distinct populations with a wide variety of metabolic
functions, but which are coordinated within a small space.
The association of various bacterial metabolisms present
in microbial mats induces the formation of chemical
microgradients (Des Marais 2003). For example, in the presence of light, oxygenic photosynthesis produces dioxygen at
the surface (up to 0.5–1 mm), thereby establishing a gradient
of dioxygen from the surface to the bottom of the mat. In
contrast, a gradient of sulfide appears due to the sulfatereducing activity in the deeper zones of the mat (1–3 mm).
This sulfide concentration decreases in the upper layers,
primarily because of the activity of aerobic and anaerobic
sulfo-oxidizing bacteria. In addition, a pH gradient is
established from the surface to the bottom of the mat, created
by the high photosynthetic activity of cyanobacteria coupled
with the low buffering capacity of microbial mats. The
absorption of solar radiations by photosynthetic organisms
and by sediments may also establish a light gradient in
microbial mats. The formation of these gradients relies on
the metabolic richness of the system and the proximity of
different micro-environments created by the superposition
of gradients. The unstable equilibrium of this system can be
observed when changing the gradient of light that affects the
activity of photosynthetic microorganisms. In fact, physicochemical gradients within microbial mats fluctuate strongly
with the circadian cycle (Fig. 14.44), typically with the
spatial rearrangements of the associated microbial
communities (van Gemerden 1993). Thus, during a circadian
cycle, vertical shifts in anoxygenic phototrophic bacteria,
denitrifying bacteria, sulfate-reducing microorganisms, and
chemolithotrophic sulfur-oxidizing bacteria, could be
observed within microbial mats.
The establishment of physical and chemical gradients at
the micrometer scale in microbial mats allows the coexistence of various metabolisms (Fig. 14.44). Cyanobacteria
are the pioneer builders of microbial mats due to their ability
to produce exopolymeric matrices. Cyanobacteria are also
considered the main producers of dioxygen on earth. Both
filamentous and unicellular forms group complex
microorganisms, with huge diversity (cf. Sect. 6.6.2).
These primary producers are typically oxygenic photosynthetic organisms, producing organic matter and dioxygen
from solar energy used to fix CO 2 . In addition, many
cyanobacteria have the ability to fix dinitrogen. These
characteristics explain why they can colonize surface
habitats that are originally poor in organic matter and in
nutrients. This colonization enriched the surrounding
medium in organic matter, in dioxygen, and in nitrogenous
compounds, allowing the development of other
microorganisms. Among these microorganisms, aerobic
heterotrophic bacteria will use the organic matter as carbon
and energy source, by performing an aerobic respiration
based on the dioxygen released by cyanobacteria. Facultative anaerobic heterotrophs are also able to develop in such
ecosystems. This is particularly the case of denitrifying
bacteria and other nitrate respiring bacteria that will benefit
from ammonium oxidizing activities by nitrifying bacteria
or by those microorganisms that are able to oxidize ammonium under anaerobic conditions (cf. Sect. 14.3.5). Fermentative bacteria, i.e. strict or facultative anaerobes, also have
an important role in microbial mats, by participating in the
degradation of organic matter and by providing organic
substrates (organic acids, alcohols) or minerals (dihydrogen)
for other anaerobic microorganisms. Among them, the activity of sulfate-reducing microorganisms is often very important in these ecosystems. From their activity, the generated
sulfide will be oxidized by anoxygenic phototrophic sulfur
bacteria and chemolithotrophic sulfur-oxidizing bacteria. In
these shallow ecosystems, high wavelengths (near-infrared)
penetrate the most deeply, and among anoxygenic photosynthetic bacteria, those containing bacteriochlorophyll a and b
are favored (phototrophic purple bacteria). Other
microorganisms such as methanogenic archaea and ironreducing or iron-oxidizing bacteria are also present. Finally,
eukaryotic microorganisms, photosynthetic or not, are also
present in these ecosystems and have a role in the regulation
of prokaryotic populations, primarily as predators. Thus, a
photosynthetic microbial mat groups, on a very narrow spatial scale (a few mm deep), all functions that sustain a proper
functioning of biogeochemical cycles, particularly those of
carbon, nitrogen, sulfur, and iron.
14.4.4.4 Deep Ocean Hydrothermal Vents
These ecosystems, located in hot mid-ocean ridges
(contraction areas with faults), are life oases characterized
by an abundant fauna thriving in inhospitable, cold, and dark
deep seafloors. In these environments, seawater enters faults
of the Earth’s crust and, subjected to high temperatures and
pressures, becomes a hot, acidic, and reduced fluid that is
enriched in metal elements, in sulfide and in dihydrogen,
before being injected back to the oceanic waters as hydrothermal vents.
Hydrothermal vents (cf. Sect. 10.3.1) are resurgences
of fluid whose characteristics may vary depending on
environmental conditions. Some of these hydrothermal
vents, called black or white smokers, consist of hot
(200–400
C) and anoxic fluid which is ejected into cold
(4
C) and oxygenated deep oceanic waters. These hydrothermal vents are enriched in H 2 S whose concentration
varies depending on the content of metallic elements
14 Biogeochemical Cycles
599
functions, but which are coordinated within a small space.
The association of various bacterial metabolisms present
in microbial mats induces the formation of chemical
microgradients (Des Marais 2003). For example, in the presence of light, oxygenic photosynthesis produces dioxygen at
the surface (up to 0.5–1 mm), thereby establishing a gradient
of dioxygen from the surface to the bottom of the mat. In
contrast, a gradient of sulfide appears due to the sulfatereducing activity in the deeper zones of the mat (1–3 mm).
This sulfide concentration decreases in the upper layers,
primarily because of the activity of aerobic and anaerobic
sulfo-oxidizing bacteria. In addition, a pH gradient is
established from the surface to the bottom of the mat, created
by the high photosynthetic activity of cyanobacteria coupled
with the low buffering capacity of microbial mats. The
absorption of solar radiations by photosynthetic organisms
and by sediments may also establish a light gradient in
microbial mats. The formation of these gradients relies on
the metabolic richness of the system and the proximity of
different micro-environments created by the superposition
of gradients. The unstable equilibrium of this system can be
observed when changing the gradient of light that affects the
activity of photosynthetic microorganisms. In fact, physicochemical gradients within microbial mats fluctuate strongly
with the circadian cycle (Fig. 14.44), typically with the
spatial rearrangements of the associated microbial
communities (van Gemerden 1993). Thus, during a circadian
cycle, vertical shifts in anoxygenic phototrophic bacteria,
denitrifying bacteria, sulfate-reducing microorganisms, and
chemolithotrophic sulfur-oxidizing bacteria, could be
observed within microbial mats.
The establishment of physical and chemical gradients at
the micrometer scale in microbial mats allows the coexistence of various metabolisms (Fig. 14.44). Cyanobacteria
are the pioneer builders of microbial mats due to their ability
to produce exopolymeric matrices. Cyanobacteria are also
considered the main producers of dioxygen on earth. Both
filamentous and unicellular forms group complex
microorganisms, with huge diversity (cf. Sect. 6.6.2).
These primary producers are typically oxygenic photosynthetic organisms, producing organic matter and dioxygen
from solar energy used to fix CO 2 . In addition, many
cyanobacteria have the ability to fix dinitrogen. These
characteristics explain why they can colonize surface
habitats that are originally poor in organic matter and in
nutrients. This colonization enriched the surrounding
medium in organic matter, in dioxygen, and in nitrogenous
compounds, allowing the development of other
microorganisms. Among these microorganisms, aerobic
heterotrophic bacteria will use the organic matter as carbon
and energy source, by performing an aerobic respiration
based on the dioxygen released by cyanobacteria. Facultative anaerobic heterotrophs are also able to develop in such
ecosystems. This is particularly the case of denitrifying
bacteria and other nitrate respiring bacteria that will benefit
from ammonium oxidizing activities by nitrifying bacteria
or by those microorganisms that are able to oxidize ammonium under anaerobic conditions (cf. Sect. 14.3.5). Fermentative bacteria, i.e. strict or facultative anaerobes, also have
an important role in microbial mats, by participating in the
degradation of organic matter and by providing organic
substrates (organic acids, alcohols) or minerals (dihydrogen)
for other anaerobic microorganisms. Among them, the activity of sulfate-reducing microorganisms is often very important in these ecosystems. From their activity, the generated
sulfide will be oxidized by anoxygenic phototrophic sulfur
bacteria and chemolithotrophic sulfur-oxidizing bacteria. In
these shallow ecosystems, high wavelengths (near-infrared)
penetrate the most deeply, and among anoxygenic photosynthetic bacteria, those containing bacteriochlorophyll a and b
are favored (phototrophic purple bacteria). Other
microorganisms such as methanogenic archaea and ironreducing or iron-oxidizing bacteria are also present. Finally,
eukaryotic microorganisms, photosynthetic or not, are also
present in these ecosystems and have a role in the regulation
of prokaryotic populations, primarily as predators. Thus, a
photosynthetic microbial mat groups, on a very narrow spatial scale (a few mm deep), all functions that sustain a proper
functioning of biogeochemical cycles, particularly those of
carbon, nitrogen, sulfur, and iron.
14.4.4.4 Deep Ocean Hydrothermal Vents
These ecosystems, located in hot mid-ocean ridges
(contraction areas with faults), are life oases characterized
by an abundant fauna thriving in inhospitable, cold, and dark
deep seafloors. In these environments, seawater enters faults
of the Earth’s crust and, subjected to high temperatures and
pressures, becomes a hot, acidic, and reduced fluid that is
enriched in metal elements, in sulfide and in dihydrogen,
before being injected back to the oceanic waters as hydrothermal vents.
Hydrothermal vents (cf. Sect. 10.3.1) are resurgences
of fluid whose characteristics may vary depending on
environmental conditions. Some of these hydrothermal
vents, called black or white smokers, consist of hot
(200–400
C) and anoxic fluid which is ejected into cold
(4
C) and oxygenated deep oceanic waters. These hydrothermal vents are enriched in H 2 S whose concentration
varies depending on the content of metallic elements
14 Biogeochemical Cycles
599