6
Benthic Respiration in Aquatic Sediments
Bo Thamdrup and Donald E. Canfield
Introduction
Sediments make up an important compartment in
the energy flow of aquatic ecosystems. On the continental shelves, 10 to 50% of the primary production reaches the sea-floor, most of which is mineralized there (Suess 1980; Wollast 1991; Canfield
1993). Through the balance between retention and
release of nutrients, benthic mineralization influences the nutrient dynamics of the ecosystem (e.g.,
Howarth 1988), and through their regulatory effect
on the burial of residual carbon and associated nutrients, mineralization processes may also affect the
global cycles of these elements (Canfield 1994;
Hedges and Kei11995).
The mineralization of organic matter, that is, its
biologically catalyzed degradation to inorganic carbon and nutrients, proceeds in sediments through
several different pathways involving different organisms. The generalized scheme involves extracellular hydrolysis, fermentative transformations,
and respiratory oxidation. In addition to the respiration with oxygen by bacteria and zoobenthos, nitrate, manganese oxides, iron oxides, and sulfate
may serve as terminal electron acceptors in bacterial respiration (Table 6.1). Only organisms that respire with oxygen or nitrate are able to carry out
the complete oxidation of hydrolysates to carbon
dioxide (C02), while the anaerobic bacteria depend
on coexisting fermentative organisms that transform the hydrolysates to secondary substrates such
as acetate and hydrogen. Some CO 2 may be released during fermentations (Schink 1988) but as
the reduced fermentation products are rapidly consumed, an overall balance of CO 2 production and
86
respiration is expected (S0rensen et al. 1981;
Parkes et al. 1989). In the absence of inorganic
electron acceptors, organic matter is mineralized
through methanogenesis without any net oxidation
of carbon (see Table 6.1).
The relative importance of the different mineralization pathways in a given sediment depends on
factors such as bottom water chemistry, sedimentation rate, and bulk sediment composition. Locations have been found where any of the pathways
in Table 6.1 dominates carbon degradation (reviewed by J0rgensen 1983; Henrichs and Reeburgh
1987; Canfield 1993; see also Aller 1990; Canfield
et al. 1993a). Sediments typically exhibit a vertical
redox zonation where the oxidants are depleted in
the order oxygen > nitrate > manganese oxide
> iron oxide> sulfate (e.g., Froelich et al. 1979;
Canfield et al. 1993a). However, the exclusion of
less favorable pathways of respiration is often not
complete, and a considerable spatial overlap between respiratory types is observed (Canfield and
Des Marais 1991; J0rgensen and Bak 1991; Canfield et al. 1993a; Brandes and Devol 1995).
Parallel to carbon oxidation, the electron acceptors are also consumed in the reoxidation of the
reduced inorganic metabolites produced during
mineralization. These include reduced manganese
and iron, sulfide, and methane (see Table 6.1), as
well as ammonia. Each reduced species can potentially be oxidized with an electron acceptor higher
in the redox sequence listed above, and many of
these pathways appear to function in sediments
through either abiotic reaction, lithotrophic bacterial catalysis, or both. This adds considerable complexity to the benthic processes (Fig. 6.1). Reoxi-
Benthic Respiration in Aquatic Sediments
Bo Thamdrup and Donald E. Canfield
Introduction
Sediments make up an important compartment in
the energy flow of aquatic ecosystems. On the continental shelves, 10 to 50% of the primary production reaches the sea-floor, most of which is mineralized there (Suess 1980; Wollast 1991; Canfield
1993). Through the balance between retention and
release of nutrients, benthic mineralization influences the nutrient dynamics of the ecosystem (e.g.,
Howarth 1988), and through their regulatory effect
on the burial of residual carbon and associated nutrients, mineralization processes may also affect the
global cycles of these elements (Canfield 1994;
Hedges and Kei11995).
The mineralization of organic matter, that is, its
biologically catalyzed degradation to inorganic carbon and nutrients, proceeds in sediments through
several different pathways involving different organisms. The generalized scheme involves extracellular hydrolysis, fermentative transformations,
and respiratory oxidation. In addition to the respiration with oxygen by bacteria and zoobenthos, nitrate, manganese oxides, iron oxides, and sulfate
may serve as terminal electron acceptors in bacterial respiration (Table 6.1). Only organisms that respire with oxygen or nitrate are able to carry out
the complete oxidation of hydrolysates to carbon
dioxide (C02), while the anaerobic bacteria depend
on coexisting fermentative organisms that transform the hydrolysates to secondary substrates such
as acetate and hydrogen. Some CO 2 may be released during fermentations (Schink 1988) but as
the reduced fermentation products are rapidly consumed, an overall balance of CO 2 production and
86
respiration is expected (S0rensen et al. 1981;
Parkes et al. 1989). In the absence of inorganic
electron acceptors, organic matter is mineralized
through methanogenesis without any net oxidation
of carbon (see Table 6.1).
The relative importance of the different mineralization pathways in a given sediment depends on
factors such as bottom water chemistry, sedimentation rate, and bulk sediment composition. Locations have been found where any of the pathways
in Table 6.1 dominates carbon degradation (reviewed by J0rgensen 1983; Henrichs and Reeburgh
1987; Canfield 1993; see also Aller 1990; Canfield
et al. 1993a). Sediments typically exhibit a vertical
redox zonation where the oxidants are depleted in
the order oxygen > nitrate > manganese oxide
> iron oxide> sulfate (e.g., Froelich et al. 1979;
Canfield et al. 1993a). However, the exclusion of
less favorable pathways of respiration is often not
complete, and a considerable spatial overlap between respiratory types is observed (Canfield and
Des Marais 1991; J0rgensen and Bak 1991; Canfield et al. 1993a; Brandes and Devol 1995).
Parallel to carbon oxidation, the electron acceptors are also consumed in the reoxidation of the
reduced inorganic metabolites produced during
mineralization. These include reduced manganese
and iron, sulfide, and methane (see Table 6.1), as
well as ammonia. Each reduced species can potentially be oxidized with an electron acceptor higher
in the redox sequence listed above, and many of
these pathways appear to function in sediments
through either abiotic reaction, lithotrophic bacterial catalysis, or both. This adds considerable complexity to the benthic processes (Fig. 6.1). Reoxi-