sediment originates mainly from the water column and
arrives by sedimentation. In shallower aquatic ecosystems,
this OM can also be produced on site by benthic
macrophytes (aquatic Magnoliophyta and macroalgae), benthic microalgae, and cyanobacteria. The benthos of lakes and
coastal systems also receive OM input from their watersheds
through advective transport. The benthic systems covered by
an aquatic vegetation of macrophytes bear strong
resemblances with the terrestrial systems.
Many benthic prokaryote species and microalgae release
large quantities of extracellular polymeric substances (EPS).
The polymers of the EPS play several important roles for the
benthic microorganisms including (1) their adhesion to the
surfaces, (2) their mobility by gliding, and (3) as an important stock of energy that can be used later under starvation
conditions.
EPS is generally very rich in carbon (C) and poor in
nitrogen (N) and phosphorus (P). Hence, the excretion of
EPS can, in some cases, also be the result of unbalanced
growth of benthic microorganisms under N and/or P limiting
conditions.
The EPS thus represents a stock of OM that represents a
source of food for any different benthic organisms, and,
therefore, EPS represents an important compartment that
should be considered for benthic microbial food webs. EPS
is particularly important in biofilms; here the polymers form
an organic matrix in which the microorganisms are embedded. Photosynthetic biofilms are a special case where
microalgae and cyanobacteria are the primary producers.
13.1.3 Terrestrial Ecosystems
As described for the aquatic ecosystems, the soils of terrestrial systems also deliver important ecosystem services to
humans. Soils occur at the interface between the atmosphere,
the lithosphere, the hydrosphere, and the biosphere. The soil
ecosystems contribute to the major biogeochemical cycles,
including the water cycle, and the major element (C, N, P,
and others) cycles. The soils are the support for agriculture,
forestry, and pastoral exploitation. These ecosystems contribute to the regulation of the climate and to detoxification
as well to erosion control (Lavelle et al. 2006). For the soil
constituents, four different organization levels can be
identified:
1. The aggregate (ranging from a couple of mm to several
dm in size), which represents a coherent group of elementary soil particles
2. The assemblage (an association of aggregates)
3. The horizon (ranging from a couple of cm to several m),
soil layers morphologically homogeneous
4. Pedological cover: three-dimensional all horizons
Because aggregation of soil particles (1) and assemblage
of soil aggregates (2) have such a profound effect on soil
structure, it is often observed that the topsoil layers show a
complex three-dimensional distribution of the main environmental variables (dioxygen partial pressure, OM contents,
redox potential). This is in contrast to most sediments, where
a one-dimensional depth stratification of these variables
overwhelms their heterogeneity in the horizontal plane.
However, sediments that are submitted to important bioturbation are an exception in this respect.
The presence of a gas phase in soils obviously results in
important differences in oxygenation among (1) the zone in
between the aggregates and (2) the zone located in the
interior of the aggregates and (3) oxic-anoxic microgradients across the aggregates (Sextone et al. 1985; Renault
and Stengel 1994). The way of how the different particles
and other constituents are ordered within and at the surface
of a soil aggregate is always very complex. However, the
resulting three-dimensional heterogeneity of environmental
conditions and the occurrence of micro-gradients vary a lot
among different soils. These three-dimensional structures
are very well developed in clearly fragmented structures
(soil layers where aggregates are visible to the naked eye)
and become progressively less important in structures built
of particles (where soil particles remain dissociated from
each other) and continuous structures (where no visible
aggregates are present).
The biological functioning of the soils encompasses the
functioning of all the organisms, which, in interaction with
the chemical and physical components of the soil, contribute
to nutrient recycling and water flows. These functions are
realized by organisms that range in size from
microorganisms (Archaea, Bacteria, unicellular heterotrophic eukaryotes) to invertebrates. Moreover, strong
interactions exist between these organisms, which contribute
to the quality of the above-mentioned ecosystem services.
Despite this obvious fact, the concept of a food web has not
shaped the studies of the soil to a comparable extent as it has
done for aquatic ecosystems. One of the main reasons
appears to be that the function of primary productivity in
terrestrial ecosystems is mainly realized by higher plants and
only to a very minor extent by microorganisms. Hence, the
functioning of the biological component of belowground
terrestrial systems is mainly dependent on the vegetation.
In the first place, the plants deliver almost all the energy to
this belowground system. In addition, through their root
systems, the plants exert an important control of the
biophysicochemical interactions in the soil and of the structuring of the soils and can, therefore, be considered as
important soil engineers*.
Another difficulty for developing food web studies in
soils is that it represents an environment comprising three
phases (water, solid, and gas). In contrast to aquatic
13 Microbial Food Webs in Aquatic and Terrestrial Ecosystems
487
arrives by sedimentation. In shallower aquatic ecosystems,
this OM can also be produced on site by benthic
macrophytes (aquatic Magnoliophyta and macroalgae), benthic microalgae, and cyanobacteria. The benthos of lakes and
coastal systems also receive OM input from their watersheds
through advective transport. The benthic systems covered by
an aquatic vegetation of macrophytes bear strong
resemblances with the terrestrial systems.
Many benthic prokaryote species and microalgae release
large quantities of extracellular polymeric substances (EPS).
The polymers of the EPS play several important roles for the
benthic microorganisms including (1) their adhesion to the
surfaces, (2) their mobility by gliding, and (3) as an important stock of energy that can be used later under starvation
conditions.
EPS is generally very rich in carbon (C) and poor in
nitrogen (N) and phosphorus (P). Hence, the excretion of
EPS can, in some cases, also be the result of unbalanced
growth of benthic microorganisms under N and/or P limiting
conditions.
The EPS thus represents a stock of OM that represents a
source of food for any different benthic organisms, and,
therefore, EPS represents an important compartment that
should be considered for benthic microbial food webs. EPS
is particularly important in biofilms; here the polymers form
an organic matrix in which the microorganisms are embedded. Photosynthetic biofilms are a special case where
microalgae and cyanobacteria are the primary producers.
13.1.3 Terrestrial Ecosystems
As described for the aquatic ecosystems, the soils of terrestrial systems also deliver important ecosystem services to
humans. Soils occur at the interface between the atmosphere,
the lithosphere, the hydrosphere, and the biosphere. The soil
ecosystems contribute to the major biogeochemical cycles,
including the water cycle, and the major element (C, N, P,
and others) cycles. The soils are the support for agriculture,
forestry, and pastoral exploitation. These ecosystems contribute to the regulation of the climate and to detoxification
as well to erosion control (Lavelle et al. 2006). For the soil
constituents, four different organization levels can be
identified:
1. The aggregate (ranging from a couple of mm to several
dm in size), which represents a coherent group of elementary soil particles
2. The assemblage (an association of aggregates)
3. The horizon (ranging from a couple of cm to several m),
soil layers morphologically homogeneous
4. Pedological cover: three-dimensional all horizons
Because aggregation of soil particles (1) and assemblage
of soil aggregates (2) have such a profound effect on soil
structure, it is often observed that the topsoil layers show a
complex three-dimensional distribution of the main environmental variables (dioxygen partial pressure, OM contents,
redox potential). This is in contrast to most sediments, where
a one-dimensional depth stratification of these variables
overwhelms their heterogeneity in the horizontal plane.
However, sediments that are submitted to important bioturbation are an exception in this respect.
The presence of a gas phase in soils obviously results in
important differences in oxygenation among (1) the zone in
between the aggregates and (2) the zone located in the
interior of the aggregates and (3) oxic-anoxic microgradients across the aggregates (Sextone et al. 1985; Renault
and Stengel 1994). The way of how the different particles
and other constituents are ordered within and at the surface
of a soil aggregate is always very complex. However, the
resulting three-dimensional heterogeneity of environmental
conditions and the occurrence of micro-gradients vary a lot
among different soils. These three-dimensional structures
are very well developed in clearly fragmented structures
(soil layers where aggregates are visible to the naked eye)
and become progressively less important in structures built
of particles (where soil particles remain dissociated from
each other) and continuous structures (where no visible
aggregates are present).
The biological functioning of the soils encompasses the
functioning of all the organisms, which, in interaction with
the chemical and physical components of the soil, contribute
to nutrient recycling and water flows. These functions are
realized by organisms that range in size from
microorganisms (Archaea, Bacteria, unicellular heterotrophic eukaryotes) to invertebrates. Moreover, strong
interactions exist between these organisms, which contribute
to the quality of the above-mentioned ecosystem services.
Despite this obvious fact, the concept of a food web has not
shaped the studies of the soil to a comparable extent as it has
done for aquatic ecosystems. One of the main reasons
appears to be that the function of primary productivity in
terrestrial ecosystems is mainly realized by higher plants and
only to a very minor extent by microorganisms. Hence, the
functioning of the biological component of belowground
terrestrial systems is mainly dependent on the vegetation.
In the first place, the plants deliver almost all the energy to
this belowground system. In addition, through their root
systems, the plants exert an important control of the
biophysicochemical interactions in the soil and of the structuring of the soils and can, therefore, be considered as
important soil engineers*.
Another difficulty for developing food web studies in
soils is that it represents an environment comprising three
phases (water, solid, and gas). In contrast to aquatic
13 Microbial Food Webs in Aquatic and Terrestrial Ecosystems
487