10.4.2.2 The Physical Chemistry of Lakes
The Great Salt Lake, located in Utah, USA, is an example of
hypersaline environment at neutral pH corresponding in
ionic composition to that of concentrated seawater. Sodium
is the dominant cation, while chlorine is the dominant anion.
Significant concentrations of sulfate are also present. In
contrast, in the Dead Sea, another hypersaline basin located
between Israel and Jordan, having a neutral pH, the sodium
concentration is relatively low, while that of magnesium is
high. Both ecosystems have salt concentrations that can
reach saturation. Regarding the water chemistry of alkaline
lakes, it is similar to that of Great Salt Lake. However,
because of the presence of large amounts of carbonates in
the surrounding rocks, pH is relatively high. Waters with pH
ranging from 10 to 12 are common in these extreme
environments. Ca
2+ and Mg
2+ ions are absent from these
lakes as they precipitate at alkaline pH under high carbonate
concentrations. Several lakes in the Great Basin of the western United States, whose salinity varies from 9 to 10 %, are
alkaline with pH values measured between 9 and 10.
When seawater salts are concentrated by evaporation,
carbonate precipitates into calcium carbonate when salinity
reaches 6 %. Thereafter, sulfate precipitates forming gypsum
deposits (calcium sulfate) when salt concentration exceeds
10 %. Beyond 25 %, sodium chloride starts to precipitate as
halite and fully precipitates at 34 % corresponding approximately to ten times the salt concentration in seawater.
Waters are subsequently enriched in magnesium and potassium which precipitate at salinities 20 times higher than that
of seawater.
Evaporated seawater is at the origin of salt deposit
formations which with time lead to the formation of
evaporites. The latter are considered as fossil hypersaline
environments which are present on all continents.
Athalassohaline environments may result from their dissolution by seawater. These extreme saline environments
become appropriate sites for a progressive adaptation of
microorganisms to high salt conditions.
The temperature of the inland lakes can widely vary
throughout the year. It can range from 30
C in winter to
over 48
C in summer in the region of the Great Salt Lake. A
large majority of the halophilic microflora inhabiting this
lake is heterotrophic, aerobic, mesophilic, or thermotolerant
with an optimum growth temperature between 40 and 55
C.
Thermotolerance was observed in particular among the halophilic phototrophic anaerobic bacteria belonging to the
genus Ectothiorhodospira. Their optimal growth temperature is between 45 and 50
C. In contrast, some halophilic
bacteria isolated from hypersaline lakes in Antarctica are
psychrophilic, but this is an exception. Within the domain
Bacteria, thermotolerance of microorganisms may result
from the presence in their cytoplasm of solutes such as
betaine that protects enzymes from heat. Although being
mesophilic, halophilic eukaryotic organisms of the genus
Dunaliella withstand high temperatures. This resistance to
high temperature is linked to the presence of glycerol, which
allows intracellular enzyme stabilization.
The hypersaline systems are anoxic environments below
the surface of the water, because of high temperatures and
salinities that limit dioxygen dissolution. This creates favorable conditions for the development of anaerobes, although
aerobes are widely present. Some microorganisms pertaining
to the domain Archaea possess gas vacuoles enabling them to
float, thus staying in direct contact with dioxygen.
Finally, the different chemical composition of hypersaline habitats selected a large variety of halophilic
microorganisms. Some are endemic while others are found
in various habitats around the world. Despite the extreme
conditions existing in hypersaline environments, primary
production is important notably via autotrophy. Organic
matter resulting from this primary production by aerobic or
anaerobic phototrophic organisms allows the development
of alternative heterotrophic extremophilic microbial
populations.
10.4.3 Origin of Organic Matter
Hypersaline ecosystems generally have limited life forms.
Indeed, the largest salt concentration tolerated by a vertebrate organism is 10 % (Tilapia spp.). Above this limit
within Eukarya, only crustaceans (Artemia salina) and
algae (Dunaliella salina) were identified. At higher
concentrations of salt, aerobic and anaerobic heterotrophic
prokaryotes, together with anaerobic phototrophic bacteria
and methanoarchaea, are known to inhabit these
environments. Plants and algae that develop along the
banks of hypersaline lakes can be a significant source of
organic matter after immersion of shoreline by water.
Invertebrates and microorganisms within the domains
Archaea or Bacteria deliver a significant source of organic
matter in these environments. They include sugars, lipids,
proteins, and even more complex carbon and energy sources
such as chitin, which are important constituents of
crustaceans. A significant proportion of mineral and organic
matter can be brought by birds when transiting by these
areas.
Intracellular compounds of low molecular weight
allowing most of these microorganisms to face osmotic
pressure are preferential substrates for halophilic/
hyperhalophilic prokaryotes.
Methylated compounds resulting from the degradation
of glycine betaine in particular are used as energy sources
by methanoarchaea (cf. Sect. 14.2.7).
It therefore appears that a broad range of potential
substrates is available in hypersaline ecosystems, and
372
J.-L. Cayol et al.
The Great Salt Lake, located in Utah, USA, is an example of
hypersaline environment at neutral pH corresponding in
ionic composition to that of concentrated seawater. Sodium
is the dominant cation, while chlorine is the dominant anion.
Significant concentrations of sulfate are also present. In
contrast, in the Dead Sea, another hypersaline basin located
between Israel and Jordan, having a neutral pH, the sodium
concentration is relatively low, while that of magnesium is
high. Both ecosystems have salt concentrations that can
reach saturation. Regarding the water chemistry of alkaline
lakes, it is similar to that of Great Salt Lake. However,
because of the presence of large amounts of carbonates in
the surrounding rocks, pH is relatively high. Waters with pH
ranging from 10 to 12 are common in these extreme
environments. Ca
2+ and Mg
2+ ions are absent from these
lakes as they precipitate at alkaline pH under high carbonate
concentrations. Several lakes in the Great Basin of the western United States, whose salinity varies from 9 to 10 %, are
alkaline with pH values measured between 9 and 10.
When seawater salts are concentrated by evaporation,
carbonate precipitates into calcium carbonate when salinity
reaches 6 %. Thereafter, sulfate precipitates forming gypsum
deposits (calcium sulfate) when salt concentration exceeds
10 %. Beyond 25 %, sodium chloride starts to precipitate as
halite and fully precipitates at 34 % corresponding approximately to ten times the salt concentration in seawater.
Waters are subsequently enriched in magnesium and potassium which precipitate at salinities 20 times higher than that
of seawater.
Evaporated seawater is at the origin of salt deposit
formations which with time lead to the formation of
evaporites. The latter are considered as fossil hypersaline
environments which are present on all continents.
Athalassohaline environments may result from their dissolution by seawater. These extreme saline environments
become appropriate sites for a progressive adaptation of
microorganisms to high salt conditions.
The temperature of the inland lakes can widely vary
throughout the year. It can range from 30
C in winter to
over 48
C in summer in the region of the Great Salt Lake. A
large majority of the halophilic microflora inhabiting this
lake is heterotrophic, aerobic, mesophilic, or thermotolerant
with an optimum growth temperature between 40 and 55
C.
Thermotolerance was observed in particular among the halophilic phototrophic anaerobic bacteria belonging to the
genus Ectothiorhodospira. Their optimal growth temperature is between 45 and 50
C. In contrast, some halophilic
bacteria isolated from hypersaline lakes in Antarctica are
psychrophilic, but this is an exception. Within the domain
Bacteria, thermotolerance of microorganisms may result
from the presence in their cytoplasm of solutes such as
betaine that protects enzymes from heat. Although being
mesophilic, halophilic eukaryotic organisms of the genus
Dunaliella withstand high temperatures. This resistance to
high temperature is linked to the presence of glycerol, which
allows intracellular enzyme stabilization.
The hypersaline systems are anoxic environments below
the surface of the water, because of high temperatures and
salinities that limit dioxygen dissolution. This creates favorable conditions for the development of anaerobes, although
aerobes are widely present. Some microorganisms pertaining
to the domain Archaea possess gas vacuoles enabling them to
float, thus staying in direct contact with dioxygen.
Finally, the different chemical composition of hypersaline habitats selected a large variety of halophilic
microorganisms. Some are endemic while others are found
in various habitats around the world. Despite the extreme
conditions existing in hypersaline environments, primary
production is important notably via autotrophy. Organic
matter resulting from this primary production by aerobic or
anaerobic phototrophic organisms allows the development
of alternative heterotrophic extremophilic microbial
populations.
10.4.3 Origin of Organic Matter
Hypersaline ecosystems generally have limited life forms.
Indeed, the largest salt concentration tolerated by a vertebrate organism is 10 % (Tilapia spp.). Above this limit
within Eukarya, only crustaceans (Artemia salina) and
algae (Dunaliella salina) were identified. At higher
concentrations of salt, aerobic and anaerobic heterotrophic
prokaryotes, together with anaerobic phototrophic bacteria
and methanoarchaea, are known to inhabit these
environments. Plants and algae that develop along the
banks of hypersaline lakes can be a significant source of
organic matter after immersion of shoreline by water.
Invertebrates and microorganisms within the domains
Archaea or Bacteria deliver a significant source of organic
matter in these environments. They include sugars, lipids,
proteins, and even more complex carbon and energy sources
such as chitin, which are important constituents of
crustaceans. A significant proportion of mineral and organic
matter can be brought by birds when transiting by these
areas.
Intracellular compounds of low molecular weight
allowing most of these microorganisms to face osmotic
pressure are preferential substrates for halophilic/
hyperhalophilic prokaryotes.
Methylated compounds resulting from the degradation
of glycine betaine in particular are used as energy sources
by methanoarchaea (cf. Sect. 14.2.7).
It therefore appears that a broad range of potential
substrates is available in hypersaline ecosystems, and
372
J.-L. Cayol et al.