isolated in sodium-carbonate-rich lakes (i.e. saturated with
Na 2 CO 3 ) in Egypt, grows at pH 11 and in salinity conditions
up to 250 psu*. Their reaction centers are located in specific
structures consisting of stacks of lamellae, due to cytoplasmic invaginations which are fixed to the cytoplasmic membrane by a single attachment point, thereby forming almost a
differentiated organelle (cf. Chap. 3.4; Fig. 3.27).
Phototrophic purple nonsulfur bacteria (Table 14.9) represent a more heterogeneous group. This diversity is
reflected in their morphology (Fig. 14.40), their modes of
cell division (binary or by budding), the types of inner
membrane structures containing various carotenoids, and in
their high metabolic diversity. Under anaerobic conditions
and in the presence of light, these organisms preferentially
use organic compounds as electron donors (photoorganotrophy), but are capable of using dihydrogen or sulfur
compounds (photolithotrophy). In this latter case, the oxidation of sulfides stops most often at the stage of elemental
sulfur, deposited extra-cellularly. Only a few species belonging to genera Rhodobacter, Rhodovulum, Rhodobium, and
Rhodopseudomonas oxidize sulfur compounds to sulfate.
The majority of these microorganisms is tolerant to
dioxygen and can perform aerobic respiration. The respiratory capabilities of these bacteria can also be expressed
under anoxic conditions, with electron acceptors such as
nitrate, dimethyl sulfoxide, or trimethylamine oxide. These
microorganisms are also capable of fermentation.
Phototrophic Green Bacteria
They include phototrophic green sulfur bacteria (phylum
Chlorobi) and multicellular filamentous green bacteria (Phylum Chloroflexi). These microorganisms exhibit bacteriochlorophyll c, d, or e, and carotenoids in the biosynthetic
series of chlorobactene or isorenieratene (Tables 14.8 and
14.9). These pigments are contained in chlorosomes, i.e.
vesicles which are attached to the inner cytoplamic membrane and act as powerful light energy sensors. Small
amounts of bacteriochlorophyll a are also present in these
bacteria, mainly localized in the reaction centers, at the
attachment point of chlorosomes to the cytoplasmic
membrane.
Green sulfur bacteria (Table 14.9) are strict anaerobes
and photolithoautotrophs. They fix CO 2 via the reversecycle of tricarboxylic acids. Few simple organic compounds
may be photo-assimilated in the presence of CO 2 and sulfide,
the latter electron donor being oxidized to sulfate during
photosynthesis. Sulfur globules (intermediate compounds
of this oxidation) are deposited outside the cells
(Fig. 14.40). Only few strains use dihydrogen or thiosulfate
as electron donors. Based on their carotenoid pigments
(Table 14.8), it is possible to distinguish between brown
and green bacteria.
The brown ones contain bacteriochlorophyll e and carotenoid from the biosynthetic series of isorenieratene, while
the green ones exhibit bacteriochlorophyll c and/or d and
carotenoids of the chlorobactene series.
Multicellular filamentous green bacteria (Table 14.9) are
represented by four genera, each comprising one or two
species. Bacterial cells form flexible and motile filaments
which allow individual cells to move by gliding. These
facultative aerobic microorganisms preferably use organic
compounds via phototrophic or chemotrophic metabolism,
reduced sulfur compounds being used very little. In the latter
case, the oxidation stops at the stage of elemental sulfur
which is deposited extracellularly, i.e. outside the filaments.
Photosynthetic pigments are bacteriochlorophylls c of d,
while the carotenoids are beta and gamma carotenes (βand γ-carotenes).
14.4.4 Sulfur Cycle in Natural Ecosystems
Changes in physical and chemical environments, including
organic matter (eutrophication level), concentration of sulfur
compounds, light, and oxygen, will influence the sulfur cycle
and the dominance of targeted microorganisms, depending
of their ecophysiological potentials. The sulfur cycle is
strongly linked to the carbon cycle: microorganisms that
produce reduced compounds (sulfate-reducers in particular)
are involved in the degradation of organic matter and are
known as chemoorganotrophs and heterotrophs, while those
oxidizing reduced sulfur compounds are often known as
para-primary chemoautotrophs or phototrophs. The concentration of oxidized sulfur compounds will eventually be a
limiting factor for the activity of sulfate-reducing
microorganisms. The concentration of the resulting reduced
sulfur compounds will determine the oxidation pathways of
these compounds by anoxygenic phototrophic and
chemolithotrophic sulfur oxidizing microorganisms. The
affinity of these microorganisms to sulfide, and their ability
to oxidize completely into sulfate or incompletely into elemental sulfur, are parameters that may constitute a decisive
advantage for some species. In these microorganisms, light
(phototrophs) or dioxygen (chemolithotrophs) will also represent key factors for the functioning of the sulfur cycle.
14.4.4.1 Stratified Lakes
Many aquatic ecosystems are stratified by the presence of a
chemocline*. Unlike coastal lagoons and some athalossohaline environments whose stratification is the result of a
halocline*, freshwater lakes often present a temporary summer stratification (holomictic lakes), due to the warming of
surface waters by solar radiation. The resulting establishment
of a temperature gradient in the water column is known as
14 Biogeochemical Cycles
593
Na 2 CO 3 ) in Egypt, grows at pH 11 and in salinity conditions
up to 250 psu*. Their reaction centers are located in specific
structures consisting of stacks of lamellae, due to cytoplasmic invaginations which are fixed to the cytoplasmic membrane by a single attachment point, thereby forming almost a
differentiated organelle (cf. Chap. 3.4; Fig. 3.27).
Phototrophic purple nonsulfur bacteria (Table 14.9) represent a more heterogeneous group. This diversity is
reflected in their morphology (Fig. 14.40), their modes of
cell division (binary or by budding), the types of inner
membrane structures containing various carotenoids, and in
their high metabolic diversity. Under anaerobic conditions
and in the presence of light, these organisms preferentially
use organic compounds as electron donors (photoorganotrophy), but are capable of using dihydrogen or sulfur
compounds (photolithotrophy). In this latter case, the oxidation of sulfides stops most often at the stage of elemental
sulfur, deposited extra-cellularly. Only a few species belonging to genera Rhodobacter, Rhodovulum, Rhodobium, and
Rhodopseudomonas oxidize sulfur compounds to sulfate.
The majority of these microorganisms is tolerant to
dioxygen and can perform aerobic respiration. The respiratory capabilities of these bacteria can also be expressed
under anoxic conditions, with electron acceptors such as
nitrate, dimethyl sulfoxide, or trimethylamine oxide. These
microorganisms are also capable of fermentation.
Phototrophic Green Bacteria
They include phototrophic green sulfur bacteria (phylum
Chlorobi) and multicellular filamentous green bacteria (Phylum Chloroflexi). These microorganisms exhibit bacteriochlorophyll c, d, or e, and carotenoids in the biosynthetic
series of chlorobactene or isorenieratene (Tables 14.8 and
14.9). These pigments are contained in chlorosomes, i.e.
vesicles which are attached to the inner cytoplamic membrane and act as powerful light energy sensors. Small
amounts of bacteriochlorophyll a are also present in these
bacteria, mainly localized in the reaction centers, at the
attachment point of chlorosomes to the cytoplasmic
membrane.
Green sulfur bacteria (Table 14.9) are strict anaerobes
and photolithoautotrophs. They fix CO 2 via the reversecycle of tricarboxylic acids. Few simple organic compounds
may be photo-assimilated in the presence of CO 2 and sulfide,
the latter electron donor being oxidized to sulfate during
photosynthesis. Sulfur globules (intermediate compounds
of this oxidation) are deposited outside the cells
(Fig. 14.40). Only few strains use dihydrogen or thiosulfate
as electron donors. Based on their carotenoid pigments
(Table 14.8), it is possible to distinguish between brown
and green bacteria.
The brown ones contain bacteriochlorophyll e and carotenoid from the biosynthetic series of isorenieratene, while
the green ones exhibit bacteriochlorophyll c and/or d and
carotenoids of the chlorobactene series.
Multicellular filamentous green bacteria (Table 14.9) are
represented by four genera, each comprising one or two
species. Bacterial cells form flexible and motile filaments
which allow individual cells to move by gliding. These
facultative aerobic microorganisms preferably use organic
compounds via phototrophic or chemotrophic metabolism,
reduced sulfur compounds being used very little. In the latter
case, the oxidation stops at the stage of elemental sulfur
which is deposited extracellularly, i.e. outside the filaments.
Photosynthetic pigments are bacteriochlorophylls c of d,
while the carotenoids are beta and gamma carotenes (βand γ-carotenes).
14.4.4 Sulfur Cycle in Natural Ecosystems
Changes in physical and chemical environments, including
organic matter (eutrophication level), concentration of sulfur
compounds, light, and oxygen, will influence the sulfur cycle
and the dominance of targeted microorganisms, depending
of their ecophysiological potentials. The sulfur cycle is
strongly linked to the carbon cycle: microorganisms that
produce reduced compounds (sulfate-reducers in particular)
are involved in the degradation of organic matter and are
known as chemoorganotrophs and heterotrophs, while those
oxidizing reduced sulfur compounds are often known as
para-primary chemoautotrophs or phototrophs. The concentration of oxidized sulfur compounds will eventually be a
limiting factor for the activity of sulfate-reducing
microorganisms. The concentration of the resulting reduced
sulfur compounds will determine the oxidation pathways of
these compounds by anoxygenic phototrophic and
chemolithotrophic sulfur oxidizing microorganisms. The
affinity of these microorganisms to sulfide, and their ability
to oxidize completely into sulfate or incompletely into elemental sulfur, are parameters that may constitute a decisive
advantage for some species. In these microorganisms, light
(phototrophs) or dioxygen (chemolithotrophs) will also represent key factors for the functioning of the sulfur cycle.
14.4.4.1 Stratified Lakes
Many aquatic ecosystems are stratified by the presence of a
chemocline*. Unlike coastal lagoons and some athalossohaline environments whose stratification is the result of a
halocline*, freshwater lakes often present a temporary summer stratification (holomictic lakes), due to the warming of
surface waters by solar radiation. The resulting establishment
of a temperature gradient in the water column is known as
14 Biogeochemical Cycles
593