substrates. Similarly enzymes
associated with membrane transport are actively synthesized
when pressure is higher. Microorganisms grown under
pressure accumulate unsaturated fatty acids in their membrane; this adaptive response has the effect of allowing a
better fluidity of the membrane and an increased transport of
substances at high pressures. In piezophiles, growth at high
pressure is possible through the synthesis of a specific protein of the outer membrane OmpH (“outer membrane protein
H”), which is part of the family of porins. This specific
protein that forms channels allows diffusion of small
molecules necessary to cell metabolism through the outer
membrane. It replaces the usual porins that are not functional
at high pressure. Therefore, when the cell is subjected to
high pressure (300 atm), this new protein is synthesized and
allows the transport of substances efficiently.
9.6.5 Adaptation to Desiccation (cf. Box 9.2)
Liquid water is the main factor affecting life. Thus, life is
possible at high temperature if liquid water is still present,
e.g., hydrothermal vents at more than 100
C, or at lower
temperature. Thus, scientists working in the field of exobiology have tried to detect the presence of water on other
planets, Mars in particular; this would be considered as a
strong indication for the possibility of life (cf. Sect. 10.8).
The lack of water in general occurs in a gradual way, the
first consequence for prokaryotic cells being an increase in
osmotic pressure. To cope, the cells have implemented
response systems via mechanisms of homeostasis of osmotic
pressure discussed above, followed by the appearance of
reactive species of oxygen causing oxidative stress also
discussed above.
Finally, in the situation of extreme lack of water,
microorganisms have adapted mainly through the synthesis
of structures such as dormancy spores or cysts, a phenomenon triggered by a nutritional deficiency. The spores are cells
with thickened walls that contain little free water and in
which respiratory activity is low (Actinobacteria) or absent
(Firmicutes). When they return to open water, the spores
germinate and transform into “vegetative” cells which
metabolism is back to the level anterior to the transformation
in spore. Spores can be classified as “endospores” in
Firmicutes when they are formed inside a mother cell or as
“exospores” formed by septation at one end of a sporophore
as in Actinobacteria. In cyanobacteria and many taxa of
Proteobacteria, there are forms of survival called cysts,
which are cells with thickened walls, tolerant to desiccation.
This term should not be confused with the term heterocyst
that refers to non-photosynthetic cells of cyanobacteria
(heterocysts) specialized in nitrogen fixation that also have
a thickened wall to prevent the diffusion of oxygen. The
term cyst refers to a sack containing cells that are generally
dormant, with a less intimate contact between the wall of the
cyst and the cells as in the case of spores; nevertheless the
two terms are considered by some as synonymous mainly
differentiated by habit. In the case of eukaryotes, the distinction between the two terms is clearer with the term spore
linked to a sexual differentiation while cysts are dormant
vegetative structures.
336
P. Normand et al.