communities (e.g., bacterial mats, red tide phenomena).
CSOP are highly diverse, comprising phylogenetically distant taxa (Archaea and Bacteria) (cf. Chapter 6.6).
Depending on the species, they are able to oxidize various
forms of reduced sulfur compounds, such as sulfides, elemental sulfur, sulfite, thiosulfate, or tetrathionate. The end
product of this oxidation is sulfate, together with intermediate compounds, primarily elemental sulfur which can be
stored as intra- or extracellular sulfur globules. It is
possible to distinguish four groups of CSOP depending on
their carbon and energy sources (Robertson and Kuenen
2006).
The first group, known as obligatory chemolithotrophs,
consists of specialized microorganisms in which the electron
donor is a mineral sulfur compound, the carbon being
obtained by CO 2 fixation via the Calvin cycle. However,
most of these microorganisms can utilize exogenous carbon
sources and ferment their cytoplasmic glycogen reserves
when environmental conditions are unfavorable. Yet, in
this group of organisms that includes bacteria
(Thiomicrospira, Thiobacillus, Acidithiobacillus) and
archaea (Sulfolobus), these heterotrophic metabolisms are
secondary and mainly represent an adaptation to adverse
environmental conditions. The second group comprises facultative chemolithotrophs which exhibit the same
possibilities compared with the first group, but can also
grow as heterotrophs or mixotrophs. The third group, the
chemolithoheterotrophs, is represented by few Thiobacillus
and Beggiatoa who are unable to fix mineral carbon but use
reduced sulfur compounds as electron donors and energy
source; they thus absolutely require an organic carbon
source for growth. Finally, the chemoorganoheterotrophs*
(Beggiatoa sp., Macromonas, Thiobacterium and Thiothrix)
constitute the fourth group. They can oxidize sulfur
compounds without apparent energetic benefit.
Dioxygen is the prime electron acceptor for CSOP, but
depending on the species, the need and tolerance to dioxygen
vary greatly. Many CSOP have metabolic potentials to grow
or survive in anoxic conditions; the electron acceptor being
nitrate (reduced to nitrite or dinitrogen) or nitrite (with
release of dinitrogen). Under these conditions, some
chemolithoautotrophic CSOP (Thiobacillus denitrificans,
Sulfurimonas denitrificans) continue to grow by using
reduced sulfur compounds as electron donors. Others lose
their ability to use sulfur compounds and then grow as
denitrifying chemoorganoheterotrophs.
CSOP live mainly at the interface between oxic
(containing dioxygen) and anoxic (containing sulfides)
environments. Their development is especially visible in
the water-sediment interfaces in both continental and oceanic sulfur sources. The development of some filamentous
forms (genera Beggiatoa, Thiotrix) can be visualized in the
form of white filaments (Fig. 14.38). Their role in the
detoxification of sulfides is especially important in the
absence of light and thus of anoxygenic photosynthesis. In
some ecosystems, such as the deep oceanic (sulfurous hot
springs) or coastal (sulfide-rich sediments) environments,
CSOP live in mutualistic symbiosis with molluscs or
worms; these latter provide a favorable environment (habitat,
substrates) to CSOP which, in turn, eliminate toxic sulfides
and produce biomass and energy used by their hosts.
In some cases, aerobic sulfur oxidation activity has
adverse effects on poorly buffered ecosystems, primarily
via production of large quantities of sulfuric acid (H 2 SO 4 )
which promotes corrosion of certain materials (concrete,
stone, metals). This property, coupled with the ability of
certain CSOP to oxidize ferrous iron, is used to improve
the recovery of ore deposits (bioleaching) or the bioremediation of soils contaminated with metals (cf. Sect. 16.9).
14.4.3.2 Phototrophic Sulfate Production:
Photosynthetic Sulfo-Oxidizing
Microorganisms
Anoxygenic phototrophic bacteria (APB) have only one
photosystem and use compounds which are more reduced
than water as electron donors during their photosynthesis. In
this case, the electron donors are hydrogen sulfide or other
reduced sulfur compounds, dihydrogen, or organic
compounds of low molecular weight. This photosynthesis,
which does not lead to the production of dioxygen, is called
anoxygenic photosynthesis (cf. Sect. 3.3.4). Therefore, it
represents an essential step in matter cycle under anoxic
conditions, wherein the couple SO 4
2À /H 2 S can be compared
with the couple O 2 /H 2 O under aerobic conditions.
Anoxygenic phototrophic bacteria are considered as paraprimary producers because they use compounds derived
from the degradation of organic material and from anaerobic
respirations (Pfennig 1975).
From a metabolic and ecological point of view, APB can
be separated into two main groups: phototrophic sulfur bacteria that mainly use sulfur compounds as electron donors
(sulfide, polysulfides, elemental sulfur, thiosulfate, sulfite),
and phototrophic nonsulfur bacteria which preferentially
photo-oxidize organic compounds (Overmann and GarciaPichel 2006). When electron donors are reduced sulfur
compounds, their oxidation during photosynthesis leads to
the formation of elemental sulfur and sulfate. From a phenotypic point of view, anoxygenic phototrophic bacteria were
classified into two major groups according to their photosynthetic pigments: purple and green phototrophic bacteria. The
differentiation of these two groups is based, on one hand, on
the ultrastructure of the photosynthetic apparatus where the
reaction centers are located and, on the other hand, on the
nature of the pigments, bacteriochlorophylls and carotenoids
which determine the natural colors of these microorganisms
(Fig. 14.39; Table 14.8).
14 Biogeochemical Cycles
589
CSOP are highly diverse, comprising phylogenetically distant taxa (Archaea and Bacteria) (cf. Chapter 6.6).
Depending on the species, they are able to oxidize various
forms of reduced sulfur compounds, such as sulfides, elemental sulfur, sulfite, thiosulfate, or tetrathionate. The end
product of this oxidation is sulfate, together with intermediate compounds, primarily elemental sulfur which can be
stored as intra- or extracellular sulfur globules. It is
possible to distinguish four groups of CSOP depending on
their carbon and energy sources (Robertson and Kuenen
2006).
The first group, known as obligatory chemolithotrophs,
consists of specialized microorganisms in which the electron
donor is a mineral sulfur compound, the carbon being
obtained by CO 2 fixation via the Calvin cycle. However,
most of these microorganisms can utilize exogenous carbon
sources and ferment their cytoplasmic glycogen reserves
when environmental conditions are unfavorable. Yet, in
this group of organisms that includes bacteria
(Thiomicrospira, Thiobacillus, Acidithiobacillus) and
archaea (Sulfolobus), these heterotrophic metabolisms are
secondary and mainly represent an adaptation to adverse
environmental conditions. The second group comprises facultative chemolithotrophs which exhibit the same
possibilities compared with the first group, but can also
grow as heterotrophs or mixotrophs. The third group, the
chemolithoheterotrophs, is represented by few Thiobacillus
and Beggiatoa who are unable to fix mineral carbon but use
reduced sulfur compounds as electron donors and energy
source; they thus absolutely require an organic carbon
source for growth. Finally, the chemoorganoheterotrophs*
(Beggiatoa sp., Macromonas, Thiobacterium and Thiothrix)
constitute the fourth group. They can oxidize sulfur
compounds without apparent energetic benefit.
Dioxygen is the prime electron acceptor for CSOP, but
depending on the species, the need and tolerance to dioxygen
vary greatly. Many CSOP have metabolic potentials to grow
or survive in anoxic conditions; the electron acceptor being
nitrate (reduced to nitrite or dinitrogen) or nitrite (with
release of dinitrogen). Under these conditions, some
chemolithoautotrophic CSOP (Thiobacillus denitrificans,
Sulfurimonas denitrificans) continue to grow by using
reduced sulfur compounds as electron donors. Others lose
their ability to use sulfur compounds and then grow as
denitrifying chemoorganoheterotrophs.
CSOP live mainly at the interface between oxic
(containing dioxygen) and anoxic (containing sulfides)
environments. Their development is especially visible in
the water-sediment interfaces in both continental and oceanic sulfur sources. The development of some filamentous
forms (genera Beggiatoa, Thiotrix) can be visualized in the
form of white filaments (Fig. 14.38). Their role in the
detoxification of sulfides is especially important in the
absence of light and thus of anoxygenic photosynthesis. In
some ecosystems, such as the deep oceanic (sulfurous hot
springs) or coastal (sulfide-rich sediments) environments,
CSOP live in mutualistic symbiosis with molluscs or
worms; these latter provide a favorable environment (habitat,
substrates) to CSOP which, in turn, eliminate toxic sulfides
and produce biomass and energy used by their hosts.
In some cases, aerobic sulfur oxidation activity has
adverse effects on poorly buffered ecosystems, primarily
via production of large quantities of sulfuric acid (H 2 SO 4 )
which promotes corrosion of certain materials (concrete,
stone, metals). This property, coupled with the ability of
certain CSOP to oxidize ferrous iron, is used to improve
the recovery of ore deposits (bioleaching) or the bioremediation of soils contaminated with metals (cf. Sect. 16.9).
14.4.3.2 Phototrophic Sulfate Production:
Photosynthetic Sulfo-Oxidizing
Microorganisms
Anoxygenic phototrophic bacteria (APB) have only one
photosystem and use compounds which are more reduced
than water as electron donors during their photosynthesis. In
this case, the electron donors are hydrogen sulfide or other
reduced sulfur compounds, dihydrogen, or organic
compounds of low molecular weight. This photosynthesis,
which does not lead to the production of dioxygen, is called
anoxygenic photosynthesis (cf. Sect. 3.3.4). Therefore, it
represents an essential step in matter cycle under anoxic
conditions, wherein the couple SO 4
2À /H 2 S can be compared
with the couple O 2 /H 2 O under aerobic conditions.
Anoxygenic phototrophic bacteria are considered as paraprimary producers because they use compounds derived
from the degradation of organic material and from anaerobic
respirations (Pfennig 1975).
From a metabolic and ecological point of view, APB can
be separated into two main groups: phototrophic sulfur bacteria that mainly use sulfur compounds as electron donors
(sulfide, polysulfides, elemental sulfur, thiosulfate, sulfite),
and phototrophic nonsulfur bacteria which preferentially
photo-oxidize organic compounds (Overmann and GarciaPichel 2006). When electron donors are reduced sulfur
compounds, their oxidation during photosynthesis leads to
the formation of elemental sulfur and sulfate. From a phenotypic point of view, anoxygenic phototrophic bacteria were
classified into two major groups according to their photosynthetic pigments: purple and green phototrophic bacteria. The
differentiation of these two groups is based, on one hand, on
the ultrastructure of the photosynthetic apparatus where the
reaction centers are located and, on the other hand, on the
nature of the pigments, bacteriochlorophylls and carotenoids
which determine the natural colors of these microorganisms
(Fig. 14.39; Table 14.8).
14 Biogeochemical Cycles
589