presence of a microfauna that consumes fungi generally, but
not always, influences the fungal diversity. This can be
explained by the combined effect of the importance of the
selective nature of consumers (see above) and of the role of
competition in structuring fungal communities (McLean and
Parkinson 2000).
Besides, organisms considered as engineers such as
earthworms can have an impact on the diversity – especially
that of fungi – when modifying the soil structure, leading to
the buildup of new habitats. McLean and Parkinson (2000)
have found, based on experiments in microcosms, that the
worm Dendrobaena octaedra could alter the structure of the
fungal community in the soil and, in some conditions,
increase the fungal diversity by decreasing the intensity of
competition. In other experiences, these same authors show
that the presence of D. octaedra can, however, reduce the
fungal diversity by promoting a fast-growing species which
becomes dominant.
Numerous studies have shown that plant species can force
diversity, abundance, and activity of microorganisms in
soils. For example, Badejo and Tian (1999) have shown in
monoculture plots in Nigeria, Africa, that the diversity of
some groups of microfauna was greater under the plant
species Leucaena leucocephala than under three other
plant species investigated. Similarly, it has been shown
that the species richness of mycorrhizal fungi on the roots
of plants is very different for five different plant species on
the same site.
Finally, the role of the composition of the vegetation on
the diversity of soil organisms was also shown for various
stages of plant successions which are dominated by different
functional types of plants (Wardle 2002).
13.5 Conclusion
The microbial food web (MFW) organisms, including
viruses, archaea, bacteria, fungi, phytoplankton, benthic
algae, and protozoa, play an essential role in the functioning
of aquatic (pelagic and benthic) and terrestrial ecosystems.
However, there are fundamental differences between the
different ecosystems in the functional role of
microorganisms. The functioning of pelagic ecosystems is
largely depending on the advection, vertical mixing, and
stratification of the water column and the associated effects.
Planktonic mode life is best suited to this dynamic environment. Therefore, pelagic aquatic ecosystems are particularly
original because microbial communities provide almost all
primary production, constitute the essential of the biomass
present, and form thereby the basis of all aquatic food webs.
In connection with the physical nature of the sediments
and soils, the functioning of benthic and terrestrial MFW is
significantly different. In terrestrial ecosystems, unlike
aquatic pelagic ecosystems and even if there are the same
groups of microorganisms in water with a diversity at least
as important, the functional basis of these ecosystems is
formed by higher plants as the main primary producers.
Moreover, the plants exercise, through their root system,
an important control on the soil structuring and on the
biophysicochemical interactions. In this context,
microorganisms play, however, an important role in terrestrial biogeochemical cycles and they act as “bioreactors,”
transforming and recycling the primary organic matter that
comes from plants. On this last point, sediments in some
cases share similarities with the soil when the bulk of primary production provides by phanerogam. The similarities
between sediments and pelagic systems can be also observed
when the essential of organic matter in the water column
comes from the resuspension of autotrophic microorganisms
living in the sediment surface or even resuspension of settled
matter on the sediment. Because of the biophysical
characteristics of soil at macro-, meso-, and microscales,
terrestrial MFW are usually apprehended in substantially
different ways, and the concept of food web has much less
impregnated studies on soil ecosystems than those in aquatic
ecosystems. The aggregation process, favoring a threedimensional structure of environmental factors, leading to
a three-dimensional distribution of microorganisms. As in
the sediment, this structuring results in a partial dissociation
of the types of microorganisms in space. In addition, the
presence of gaseous dioxygen in the soil, associated with
aggregation mechanisms, has the effect of promoting microscale coexistence of aerobic and anoxic zones. These microgradients of oxygenation guide the “colonization” of
aggregates by microorganisms according to their strict aerobic, aerobic-anaerobic, or strict anaerobic characteristics.
Such oxygenation gradients exist also in the immediate
vicinity of the roots and affect the distribution of
microorganisms according to the availability of electron
acceptors that can be used in the respiration process.
It should be noted, however, that the pelagic aquatic
environments are not either strictly homogeneous, particularly because of microaggregates that can lead to the presence
of anoxic zones in microscale within an overall aerobic water
column. More generally, the structure of MFW in aquatic
(pelagic and benthic) and terrestrial environments reflects
the adaptation of microorganisms against physical, chemical,
and biological environmental forcing factors. Understanding
the mechanisms responsible for these adaptations to changing
environmental conditions is a major challenge for the
coming years.
506
B. Mostajir et al.
not always, influences the fungal diversity. This can be
explained by the combined effect of the importance of the
selective nature of consumers (see above) and of the role of
competition in structuring fungal communities (McLean and
Parkinson 2000).
Besides, organisms considered as engineers such as
earthworms can have an impact on the diversity – especially
that of fungi – when modifying the soil structure, leading to
the buildup of new habitats. McLean and Parkinson (2000)
have found, based on experiments in microcosms, that the
worm Dendrobaena octaedra could alter the structure of the
fungal community in the soil and, in some conditions,
increase the fungal diversity by decreasing the intensity of
competition. In other experiences, these same authors show
that the presence of D. octaedra can, however, reduce the
fungal diversity by promoting a fast-growing species which
becomes dominant.
Numerous studies have shown that plant species can force
diversity, abundance, and activity of microorganisms in
soils. For example, Badejo and Tian (1999) have shown in
monoculture plots in Nigeria, Africa, that the diversity of
some groups of microfauna was greater under the plant
species Leucaena leucocephala than under three other
plant species investigated. Similarly, it has been shown
that the species richness of mycorrhizal fungi on the roots
of plants is very different for five different plant species on
the same site.
Finally, the role of the composition of the vegetation on
the diversity of soil organisms was also shown for various
stages of plant successions which are dominated by different
functional types of plants (Wardle 2002).
13.5 Conclusion
The microbial food web (MFW) organisms, including
viruses, archaea, bacteria, fungi, phytoplankton, benthic
algae, and protozoa, play an essential role in the functioning
of aquatic (pelagic and benthic) and terrestrial ecosystems.
However, there are fundamental differences between the
different ecosystems in the functional role of
microorganisms. The functioning of pelagic ecosystems is
largely depending on the advection, vertical mixing, and
stratification of the water column and the associated effects.
Planktonic mode life is best suited to this dynamic environment. Therefore, pelagic aquatic ecosystems are particularly
original because microbial communities provide almost all
primary production, constitute the essential of the biomass
present, and form thereby the basis of all aquatic food webs.
In connection with the physical nature of the sediments
and soils, the functioning of benthic and terrestrial MFW is
significantly different. In terrestrial ecosystems, unlike
aquatic pelagic ecosystems and even if there are the same
groups of microorganisms in water with a diversity at least
as important, the functional basis of these ecosystems is
formed by higher plants as the main primary producers.
Moreover, the plants exercise, through their root system,
an important control on the soil structuring and on the
biophysicochemical interactions. In this context,
microorganisms play, however, an important role in terrestrial biogeochemical cycles and they act as “bioreactors,”
transforming and recycling the primary organic matter that
comes from plants. On this last point, sediments in some
cases share similarities with the soil when the bulk of primary production provides by phanerogam. The similarities
between sediments and pelagic systems can be also observed
when the essential of organic matter in the water column
comes from the resuspension of autotrophic microorganisms
living in the sediment surface or even resuspension of settled
matter on the sediment. Because of the biophysical
characteristics of soil at macro-, meso-, and microscales,
terrestrial MFW are usually apprehended in substantially
different ways, and the concept of food web has much less
impregnated studies on soil ecosystems than those in aquatic
ecosystems. The aggregation process, favoring a threedimensional structure of environmental factors, leading to
a three-dimensional distribution of microorganisms. As in
the sediment, this structuring results in a partial dissociation
of the types of microorganisms in space. In addition, the
presence of gaseous dioxygen in the soil, associated with
aggregation mechanisms, has the effect of promoting microscale coexistence of aerobic and anoxic zones. These microgradients of oxygenation guide the “colonization” of
aggregates by microorganisms according to their strict aerobic, aerobic-anaerobic, or strict anaerobic characteristics.
Such oxygenation gradients exist also in the immediate
vicinity of the roots and affect the distribution of
microorganisms according to the availability of electron
acceptors that can be used in the respiration process.
It should be noted, however, that the pelagic aquatic
environments are not either strictly homogeneous, particularly because of microaggregates that can lead to the presence
of anoxic zones in microscale within an overall aerobic water
column. More generally, the structure of MFW in aquatic
(pelagic and benthic) and terrestrial environments reflects
the adaptation of microorganisms against physical, chemical,
and biological environmental forcing factors. Understanding
the mechanisms responsible for these adaptations to changing
environmental conditions is a major challenge for the
coming years.
506
B. Mostajir et al.