Therefore, their mineral salts will be in similar
proportions and percentages to those contained in the
sea water, at least until the precipitation thresholds are
not reached. Hypersaline waters obtained by partial evaporation of sea water are called “thalassohaline,” while
those obtained by the dissolution of salty fossil deposits
are called “athalassohaline.” These latter may have different proportions of salts well different from those of
seawater according to the nature of the deposits. Some
contain a high percentage of sodium carbonate and are
very alkaline (alkaline lakes).
Hypersaline environments have been formed over a
long period in the history of the Earth. Some authors have
shown that the salinity of seawater has been constant for
at least 200 million years, due to a subtle balance between
the dissolution of igneous rocks, the contribution of
elements in the Earth’s crust by oceanic ridges, and
chemical reactions in water.
When seawater is concentrated by evaporation, all
salts present an increase of their concentrations in the
same proportions until reaching their respective precipitation thresholds. Carbonates precipitate as calcium carbonate when the salinity reaches 6 %. Then, sulfate
precipitate and form deposits of gypsum (calcium sulfate)
when the salinity exceeds 10 %. Above 25 % sodium
chloride begins to precipitate and form halite precipitates
up to 35 % where it is fully precipitated (10 times the
concentration of sea water). The waters are then enriched
in magnesium and potassium salts which precipitate at
salinities 20 times superior to those of the sea water.
Completely evaporated sea water is the origin of deposits of
salt formations which constitute through time evaporites.
These last are considered as hypersaline fossil environments
and are present on all continents. Their dissolution in water
can be at the origin of athalassohaline environments. All
these hypersaline environments formed during the evolution of the Earth represent reservoirs for an evolution over a
long-term of halophilic life.
• The halophilic bacteria and archaea
Regarding extreme halophilic prokaryotes belonging to
the domain Bacteria, most were isolated from anoxic
hypersaline environments. They are included in two
families: Haloanaerobiaceae (fermentative bacteria)
and
Ectothiorhodospiraceae
with
the
genus
Halorhodospira (phototrophic purple bacteria). These
bacteria are isolated from environments with salinities
of 20–25 %. Some are halophilic and alkaliphilic, and
are isolated from alkaline lakes with pH superior to pH 9.
Extreme halophilic archaea (also called halobacteria or
haloarchaea) are found in hypersaline habitats such as salt
marshes, salt lakes and the Dead Sea, soda lakes, and
salty foods (fish, meat).
These microorganisms require NaCl concentrations of
at least 1.5 M (9 %) to develop. They live well in
environments with NaCl concentrations between 15 and
30 %. Some are able to multiply until the saturation limit
of NaCl concentration (35 %). Extreme halophilic
archaea isolated from hypersaline thalassohaline environments are neutrophiles (Halobacterium); those isolated
from athalassohaline environments can be, depending
on the environment, neutrophilic or alkalophilic (Natronobacter, Natronococcus, etc.).
In halophilic bacteria, some “marine” genes that encode
the active transport and biosynthesis of betaines and
the active transport of potassium have been identified.
Osmoregulation processes are controlled by the osmotic
pressure that acts as a signal for the synthesis of transport
systems and porins via the system omp. The accumulation
of potassium is regulated by the kdp operon; accumulation
of proline as precursor of glycine betaines in halophilic
chemoorganotrophic bacteria is controlled by proU.
9.6.3.2 Responses to pH
Prokaryotes are present in environments with pH extremes,
either very acidic (pH 1) or very alkaline (pH 11), but
the majority of them live a pH around neutrality even
moderately acidic or alkaline, within a range from pH 5 to
pH 8. Among bacteria and archaea, it is possible to distinguish acidophiles*, neutrophiles*, and alkaliphiles* by
their ability to grow at pH acidic, neutral, or alkaline.
For each species, we can define a minimum growth pH, an
optimum pH, and a maximum pH beyond which growth is
no longer possible. Acidophilic prokaryotes can be divided
into moderately or strictly acidophilic as the optimum pH is
below pH 5.5, and extremely acidophiles with pH lower than
pH 3. In general, pH for neutrophilic prokaryotes ranges
from pH 5.5 to pH 8.5, with an optimum between pH 6.8
and 7.2. Alkaliphilic prokaryotes can be either tolerant
alkaliphilic (pH optimum at pH 7 and trends has higher
values), moderately alkaliphilic when their pH range for
growth is above pH 7 and below 10, and extreme
alkalophilic when growth pH is between pH 9 and pH 12
or more (Fig. 9.26).
When an organism develops, it modifies rapidly the pH of
its environment through its metabolism by absorbing or
excreting substrates or products more or less acidic or alkaline: excretions of protons (H
+
), excretion of acidic or alkaline products, absorption of CO 2 , and uses of acidic or
alkaline substrates. It will therefore adapt to variations in
pH, but these must remain within the range of tolerable pH
so that it can continue to develop.
On our planet, there are many acidic environments such
as some hot springs, volcanic soils, some lakes, hydrothermal vents at the bottom of oceans, drainage of mining
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
333
proportions and percentages to those contained in the
sea water, at least until the precipitation thresholds are
not reached. Hypersaline waters obtained by partial evaporation of sea water are called “thalassohaline,” while
those obtained by the dissolution of salty fossil deposits
are called “athalassohaline.” These latter may have different proportions of salts well different from those of
seawater according to the nature of the deposits. Some
contain a high percentage of sodium carbonate and are
very alkaline (alkaline lakes).
Hypersaline environments have been formed over a
long period in the history of the Earth. Some authors have
shown that the salinity of seawater has been constant for
at least 200 million years, due to a subtle balance between
the dissolution of igneous rocks, the contribution of
elements in the Earth’s crust by oceanic ridges, and
chemical reactions in water.
When seawater is concentrated by evaporation, all
salts present an increase of their concentrations in the
same proportions until reaching their respective precipitation thresholds. Carbonates precipitate as calcium carbonate when the salinity reaches 6 %. Then, sulfate
precipitate and form deposits of gypsum (calcium sulfate)
when the salinity exceeds 10 %. Above 25 % sodium
chloride begins to precipitate and form halite precipitates
up to 35 % where it is fully precipitated (10 times the
concentration of sea water). The waters are then enriched
in magnesium and potassium salts which precipitate at
salinities 20 times superior to those of the sea water.
Completely evaporated sea water is the origin of deposits of
salt formations which constitute through time evaporites.
These last are considered as hypersaline fossil environments
and are present on all continents. Their dissolution in water
can be at the origin of athalassohaline environments. All
these hypersaline environments formed during the evolution of the Earth represent reservoirs for an evolution over a
long-term of halophilic life.
• The halophilic bacteria and archaea
Regarding extreme halophilic prokaryotes belonging to
the domain Bacteria, most were isolated from anoxic
hypersaline environments. They are included in two
families: Haloanaerobiaceae (fermentative bacteria)
and
Ectothiorhodospiraceae
with
the
genus
Halorhodospira (phototrophic purple bacteria). These
bacteria are isolated from environments with salinities
of 20–25 %. Some are halophilic and alkaliphilic, and
are isolated from alkaline lakes with pH superior to pH 9.
Extreme halophilic archaea (also called halobacteria or
haloarchaea) are found in hypersaline habitats such as salt
marshes, salt lakes and the Dead Sea, soda lakes, and
salty foods (fish, meat).
These microorganisms require NaCl concentrations of
at least 1.5 M (9 %) to develop. They live well in
environments with NaCl concentrations between 15 and
30 %. Some are able to multiply until the saturation limit
of NaCl concentration (35 %). Extreme halophilic
archaea isolated from hypersaline thalassohaline environments are neutrophiles (Halobacterium); those isolated
from athalassohaline environments can be, depending
on the environment, neutrophilic or alkalophilic (Natronobacter, Natronococcus, etc.).
In halophilic bacteria, some “marine” genes that encode
the active transport and biosynthesis of betaines and
the active transport of potassium have been identified.
Osmoregulation processes are controlled by the osmotic
pressure that acts as a signal for the synthesis of transport
systems and porins via the system omp. The accumulation
of potassium is regulated by the kdp operon; accumulation
of proline as precursor of glycine betaines in halophilic
chemoorganotrophic bacteria is controlled by proU.
9.6.3.2 Responses to pH
Prokaryotes are present in environments with pH extremes,
either very acidic (pH 1) or very alkaline (pH 11), but
the majority of them live a pH around neutrality even
moderately acidic or alkaline, within a range from pH 5 to
pH 8. Among bacteria and archaea, it is possible to distinguish acidophiles*, neutrophiles*, and alkaliphiles* by
their ability to grow at pH acidic, neutral, or alkaline.
For each species, we can define a minimum growth pH, an
optimum pH, and a maximum pH beyond which growth is
no longer possible. Acidophilic prokaryotes can be divided
into moderately or strictly acidophilic as the optimum pH is
below pH 5.5, and extremely acidophiles with pH lower than
pH 3. In general, pH for neutrophilic prokaryotes ranges
from pH 5.5 to pH 8.5, with an optimum between pH 6.8
and 7.2. Alkaliphilic prokaryotes can be either tolerant
alkaliphilic (pH optimum at pH 7 and trends has higher
values), moderately alkaliphilic when their pH range for
growth is above pH 7 and below 10, and extreme
alkalophilic when growth pH is between pH 9 and pH 12
or more (Fig. 9.26).
When an organism develops, it modifies rapidly the pH of
its environment through its metabolism by absorbing or
excreting substrates or products more or less acidic or alkaline: excretions of protons (H
+
), excretion of acidic or alkaline products, absorption of CO 2 , and uses of acidic or
alkaline substrates. It will therefore adapt to variations in
pH, but these must remain within the range of tolerable pH
so that it can continue to develop.
On our planet, there are many acidic environments such
as some hot springs, volcanic soils, some lakes, hydrothermal vents at the bottom of oceans, drainage of mining
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
333