fluctuations in the concentrations of their electron donors
and acceptors (Schulz and Jorgensen 2001).
In non-permeable sediments as muds, the movement of
the interstitial water is extremely slow and the solute transport mainly depends on molecular diffusion. The characteristic diffusion time for O 2 to travel 1 μm by diffusion is equal
to 0.3 ms, while the time to travel 1 cm is equal to 7 h. The
microhabitat becomes anoxic when the diffusive delivery is
lower than the local demand. When comparing soils and
sediments, it is important to consider that the diffusion
coefficient of O 2 in air exceeds its value in water by a factor
of 10
4 . Hence, the diffusive transport of O 2 is much more
efficient in soils that are not saturated with water than in
sediments and in the waterlogged soils of wetlands. As a
result, in many benthic systems, anoxic zones occur in close
proximity to oxic microhabitats, which implies that functional groups of prokaryotes tend to stratify along redox
gradient (cf. Sect. 14.2.4). The heterotrophic archaea and
bacteria that live in the anoxic parts of the sediments function in concert based on their trophic interactions that are
relatively well known. Hence, the methanogenic archaea and
the sulfate-reducing bacteria are only capable to catabolize
dihydrogen gas and a small number of low-weight
compounds and thus cannot mineralize all the OM in the
anoxic sediment. These prokaryotes are organized in consortia together with fermenting and acetogenic bacteria, which
produce the intermediate metabolites (low-weight fatty
acids, acetic acid, and dihydrogen) that are used by the
sulfate-reducing bacteria and the methanogenic archaea (cf.
Sect. 14.2.4). The products of the anaerobic metabolism, i.e.,
NH 4
+
, Mn
2+ , Fe
2+ , H 2 S, and CH 4 , can diffuse into the oxic
habitats of the sediment or directly into the water column,
where they can be used as electron donors for chemolithoautotrophic prokaryotes.
The direct strong coupling between aerobic and anaerobic
microbial processes is, therefore, another characteristic feature of benthic systems. The coupling between nitrification
and denitrification represents a nice example of such a strong
coupling. Osmotrophy is also particularly widespread
among the unicellular eukaryotes thriving in sediments
where they may benefit from the excretion by and lysis of
other microorganisms as well as from their hydrolytic
activities which results in hydrolysis of extracellular
polymers.
Macrofauna (i.e., macrobenthic invertebrates) are divided
into different functional feeding groups according to their
trophic behavior (Tachet et al. 2010). Hence, macrofauna
can be classified (Fig. 13.5) according to their trophic behavior (functional feeding groups). “Scrapers” collect microbial
patches by removing them from their supports before ingestion (e.g., gastropods). “Gathering collectors” (e.g., chironomid larvae) ingest superficial particles and their associated
microbiota. Thus, these two groups potentially feed on both
microalgae, cyanobacteria, and other bacteria as well as their
associated EPS. The benthic “filtering collectors” (e.g.,
Simuliidae larvae) feed by filtering the overlaying water.
Thus, these invertebrates feed on plankton and constitute a
trophic link between benthic and pelagic communities.
However, benthic microorganisms that are resuspended in
the overlaying water are also used as food sources by the
benthic “filtering collectors.” Meiofauna actively move
through sediments and biofilms and may consume
microalgae such as diatoms, bacteria, and EPS (Mathieu
et al. 2007; Buffan-Dubau and Carman 2000; Majdi et al.
2012b). Nematodes, for example, are divided into four feeding groups based on morphological characteristics (Moens
et al. 2006). “Deposit feeders or swallowers” feed on bacteria and unicellular eukaryotes that are swallowed whole.
Epistrate feeders (tear-and-swallow feeders having no
teeth) feed on bacteria, unicellular eukaryotes, diatoms,
and other microalgae. “Chewers” (having a small tooth)
feed on unicellular eukaryotes and smaller meiofauna. “Suction feeders” (voluminous buccal cavity with teeth and
denticles) are potentially omnivorous using algae, fungi,
plants, and animals. Heterotrophic unicellular eukaryotes
used microalgae, cyanobacteria and other bacteria, and
DOM as trophic sources by phagocytosis and osmotrophy,
respectively. Most of benthic ciliates are bacterivores; however, they are also detritivores and often even selective
algivorous grazers feeding on diatoms of selected size
(Giere 2009).
Faunal activity in sediments is not only linked to their
trophic role since benthic invertebrates profoundly alter the
distribution of sediment particles, solutes, and microbial
communities, through sediment reworking and irrigation.
They particularly burrow galleries and actively ventilate
them, contributing to enhance sediment irrigation. Thus,
faunal activity directly affects the decomposition and
remineralization of organic matter in sediments (e.g.,
Mathieu et al. 2007; Pischedda et al. 2012). These whole
activities are described using the term “bioturbation.” The
burrowing activity of benthic macrofauna induces a very
complex spatial structure of sediments with relatively welled
oxygenated micro-horizons such as the worm burrows and
anoxic zones (Pischedda et al. 2012). Some meiofaunal
invertebrates (e.g., nematodes) have also an intensive
burrowing activity modifying the oxygen turnover in
biofilms (Riemann and Helmke 2002; Mathieu et al. 2007).
Within permeable sediments, which are composed of
sand, gravels, and pebbles, advection currents, which percolate through these sediments by hydrodynamic forces, can
facilitate the transport of dissolved O 2 and other solutes.
Detritus plays a key role in benthic ecosystems. Sediments
covered by aquatic vegetation can be compared to terrestrial
soil systems. The larger herbivores can be represented by
Anatidae. Nevertheless, a large part of the macrophyte
498
B. Mostajir et al.
and acceptors (Schulz and Jorgensen 2001).
In non-permeable sediments as muds, the movement of
the interstitial water is extremely slow and the solute transport mainly depends on molecular diffusion. The characteristic diffusion time for O 2 to travel 1 μm by diffusion is equal
to 0.3 ms, while the time to travel 1 cm is equal to 7 h. The
microhabitat becomes anoxic when the diffusive delivery is
lower than the local demand. When comparing soils and
sediments, it is important to consider that the diffusion
coefficient of O 2 in air exceeds its value in water by a factor
of 10
4 . Hence, the diffusive transport of O 2 is much more
efficient in soils that are not saturated with water than in
sediments and in the waterlogged soils of wetlands. As a
result, in many benthic systems, anoxic zones occur in close
proximity to oxic microhabitats, which implies that functional groups of prokaryotes tend to stratify along redox
gradient (cf. Sect. 14.2.4). The heterotrophic archaea and
bacteria that live in the anoxic parts of the sediments function in concert based on their trophic interactions that are
relatively well known. Hence, the methanogenic archaea and
the sulfate-reducing bacteria are only capable to catabolize
dihydrogen gas and a small number of low-weight
compounds and thus cannot mineralize all the OM in the
anoxic sediment. These prokaryotes are organized in consortia together with fermenting and acetogenic bacteria, which
produce the intermediate metabolites (low-weight fatty
acids, acetic acid, and dihydrogen) that are used by the
sulfate-reducing bacteria and the methanogenic archaea (cf.
Sect. 14.2.4). The products of the anaerobic metabolism, i.e.,
NH 4
+
, Mn
2+ , Fe
2+ , H 2 S, and CH 4 , can diffuse into the oxic
habitats of the sediment or directly into the water column,
where they can be used as electron donors for chemolithoautotrophic prokaryotes.
The direct strong coupling between aerobic and anaerobic
microbial processes is, therefore, another characteristic feature of benthic systems. The coupling between nitrification
and denitrification represents a nice example of such a strong
coupling. Osmotrophy is also particularly widespread
among the unicellular eukaryotes thriving in sediments
where they may benefit from the excretion by and lysis of
other microorganisms as well as from their hydrolytic
activities which results in hydrolysis of extracellular
polymers.
Macrofauna (i.e., macrobenthic invertebrates) are divided
into different functional feeding groups according to their
trophic behavior (Tachet et al. 2010). Hence, macrofauna
can be classified (Fig. 13.5) according to their trophic behavior (functional feeding groups). “Scrapers” collect microbial
patches by removing them from their supports before ingestion (e.g., gastropods). “Gathering collectors” (e.g., chironomid larvae) ingest superficial particles and their associated
microbiota. Thus, these two groups potentially feed on both
microalgae, cyanobacteria, and other bacteria as well as their
associated EPS. The benthic “filtering collectors” (e.g.,
Simuliidae larvae) feed by filtering the overlaying water.
Thus, these invertebrates feed on plankton and constitute a
trophic link between benthic and pelagic communities.
However, benthic microorganisms that are resuspended in
the overlaying water are also used as food sources by the
benthic “filtering collectors.” Meiofauna actively move
through sediments and biofilms and may consume
microalgae such as diatoms, bacteria, and EPS (Mathieu
et al. 2007; Buffan-Dubau and Carman 2000; Majdi et al.
2012b). Nematodes, for example, are divided into four feeding groups based on morphological characteristics (Moens
et al. 2006). “Deposit feeders or swallowers” feed on bacteria and unicellular eukaryotes that are swallowed whole.
Epistrate feeders (tear-and-swallow feeders having no
teeth) feed on bacteria, unicellular eukaryotes, diatoms,
and other microalgae. “Chewers” (having a small tooth)
feed on unicellular eukaryotes and smaller meiofauna. “Suction feeders” (voluminous buccal cavity with teeth and
denticles) are potentially omnivorous using algae, fungi,
plants, and animals. Heterotrophic unicellular eukaryotes
used microalgae, cyanobacteria and other bacteria, and
DOM as trophic sources by phagocytosis and osmotrophy,
respectively. Most of benthic ciliates are bacterivores; however, they are also detritivores and often even selective
algivorous grazers feeding on diatoms of selected size
(Giere 2009).
Faunal activity in sediments is not only linked to their
trophic role since benthic invertebrates profoundly alter the
distribution of sediment particles, solutes, and microbial
communities, through sediment reworking and irrigation.
They particularly burrow galleries and actively ventilate
them, contributing to enhance sediment irrigation. Thus,
faunal activity directly affects the decomposition and
remineralization of organic matter in sediments (e.g.,
Mathieu et al. 2007; Pischedda et al. 2012). These whole
activities are described using the term “bioturbation.” The
burrowing activity of benthic macrofauna induces a very
complex spatial structure of sediments with relatively welled
oxygenated micro-horizons such as the worm burrows and
anoxic zones (Pischedda et al. 2012). Some meiofaunal
invertebrates (e.g., nematodes) have also an intensive
burrowing activity modifying the oxygen turnover in
biofilms (Riemann and Helmke 2002; Mathieu et al. 2007).
Within permeable sediments, which are composed of
sand, gravels, and pebbles, advection currents, which percolate through these sediments by hydrodynamic forces, can
facilitate the transport of dissolved O 2 and other solutes.
Detritus plays a key role in benthic ecosystems. Sediments
covered by aquatic vegetation can be compared to terrestrial
soil systems. The larger herbivores can be represented by
Anatidae. Nevertheless, a large part of the macrophyte
498
B. Mostajir et al.