It should be noted that the behavior of predators is also an
important factor to consider when assessing the control
systems of benthic trophic webs. So, feeding of gastropods
and insect larvae (e.g., Trichoptera: Psychomyiidae) in
marine and freshwater systems is linked to their movements
and life cycle, respectively. Their feeding activity on
biofilms often induces drastic effects (e.g., Lawrence et al.
2002; Majdi et al. 2012a) and, for a matter of fact, is called
“trophic raid.” After a trophic raid, a new colonization cycle
can start allowing the growth of a new biofilm when the
macrofaunal grazing pressure is low.
Predation pressure exerted by macrobenthic invertebrates
often avoids an accumulation of detritic OM as observed in
microbial mats. Microbial mats mainly occur in an area where
predation pressure is low, i.e., in extreme environments
(hyperhaline waters, high and low temperatures, high hydrodynamics). Moreover, it must be noticed that during the
Precambrian, i.e., before eukaryotic metazoan occurrence,
cyanobacterial mats were largely widespread (cf. Chap. 4).
Production of benthic photosynthetic microorganisms in
intertidal superficial sediments is mainly submitted to “bottom-up” control processes. This means that their production
is mainly limited by nutrient availability and light. However,
studies have shown that biomass and production of benthic
microorganisms are tightly linked to the rate of the grazing
and bioturbation activities exerted by meiofaunal consumers
(Buffan-Dubau and Carman 2000; Mathieu et al. 2007,
2012b). Grazing pressure is not the main control process;
however, production of benthic microphytes is indirectly
enhanced by meiofaunal activity modifying “bottom-up”
control potential effect (i.e., enhance light penetration and
nutrient regeneration) in coastal superficial sediments and in
biofilms. The sediment reworking resulting from meiofaunal
activity can also increase the porosity and the solute transport
rates in sediments and microbial aggregates, improving
availability of dissolved nutrients for photoautotrophic
microorganisms (Aller and Aller 1992; Mathieu et al. 2007).
The feeding activity of meiofauna induces indirect
effects. The most significant are described for nematode
activity. They produce copious mucous secretions that in
some cases attract bacteria, stimulate their growth, and
modify the bacterial community composition and diversity
in river superficial sediments (Moens et al. 2006).
13.4.3 Factors Controlling Terrestrial Microbial
Food Webs
Given the fact that almost all of the primary production of
terrestrial ecosystems come from higher plants, it is obviously necessary to consider plants as central to the regulation
of soil organisms and their interactions, at least as important
as environmental factors and disturbances.
13.4.3.1 Regulation by Environmental Factors
and Perturbations
As in aquatic environments, the microbial food web of soil
depends on both “top-down” and “bottom-up” controlling
factors. It is generally accepted that bacteria are primarily
regulated by predation, while fungi are mainly regulated by
the resources. However, there are many cases of “top-down”
control of fungi. For example, the presence of protozoa and
nematodes in the rhizosphere of trees reduces the colonization of tree roots by mycorrhizal fungi (Wardle 2002).
Diversity within a given group in the soil depends on many
factors such as resource availability, temperature, pH, or
water status, as well as disturbances such as setting culture
or rapid and extreme climate events.
Some studies show that an increase in resources is
accompanied by a stimulation of microbial functional diversity of soil (Degens and Harris 2000). This observation goes
against the theory developed for the plants that the increase
in resources should lead to a reduction in biodiversity. In the
study by Degens and Harris (2000), diversity is measured
in vitro by the ability of microorganisms in the soil to use
substrates made, and it is likely that the observed results
reflect further metabolic abilities of bacteria relative to
fungi. The bacteria are indeed able to grow rapidly on
the substrates and are, therefore, less regulated by the
competition that groups with slower growth like fungi. Yet,
it is generally recognized that the diversity of soil fauna is
maintained even when conditions become more favorable.
Yeates and Bongers (1999) found that the diversity of
nematodes (assuming that this group is poorly regulated by
competition) gradually decreases with an increase in the
concentration of heavy metals and is not reduced by dietary
calcium ground. More generally, the diversity of many
groups in the soil is more important on a humus of Fagus
sylvatica where relatively favorable conditions prevail for
the activity of organisms (pH: 4.3–6.8, high cation exchange
capacity) relative to the humus in the same vegetation where
the conditions are much less favorable (pH: 3–4, with low
cation exchange capacity). Most studies tend to show that
the diversity of decomposers does not decrease with the
improvement of environmental conditions. These organisms
do not seem, from this point of view, to follow the models
most frequently observed in plants. In contrast, and as noted
earlier in this paragraph, certain groups such as fungi can be
controlled by the competition. Thereby, Egerton-Warburton
and Allen (2000) show that the addition of N reduces the
diversity of fungi. This result suggests that the increased
resources promote competitive exclusion*.
13.4.3.2 Regulation by Biotic Factors
Studies on the biotic regulation of soil organisms and their
activities are scarce, with the notable exception of the
interactions between plants and microorganisms. The
13 Microbial Food Webs in Aquatic and Terrestrial Ecosystems
505
important factor to consider when assessing the control
systems of benthic trophic webs. So, feeding of gastropods
and insect larvae (e.g., Trichoptera: Psychomyiidae) in
marine and freshwater systems is linked to their movements
and life cycle, respectively. Their feeding activity on
biofilms often induces drastic effects (e.g., Lawrence et al.
2002; Majdi et al. 2012a) and, for a matter of fact, is called
“trophic raid.” After a trophic raid, a new colonization cycle
can start allowing the growth of a new biofilm when the
macrofaunal grazing pressure is low.
Predation pressure exerted by macrobenthic invertebrates
often avoids an accumulation of detritic OM as observed in
microbial mats. Microbial mats mainly occur in an area where
predation pressure is low, i.e., in extreme environments
(hyperhaline waters, high and low temperatures, high hydrodynamics). Moreover, it must be noticed that during the
Precambrian, i.e., before eukaryotic metazoan occurrence,
cyanobacterial mats were largely widespread (cf. Chap. 4).
Production of benthic photosynthetic microorganisms in
intertidal superficial sediments is mainly submitted to “bottom-up” control processes. This means that their production
is mainly limited by nutrient availability and light. However,
studies have shown that biomass and production of benthic
microorganisms are tightly linked to the rate of the grazing
and bioturbation activities exerted by meiofaunal consumers
(Buffan-Dubau and Carman 2000; Mathieu et al. 2007,
2012b). Grazing pressure is not the main control process;
however, production of benthic microphytes is indirectly
enhanced by meiofaunal activity modifying “bottom-up”
control potential effect (i.e., enhance light penetration and
nutrient regeneration) in coastal superficial sediments and in
biofilms. The sediment reworking resulting from meiofaunal
activity can also increase the porosity and the solute transport
rates in sediments and microbial aggregates, improving
availability of dissolved nutrients for photoautotrophic
microorganisms (Aller and Aller 1992; Mathieu et al. 2007).
The feeding activity of meiofauna induces indirect
effects. The most significant are described for nematode
activity. They produce copious mucous secretions that in
some cases attract bacteria, stimulate their growth, and
modify the bacterial community composition and diversity
in river superficial sediments (Moens et al. 2006).
13.4.3 Factors Controlling Terrestrial Microbial
Food Webs
Given the fact that almost all of the primary production of
terrestrial ecosystems come from higher plants, it is obviously necessary to consider plants as central to the regulation
of soil organisms and their interactions, at least as important
as environmental factors and disturbances.
13.4.3.1 Regulation by Environmental Factors
and Perturbations
As in aquatic environments, the microbial food web of soil
depends on both “top-down” and “bottom-up” controlling
factors. It is generally accepted that bacteria are primarily
regulated by predation, while fungi are mainly regulated by
the resources. However, there are many cases of “top-down”
control of fungi. For example, the presence of protozoa and
nematodes in the rhizosphere of trees reduces the colonization of tree roots by mycorrhizal fungi (Wardle 2002).
Diversity within a given group in the soil depends on many
factors such as resource availability, temperature, pH, or
water status, as well as disturbances such as setting culture
or rapid and extreme climate events.
Some studies show that an increase in resources is
accompanied by a stimulation of microbial functional diversity of soil (Degens and Harris 2000). This observation goes
against the theory developed for the plants that the increase
in resources should lead to a reduction in biodiversity. In the
study by Degens and Harris (2000), diversity is measured
in vitro by the ability of microorganisms in the soil to use
substrates made, and it is likely that the observed results
reflect further metabolic abilities of bacteria relative to
fungi. The bacteria are indeed able to grow rapidly on
the substrates and are, therefore, less regulated by the
competition that groups with slower growth like fungi. Yet,
it is generally recognized that the diversity of soil fauna is
maintained even when conditions become more favorable.
Yeates and Bongers (1999) found that the diversity of
nematodes (assuming that this group is poorly regulated by
competition) gradually decreases with an increase in the
concentration of heavy metals and is not reduced by dietary
calcium ground. More generally, the diversity of many
groups in the soil is more important on a humus of Fagus
sylvatica where relatively favorable conditions prevail for
the activity of organisms (pH: 4.3–6.8, high cation exchange
capacity) relative to the humus in the same vegetation where
the conditions are much less favorable (pH: 3–4, with low
cation exchange capacity). Most studies tend to show that
the diversity of decomposers does not decrease with the
improvement of environmental conditions. These organisms
do not seem, from this point of view, to follow the models
most frequently observed in plants. In contrast, and as noted
earlier in this paragraph, certain groups such as fungi can be
controlled by the competition. Thereby, Egerton-Warburton
and Allen (2000) show that the addition of N reduces the
diversity of fungi. This result suggests that the increased
resources promote competitive exclusion*.
13.4.3.2 Regulation by Biotic Factors
Studies on the biotic regulation of soil organisms and their
activities are scarce, with the notable exception of the
interactions between plants and microorganisms. The
13 Microbial Food Webs in Aquatic and Terrestrial Ecosystems
505