sulfide (reduction of the electron acceptor) and sulfate (oxidation of the electron donor) (Fig. 14.36). Thus, sulfatereducers exhibit a great number of metabolic potentials
allowing them to thrive in highly diverse environmental
situations, and play a major role in the transformation of
organic and inorganic compounds.
The phylogenetic dispersion of sulfate-reducers makes
difficult the study of this community based exclusively on
the 16S rRNA gene (cf. Sect. 6.6.2), which led to the investigation of novel molecular markers among specific sulfatereducing enzymes. Among these enzymes, adenosine
phosphosulfate reductase (APS reductase) and sulfite reductase catalyze the reduction of APS to AMP and sulfite, and
the dissimilatory reduction of sulfite to sulfide, respectively
(cf. Sect. 3.3.2). The genes encoding these enzymes, including dsrAB genes, are becoming increasingly used for the
study of the communities of sulfate-reducers in natural
environments (Wagner et al. 2005).
14.4.2.3 Other Sulfide-Producing Metabolisms
Micro-organisms which are unable to use sulfate as an acceptor
of electrons participate in the production of reduced sulfur
compounds by dissimilatory reduction of elemental sulfur.
Sulfur reducers constitute a physiologically disparate group of
microorganisms, comprising both Bacteria (genera Desulforomonas, Desulfurella) and Archaea (Crenarchaeota and
Euryarchaeota). Some of these microorganisms completely
oxidize simple organic substrates (acetate, lactate, pyruvate,
succinate, ethanol). Those belonging to the genus Desulfurella
are strict sulfur reducers, while in the genus Desulforomonas,
fumarate or malate may be used as electron acceptors by certain
strains (Widdel and Pfennig 1992). Some fermentative sulfur
reducers are not involved in respiration but rather use sulfur as
an electron trap to increase energy production and synthesize
additional ATP through substrate phosphorylation.
Sulfur reducers are abundant in inland and oceanic geothermal environments, rich in elemental sulfur. Indeed, many
sulfo-reducing species are extreme (especially among
Archaea) or moderate (bacteria) thermophiles, although
mesophilic or even psychrophilic sulfo-reducing species
exist. They are also abundant in marine or brackish waters
where sulfides produced by sulfate-reducers are chemically or
biologically oxidized, contributing to the formation of elemental sulfur used by sulfo-reducing bacteria (Fig. 14.36). These
bacteria can live in mutualistic association with phototrophic
green bacteria, based on the oxidative reduction of sulfur.
Finally, some microorganisms, such as phototrophic purple sulfur bacteria, chemolithotrophic Beggiatoa and certain
cyanobacteria, have the possibility of exploiting the sulforeduction as a mechanism to conserve energy, the sulfur
being utilized as the final electron acceptor during oxidation
of cellular polysaccharide reserves in the dark.
Another group of organisms, the thiosulfate-reducers, are
specialized in the use of thiosulfate as electron acceptor.
These micro-organisms have a fermentative metabolism. In
thiosulfate-reducers, and in fermentative microorganisms
which reduce sulfur, the sulfur compound react by trapping
electrons and contributes to the reduction of high dihydrogen
concentrations generated by fermentative metabolism. During the fermentation of substrates, the reduction of sulfur
compounds increases the oxidation level of fermentative
products, thereby enhancing the production of energy and
the growth rates of microorganisms. This is not a respiratory
process but the use of thiosulfate as an electron trap by
fermentative microorganisms (which is similar in some
sulfur reducers), involves strict anaerobic thermophiles, primarily those from the genera Thermotoga, Thermosipho and
Fervidobacterium. The thiosulfate-reduction by these nonsulfate-reducers has been identified in soil and subsoil
ecosystems. They seem to have a corrosion power far superior to that of sulfate- or sulfur reducers (Box 14.12).
14.4.3 The Production of Sulfate
The oxidation of sulfides or of other sulfur-containing
compounds of higher oxidation state depends on both chemical and biological processes. In the presence of dioxygen,
sulfide is chemically oxidized to elementary sulfur or thiosulfate. Similarly, sulfite, which is a highly reactive compound is
rapidly oxidized to sulfate in the presence of molecular oxygen. In anoxic environments, sulfide can be trapped in the
sediments in the presence of ferrous iron to form iron sulfide
(FeS) and pyrite (FeS 2 ), while ferric iron reacts with sulfide to
form elementary sulfur. The biological oxidation of sulfur
compounds involves many bacterial groups with diverse
metabolisms: chemolithotrophic sulfo-oxidizing prokaryotes
(CSOP), anoxygenic phototrophic bacteria (APB), sulfatereducing bacteria, and cyanobacteria. While the two first
groups are typical sulfur-oxidizing bacteria (i.e. the sulfur
compounds are used as electron donors), the contribution of
microorganisms in the two other groups to the oxidation of
sulfur compounds are occasional or circumstantial. Hence,
sulfate-reducing bacteria capable of dismutation contribute
to the production of both reduced and oxidized sulfur
compounds. In the presence of sulfide which is an inhibitor
of the photosystem II, some cyanobacteria can also carry out
an anoxygenic photosynthesis identical to that of APB by
photometabolizing sulfide (electron donor) through the photosystem I (Cohen et al. 1975).
14.4.3.1 Chemotrophic Production of Sulfate:
Colorless Sulfo-Oxidizing Microorganisms
Chemolithotrophic sulfo-oxidizing prokaryotes (CSOP) are
also called colorless sulfo-oxidizing microorganisms, by
opposition to phototrophic bacteria in which the presence
of photosynthetic pigments provides a color to cellular
suspensions and, in some cases, to the whole natural
588
J.-C. Bertrand et al.
allowing them to thrive in highly diverse environmental
situations, and play a major role in the transformation of
organic and inorganic compounds.
The phylogenetic dispersion of sulfate-reducers makes
difficult the study of this community based exclusively on
the 16S rRNA gene (cf. Sect. 6.6.2), which led to the investigation of novel molecular markers among specific sulfatereducing enzymes. Among these enzymes, adenosine
phosphosulfate reductase (APS reductase) and sulfite reductase catalyze the reduction of APS to AMP and sulfite, and
the dissimilatory reduction of sulfite to sulfide, respectively
(cf. Sect. 3.3.2). The genes encoding these enzymes, including dsrAB genes, are becoming increasingly used for the
study of the communities of sulfate-reducers in natural
environments (Wagner et al. 2005).
14.4.2.3 Other Sulfide-Producing Metabolisms
Micro-organisms which are unable to use sulfate as an acceptor
of electrons participate in the production of reduced sulfur
compounds by dissimilatory reduction of elemental sulfur.
Sulfur reducers constitute a physiologically disparate group of
microorganisms, comprising both Bacteria (genera Desulforomonas, Desulfurella) and Archaea (Crenarchaeota and
Euryarchaeota). Some of these microorganisms completely
oxidize simple organic substrates (acetate, lactate, pyruvate,
succinate, ethanol). Those belonging to the genus Desulfurella
are strict sulfur reducers, while in the genus Desulforomonas,
fumarate or malate may be used as electron acceptors by certain
strains (Widdel and Pfennig 1992). Some fermentative sulfur
reducers are not involved in respiration but rather use sulfur as
an electron trap to increase energy production and synthesize
additional ATP through substrate phosphorylation.
Sulfur reducers are abundant in inland and oceanic geothermal environments, rich in elemental sulfur. Indeed, many
sulfo-reducing species are extreme (especially among
Archaea) or moderate (bacteria) thermophiles, although
mesophilic or even psychrophilic sulfo-reducing species
exist. They are also abundant in marine or brackish waters
where sulfides produced by sulfate-reducers are chemically or
biologically oxidized, contributing to the formation of elemental sulfur used by sulfo-reducing bacteria (Fig. 14.36). These
bacteria can live in mutualistic association with phototrophic
green bacteria, based on the oxidative reduction of sulfur.
Finally, some microorganisms, such as phototrophic purple sulfur bacteria, chemolithotrophic Beggiatoa and certain
cyanobacteria, have the possibility of exploiting the sulforeduction as a mechanism to conserve energy, the sulfur
being utilized as the final electron acceptor during oxidation
of cellular polysaccharide reserves in the dark.
Another group of organisms, the thiosulfate-reducers, are
specialized in the use of thiosulfate as electron acceptor.
These micro-organisms have a fermentative metabolism. In
thiosulfate-reducers, and in fermentative microorganisms
which reduce sulfur, the sulfur compound react by trapping
electrons and contributes to the reduction of high dihydrogen
concentrations generated by fermentative metabolism. During the fermentation of substrates, the reduction of sulfur
compounds increases the oxidation level of fermentative
products, thereby enhancing the production of energy and
the growth rates of microorganisms. This is not a respiratory
process but the use of thiosulfate as an electron trap by
fermentative microorganisms (which is similar in some
sulfur reducers), involves strict anaerobic thermophiles, primarily those from the genera Thermotoga, Thermosipho and
Fervidobacterium. The thiosulfate-reduction by these nonsulfate-reducers has been identified in soil and subsoil
ecosystems. They seem to have a corrosion power far superior to that of sulfate- or sulfur reducers (Box 14.12).
14.4.3 The Production of Sulfate
The oxidation of sulfides or of other sulfur-containing
compounds of higher oxidation state depends on both chemical and biological processes. In the presence of dioxygen,
sulfide is chemically oxidized to elementary sulfur or thiosulfate. Similarly, sulfite, which is a highly reactive compound is
rapidly oxidized to sulfate in the presence of molecular oxygen. In anoxic environments, sulfide can be trapped in the
sediments in the presence of ferrous iron to form iron sulfide
(FeS) and pyrite (FeS 2 ), while ferric iron reacts with sulfide to
form elementary sulfur. The biological oxidation of sulfur
compounds involves many bacterial groups with diverse
metabolisms: chemolithotrophic sulfo-oxidizing prokaryotes
(CSOP), anoxygenic phototrophic bacteria (APB), sulfatereducing bacteria, and cyanobacteria. While the two first
groups are typical sulfur-oxidizing bacteria (i.e. the sulfur
compounds are used as electron donors), the contribution of
microorganisms in the two other groups to the oxidation of
sulfur compounds are occasional or circumstantial. Hence,
sulfate-reducing bacteria capable of dismutation contribute
to the production of both reduced and oxidized sulfur
compounds. In the presence of sulfide which is an inhibitor
of the photosystem II, some cyanobacteria can also carry out
an anoxygenic photosynthesis identical to that of APB by
photometabolizing sulfide (electron donor) through the photosystem I (Cohen et al. 1975).
14.4.3.1 Chemotrophic Production of Sulfate:
Colorless Sulfo-Oxidizing Microorganisms
Chemolithotrophic sulfo-oxidizing prokaryotes (CSOP) are
also called colorless sulfo-oxidizing microorganisms, by
opposition to phototrophic bacteria in which the presence
of photosynthetic pigments provides a color to cellular
suspensions and, in some cases, to the whole natural
588
J.-C. Bertrand et al.