despite existing unfavorable physicochemical conditions,
they allow growth of adapted microorganisms via autotrophic, heterotrophic, or the phototrophic ways both under
aerobic and anaerobic conditions.
10.4.3.1 Degradation of Organic Matter
in Hypersaline Environments
In most habitats, the macromolecules are decomposed by
hydrolytic and fermentative bacteria into simpler products
such as organic acids, fatty acids, alcohols, dihydrogen, and
carbon dioxide. The dihydrogen can then be used as an
electron donor for different types of prokaryotes including
homoacetogenic bacteria, sulfate-reducing bacteria, or
methanoarchaea leading to an optimized mineralization of
organic matter via acetate which can be completely
converted to methane or sulfide and carbon dioxide. Increase
in salinity leads to dihydrogen and various volatile fatty
acids accumulation in sediments. These results suggest that
oxidation of organic matter is incomplete in high saline
environments in contrast to other ecosystems such as marine
environments or digesters. Beyond salinity higher than
15 %, mineralization of organic compounds through
methanogenesis is limited because of the absence or low
activity of methanoarchaea regarding dihydrogen and acetate utilization. In contrast, hydrogenotrophic sulfatereducing activities still occur but decrease while increasing
salinity up to saturation. It is noteworthy that analysis of the
Great Salt Lake sediments demonstrated that up to 200 mM
dihydrogen accumulated in deep layers.
These hydrogenotrophic activities, reduced in case of
sulfate-reducing bacteria or absent in methanoarchaea, lead
to an accumulation of fermentation products that are ultimately preserved in sediments and may result at geological
scales in oil formation.
It was evidenced that substrates such as methylamines
were used only by methanoarchaea and not by sulfatereducing bacteria, known as methanogenic competitors for
dihydrogen in most of the habitats of the planet. This gas and
formate are preferably used by sulfate-reducing bacteria
when sulfate is not limiting in the environment, and this is
particularly the case in hypersaline environments.
Bacterial sulfate reduction is therefore an important process in the mineralization of organic matter in these salted
and anoxic environments with a peculiar emphasis with
regard to dihydrogen oxidation. However, beside
dihydrogen, the oxidation of volatile fatty acids such as
acetate and butyrate by sulfate-reducing bacteria has not
been established at salinity that exceeds 19 %. Consequently, there is an incomplete anaerobic oxidation of
organic matter in hypersaline environments. This oxidation
may lead to acetate accumulation in such environments,
reaching up to several hundred millimolars.
10.4.3.2 The Compatible Solutes
A difference of solute concentration on both sides of a cell
membrane causes a force called “osmotic pressure.” This
pressure has been estimated at about 0.1 bar in freshwater
and to more than 100 bars in hypersaline environments. In
order to maintain their viability, cells must always adjust the
intracytoplasmic osmotic pressure to that of its immediate
environment. For this purpose, halophilic microorganisms
have developed two strategies to prevent osmotic shocks:
1. The transport of ions across the membrane by means of
ion pumps (salt-in strategy).
2. The synthesis or accumulation of organic molecules.
Many halophilic or halotolerant prokaryotic species accumulate organic osmolytes by synthesizing or transporting
them from the surrounding environment (compatiblesolute strategy).
In both cases, these mechanisms are of a high energy cost
to the cell that usually prefers transport to biosynthesis when
osmolytes are already present in the environment. The
organic molecules involved are generally of low molecular
weight. They are polar, highly soluble, neutral at physiological pH, and unable to cross the cell membrane without the
intervention of specific transporters. They include amino
acids and derivatives, betaines, and other polyols which do
not interfere with the cell mechanisms. The term “compatible solutes” was introduced to define these protective
molecules (cf. Sect. 9.6.3, Fig. 10.9).
Accumulation of ions was evidenced in aerobic,
extremely
halophilic
Archaea
of
the
family
Halobacteriaceae but also in anaerobic bacteria. This is the
case of K
+ which replaces advantageously Na
+ in the cytoplasm as the latter is toxic for cells. Cl
À is the dominant
anion in the cytoplasm.
10.4.4 Moderate to Extremely Halophilic
Bacteria
Anaerobic halophilic microorganisms belong to the order of
Halanaerobiales (Ollivier et al. 1994) which includes two
families Halanaerobiaceae and Halobacteroidaceae.
All species of these families ferment carbohydrates with
the exception of a homoacetogenic bacterium
(Acetohalobium arabaticum) that reduces CO 2 by producing
acetate and grows on betaine and trimethylamine. More
recently, Fuchsiella alkaliacetigena, the first alkaliphilic
anaerobic hydrogenotrophic homoacetogenic bacterium,
was isolated from sediments of the soda-depositing soda
lake in Russia. Halocella cellulolytica is the only species
capable of cellulolysis among all described species. Species
belonging to the genera Orenia and Sporohalobacter are the
only ones able to sporulate. This phenotypic feature
differentiates them from all other species.
10 The Extreme Conditions of Life on the Planet and Exobiology
373
they allow growth of adapted microorganisms via autotrophic, heterotrophic, or the phototrophic ways both under
aerobic and anaerobic conditions.
10.4.3.1 Degradation of Organic Matter
in Hypersaline Environments
In most habitats, the macromolecules are decomposed by
hydrolytic and fermentative bacteria into simpler products
such as organic acids, fatty acids, alcohols, dihydrogen, and
carbon dioxide. The dihydrogen can then be used as an
electron donor for different types of prokaryotes including
homoacetogenic bacteria, sulfate-reducing bacteria, or
methanoarchaea leading to an optimized mineralization of
organic matter via acetate which can be completely
converted to methane or sulfide and carbon dioxide. Increase
in salinity leads to dihydrogen and various volatile fatty
acids accumulation in sediments. These results suggest that
oxidation of organic matter is incomplete in high saline
environments in contrast to other ecosystems such as marine
environments or digesters. Beyond salinity higher than
15 %, mineralization of organic compounds through
methanogenesis is limited because of the absence or low
activity of methanoarchaea regarding dihydrogen and acetate utilization. In contrast, hydrogenotrophic sulfatereducing activities still occur but decrease while increasing
salinity up to saturation. It is noteworthy that analysis of the
Great Salt Lake sediments demonstrated that up to 200 mM
dihydrogen accumulated in deep layers.
These hydrogenotrophic activities, reduced in case of
sulfate-reducing bacteria or absent in methanoarchaea, lead
to an accumulation of fermentation products that are ultimately preserved in sediments and may result at geological
scales in oil formation.
It was evidenced that substrates such as methylamines
were used only by methanoarchaea and not by sulfatereducing bacteria, known as methanogenic competitors for
dihydrogen in most of the habitats of the planet. This gas and
formate are preferably used by sulfate-reducing bacteria
when sulfate is not limiting in the environment, and this is
particularly the case in hypersaline environments.
Bacterial sulfate reduction is therefore an important process in the mineralization of organic matter in these salted
and anoxic environments with a peculiar emphasis with
regard to dihydrogen oxidation. However, beside
dihydrogen, the oxidation of volatile fatty acids such as
acetate and butyrate by sulfate-reducing bacteria has not
been established at salinity that exceeds 19 %. Consequently, there is an incomplete anaerobic oxidation of
organic matter in hypersaline environments. This oxidation
may lead to acetate accumulation in such environments,
reaching up to several hundred millimolars.
10.4.3.2 The Compatible Solutes
A difference of solute concentration on both sides of a cell
membrane causes a force called “osmotic pressure.” This
pressure has been estimated at about 0.1 bar in freshwater
and to more than 100 bars in hypersaline environments. In
order to maintain their viability, cells must always adjust the
intracytoplasmic osmotic pressure to that of its immediate
environment. For this purpose, halophilic microorganisms
have developed two strategies to prevent osmotic shocks:
1. The transport of ions across the membrane by means of
ion pumps (salt-in strategy).
2. The synthesis or accumulation of organic molecules.
Many halophilic or halotolerant prokaryotic species accumulate organic osmolytes by synthesizing or transporting
them from the surrounding environment (compatiblesolute strategy).
In both cases, these mechanisms are of a high energy cost
to the cell that usually prefers transport to biosynthesis when
osmolytes are already present in the environment. The
organic molecules involved are generally of low molecular
weight. They are polar, highly soluble, neutral at physiological pH, and unable to cross the cell membrane without the
intervention of specific transporters. They include amino
acids and derivatives, betaines, and other polyols which do
not interfere with the cell mechanisms. The term “compatible solutes” was introduced to define these protective
molecules (cf. Sect. 9.6.3, Fig. 10.9).
Accumulation of ions was evidenced in aerobic,
extremely
halophilic
Archaea
of
the
family
Halobacteriaceae but also in anaerobic bacteria. This is the
case of K
+ which replaces advantageously Na
+ in the cytoplasm as the latter is toxic for cells. Cl
À is the dominant
anion in the cytoplasm.
10.4.4 Moderate to Extremely Halophilic
Bacteria
Anaerobic halophilic microorganisms belong to the order of
Halanaerobiales (Ollivier et al. 1994) which includes two
families Halanaerobiaceae and Halobacteroidaceae.
All species of these families ferment carbohydrates with
the exception of a homoacetogenic bacterium
(Acetohalobium arabaticum) that reduces CO 2 by producing
acetate and grows on betaine and trimethylamine. More
recently, Fuchsiella alkaliacetigena, the first alkaliphilic
anaerobic hydrogenotrophic homoacetogenic bacterium,
was isolated from sediments of the soda-depositing soda
lake in Russia. Halocella cellulolytica is the only species
capable of cellulolysis among all described species. Species
belonging to the genera Orenia and Sporohalobacter are the
only ones able to sporulate. This phenotypic feature
differentiates them from all other species.
10 The Extreme Conditions of Life on the Planet and Exobiology
373