epilimnion), and light (intensity and type of light radiation).
These communities are maintained as long as the water
column remains stratified.
Sometimes, stratification also occurs in marine water
systems with the occurrence of a minimum oxygen zone in
the water column. It has been demonstrated that a cryptic
sulfur cycle is extremely active in these zones where sulfatereducing organisms take advantage of low oxygen
concentrations to develop.
14.4.4.2 Lagoons and Other Coastal Environments
Located at the sea-continent interface, coastal environments
receive freshwater inflows (rivers, rain, and run-off waters), as
well as inputs of marine waters. The former are most often
rich in both organic matter (urban or industrial effluents) and
nutrients from the leaching of agricultural soils. This explains
the high productivity of these environments. The aerobic
mineralization of both allochthonous and autochthonous
organic matter creates a high biological oxygen demand
(BOD), promoting the establishment of anoxic zones in
sediments or deep waters of these lagoons. The presence of
sulfate from seawater promotes the development of sulfatereducers and the accumulation of toxic sulfides in the
sediments and possibly in the upper waters (Caumette 1986).
It is important to distinguish shallow lagoons with homogeneous water column, from deep lagoons or estuaries where
the water column can undergo stratification, most often due
to vertical gradients in salinity. These gradients result from
the superposition of a low-salinity surface layer over a highsalinity and denser deep layer of waters. For deep lagoons,
we must also distinguish holomictic lagoons in which there
is mixing of the water column (one or more times per year),
from meromictic lagoons with a permanent stratification. In
these environments, processes related to the sulfur cycle are
similar to those described for stratified lakes.
In shallow lagoons where light penetrates to the sediment
surface, the interface between the oxic and anoxic zones is
localized, mostly often, in the few first millimeters of the
water column. Sulfate-reducing activity is the most important
metabolism in these shallow sediments (input of organic
matter and of sulfate from the overlying water), where
phototrophic sulfur-oxidizing (need of light) and/or
chemolithotrophic sulfur-oxidizing (need of dioxygen)
microorganisms develop in the water-sediment interface.
However, the low depth of the water column causes high
instability of these environments which are subjected to strong
and rapid variations in the sulfide and dioxygen gradients.
Thus, during warm and calm periods, anoxia can develop
in the whole water column (Fig. 14.42). This promotes a
shift in the oxic–anoxic boundary up to the air–water interface. The entire water column is then invaded by sulfide
derived from the sulfate reduction. These phenomena are
called dystrophic crises* (Caumette 1986). Phototrophic
purple sulfur bacteria subsequently grow and heavily
colonize the water column, causing red colored water and
oxidizing the accumulated sulfide to sulfate (Fig. 14.42).
Indeed, high light intensity reaching the water column
promotes the development of phototrophic purple sulfur
bacteria that outcompete the green sulfur bacteria. These
red-water phenomena are temporary and last until the disappearance of sulfide in the water column. Biotopes other than
shallow lagoons, such as seawater pools, swamps, and sheltered parts of bay and estuaries, are susceptible to the occurrence of these red water phenomena.
The microorganisms responsible for these red waters are
often very versatile in terms of their metabolic capacity, and
are adapted to enhanced environmental fluctuations (De Wit
and van Gemerden 1990). Thus, among these organisms, are
found phototrophic purple sulfur bacteria of the genus
Thiocapsa, which can tolerate high concentrations of sulfide
(2–8 mM), store elemental sulfur in the form of intracellular
sulfur, and tolerate dioxygen. Their ability to store sulfur is
an advantage for these microorganisms which can use it in
the absence of light, and in the presence of dioxygen by
respiration. In addition, in the dark and under anaerobic
conditions, this sulfur may be used as an electron acceptor
during endogenous fermentation of polysacharidic reserves.
Dioxygen inhibits the biosynthesis of photosynthetic
pigments (bacteriochlorophylls) in phototrophic bacteria
and thus their growth. However, the microorganisms in the
genus Thiocapsa tolerate the presence of dioxygen and,
under these conditions, can grow via chemoautotrophic or
mixotrophic metabolisms, or by oxidizing sulfide, thiosulfate or sulfur (intra- or extra-cellular). However, these
organisms are not competitive with colorless chemolithotrophic sulfur-oxidizing bacteria whose generation
time is shorter. During bacterium-forced red tide phenomena, phototrophic bacteria dominate, because of the absence
dioxygen.
During dystrophic crises that lead to the appearance of
red bacterial water, the development of anoxygenic
phototrophic bacteria ensures a resiliency toward the initial
environmental conditions. Indeed, these phenomena are only
the visible part of a set of processes involved in the mineralization of excess organic materials in highly productive
environments. They also highlight the role of anoxygenic
phototrophic bacteria in the re-oxidation of toxic sulfides
from an intense sulfate-reducing activity. All of these processes are highlighted in Winograsky columns (Box 14.13).
The activity of these microorganisms results in an important
biomass production which is at the basis of the establishment
of a new food chain, at the end of the dystrophic crisis. The
biomass production by anoxygenic phototrophic bacteria is
considered a para-primary production. In the case of a dystrophic crisis or not, this biomass production can serve, in
addition or not to primary production, as the first level of a
food chain which develops in parallel to the classical trophic
chain (Fig. 14.43).
14 Biogeochemical Cycles
595
These communities are maintained as long as the water
column remains stratified.
Sometimes, stratification also occurs in marine water
systems with the occurrence of a minimum oxygen zone in
the water column. It has been demonstrated that a cryptic
sulfur cycle is extremely active in these zones where sulfatereducing organisms take advantage of low oxygen
concentrations to develop.
14.4.4.2 Lagoons and Other Coastal Environments
Located at the sea-continent interface, coastal environments
receive freshwater inflows (rivers, rain, and run-off waters), as
well as inputs of marine waters. The former are most often
rich in both organic matter (urban or industrial effluents) and
nutrients from the leaching of agricultural soils. This explains
the high productivity of these environments. The aerobic
mineralization of both allochthonous and autochthonous
organic matter creates a high biological oxygen demand
(BOD), promoting the establishment of anoxic zones in
sediments or deep waters of these lagoons. The presence of
sulfate from seawater promotes the development of sulfatereducers and the accumulation of toxic sulfides in the
sediments and possibly in the upper waters (Caumette 1986).
It is important to distinguish shallow lagoons with homogeneous water column, from deep lagoons or estuaries where
the water column can undergo stratification, most often due
to vertical gradients in salinity. These gradients result from
the superposition of a low-salinity surface layer over a highsalinity and denser deep layer of waters. For deep lagoons,
we must also distinguish holomictic lagoons in which there
is mixing of the water column (one or more times per year),
from meromictic lagoons with a permanent stratification. In
these environments, processes related to the sulfur cycle are
similar to those described for stratified lakes.
In shallow lagoons where light penetrates to the sediment
surface, the interface between the oxic and anoxic zones is
localized, mostly often, in the few first millimeters of the
water column. Sulfate-reducing activity is the most important
metabolism in these shallow sediments (input of organic
matter and of sulfate from the overlying water), where
phototrophic sulfur-oxidizing (need of light) and/or
chemolithotrophic sulfur-oxidizing (need of dioxygen)
microorganisms develop in the water-sediment interface.
However, the low depth of the water column causes high
instability of these environments which are subjected to strong
and rapid variations in the sulfide and dioxygen gradients.
Thus, during warm and calm periods, anoxia can develop
in the whole water column (Fig. 14.42). This promotes a
shift in the oxic–anoxic boundary up to the air–water interface. The entire water column is then invaded by sulfide
derived from the sulfate reduction. These phenomena are
called dystrophic crises* (Caumette 1986). Phototrophic
purple sulfur bacteria subsequently grow and heavily
colonize the water column, causing red colored water and
oxidizing the accumulated sulfide to sulfate (Fig. 14.42).
Indeed, high light intensity reaching the water column
promotes the development of phototrophic purple sulfur
bacteria that outcompete the green sulfur bacteria. These
red-water phenomena are temporary and last until the disappearance of sulfide in the water column. Biotopes other than
shallow lagoons, such as seawater pools, swamps, and sheltered parts of bay and estuaries, are susceptible to the occurrence of these red water phenomena.
The microorganisms responsible for these red waters are
often very versatile in terms of their metabolic capacity, and
are adapted to enhanced environmental fluctuations (De Wit
and van Gemerden 1990). Thus, among these organisms, are
found phototrophic purple sulfur bacteria of the genus
Thiocapsa, which can tolerate high concentrations of sulfide
(2–8 mM), store elemental sulfur in the form of intracellular
sulfur, and tolerate dioxygen. Their ability to store sulfur is
an advantage for these microorganisms which can use it in
the absence of light, and in the presence of dioxygen by
respiration. In addition, in the dark and under anaerobic
conditions, this sulfur may be used as an electron acceptor
during endogenous fermentation of polysacharidic reserves.
Dioxygen inhibits the biosynthesis of photosynthetic
pigments (bacteriochlorophylls) in phototrophic bacteria
and thus their growth. However, the microorganisms in the
genus Thiocapsa tolerate the presence of dioxygen and,
under these conditions, can grow via chemoautotrophic or
mixotrophic metabolisms, or by oxidizing sulfide, thiosulfate or sulfur (intra- or extra-cellular). However, these
organisms are not competitive with colorless chemolithotrophic sulfur-oxidizing bacteria whose generation
time is shorter. During bacterium-forced red tide phenomena, phototrophic bacteria dominate, because of the absence
dioxygen.
During dystrophic crises that lead to the appearance of
red bacterial water, the development of anoxygenic
phototrophic bacteria ensures a resiliency toward the initial
environmental conditions. Indeed, these phenomena are only
the visible part of a set of processes involved in the mineralization of excess organic materials in highly productive
environments. They also highlight the role of anoxygenic
phototrophic bacteria in the re-oxidation of toxic sulfides
from an intense sulfate-reducing activity. All of these processes are highlighted in Winograsky columns (Box 14.13).
The activity of these microorganisms results in an important
biomass production which is at the basis of the establishment
of a new food chain, at the end of the dystrophic crisis. The
biomass production by anoxygenic phototrophic bacteria is
considered a para-primary production. In the case of a dystrophic crisis or not, this biomass production can serve, in
addition or not to primary production, as the first level of a
food chain which develops in parallel to the classical trophic
chain (Fig. 14.43).
14 Biogeochemical Cycles
595