44
of decomposition, microorganisms produce secondary organic matter, including
various cellular components, extracellular polymers, and organic metabolic waste.
The fraction of OC carried over to secondary organic products as the proportion of
OC in the initial substrate consumed by microorganisms is referred to as the conversion factor. The conversion factor varies among substrates, microorganisms, and
available electron acceptors and ranges between 10% and 50% for many aerobic
bacteria (Del Giorgio and Cole 1998). The secondary organic products of microorganisms may be utilized by other microorganisms, which produce their own secondary products based on their own conversion factor. Thus, the overall process of
early diagenesis of degradable organic matter can be regarded as a microbial food
web. Among the OC initially supplied to the sediment, the degradable fraction is
processed through this diagenetic food web and eventually either remineralized to
inorganic carbon or temporarily stored as microbial secondary products (e.g.,
Lomstein et al. 2009; Cyle et al. 2016), while inherently recalcitrant fractions tend
to remain intact in the sediment for a long time.
An important factor influencing the stability of organic molecules in the sediment is the role of the mineral matrix as an adsorbent for organic matter (Keil and
Mayer 2014). The majority of OC in marine sediments exists in close association
with the surface of sediment mineral particles. This association is demonstrated by
the fact that most of the OC in bulk coastal sediments is associated with relatively
heavy density fractions (e.g., >2.0 g cm
−3
) when sediment grains are sorted by density (Bock and Mayer 2000; Arnarson and Keil 2007). Since the density range of
most natural organic matter is 1.0–1.5 g cm
−3
and those of clastic aluminosilicates
and biogenic carbonates are 2.3–2.7 g cm
−3
and 2.7–2.9 g cm
−3
, respectively, the
accumulation of OC in the density fraction >2.0 g cm
−3
indicates that most OC is
bound to clastic and/or carbonate sediment minerals. Another line of indirect
evidence for the close association between OC and mineral surfaces is the positive
correlation between the OC concentration and specific surface area (SSA) commonly
found for continental shelf sediments (Keil et al. 1994a; Mayer 1994; Bergamaschi
et al. 1997) and coastal carbonate sediments (Suess 1973). The amount of OC
preserved per unit surface area of sediment particles is referred to as OC loading.
OC loading converges within a narrow range of ~60 μmol C m
−2
for typical
continental shelf sediments (Mayer 1994) and a slightly higher range for carbonate
sediments (Suess 1973). The accumulation of OC in heavier (>2.0 g cm
−3
) density
fractions (Fig. 2.4) and the close correlation between OC and SSA (Fig. 2.5; average OC loading of 61.5 μmol m
−2
) have been demonstrated for temperate seagrass
(Zostera marina) meadow sediments as well as coastal sediments near these meadows (Miyajima et al. 2017). Thus, it can be assumed that the sediment in seagrass
meadows shares a common OC sequestration mechanism with pelagic continental
shelf sediments.
The above empirical relationships imply that (i) OC can be preserved in sediments via adsorption to mineral surfaces even if it is not inherently recalcitrant, and
(ii) the maximal amount of adsorbed OC per unit area of mineral surface is limited
based on the mineralogy of the sediment matrix. As a corollary of (ii), it may be
expected that when the rate of OC supply to the sediment exceeds the supply rate of
T. Miyajima and M. Hamaguchi
of decomposition, microorganisms produce secondary organic matter, including
various cellular components, extracellular polymers, and organic metabolic waste.
The fraction of OC carried over to secondary organic products as the proportion of
OC in the initial substrate consumed by microorganisms is referred to as the conversion factor. The conversion factor varies among substrates, microorganisms, and
available electron acceptors and ranges between 10% and 50% for many aerobic
bacteria (Del Giorgio and Cole 1998). The secondary organic products of microorganisms may be utilized by other microorganisms, which produce their own secondary products based on their own conversion factor. Thus, the overall process of
early diagenesis of degradable organic matter can be regarded as a microbial food
web. Among the OC initially supplied to the sediment, the degradable fraction is
processed through this diagenetic food web and eventually either remineralized to
inorganic carbon or temporarily stored as microbial secondary products (e.g.,
Lomstein et al. 2009; Cyle et al. 2016), while inherently recalcitrant fractions tend
to remain intact in the sediment for a long time.
An important factor influencing the stability of organic molecules in the sediment is the role of the mineral matrix as an adsorbent for organic matter (Keil and
Mayer 2014). The majority of OC in marine sediments exists in close association
with the surface of sediment mineral particles. This association is demonstrated by
the fact that most of the OC in bulk coastal sediments is associated with relatively
heavy density fractions (e.g., >2.0 g cm
−3
) when sediment grains are sorted by density (Bock and Mayer 2000; Arnarson and Keil 2007). Since the density range of
most natural organic matter is 1.0–1.5 g cm
−3
and those of clastic aluminosilicates
and biogenic carbonates are 2.3–2.7 g cm
−3
and 2.7–2.9 g cm
−3
, respectively, the
accumulation of OC in the density fraction >2.0 g cm
−3
indicates that most OC is
bound to clastic and/or carbonate sediment minerals. Another line of indirect
evidence for the close association between OC and mineral surfaces is the positive
correlation between the OC concentration and specific surface area (SSA) commonly
found for continental shelf sediments (Keil et al. 1994a; Mayer 1994; Bergamaschi
et al. 1997) and coastal carbonate sediments (Suess 1973). The amount of OC
preserved per unit surface area of sediment particles is referred to as OC loading.
OC loading converges within a narrow range of ~60 μmol C m
−2
for typical
continental shelf sediments (Mayer 1994) and a slightly higher range for carbonate
sediments (Suess 1973). The accumulation of OC in heavier (>2.0 g cm
−3
) density
fractions (Fig. 2.4) and the close correlation between OC and SSA (Fig. 2.5; average OC loading of 61.5 μmol m
−2
) have been demonstrated for temperate seagrass
(Zostera marina) meadow sediments as well as coastal sediments near these meadows (Miyajima et al. 2017). Thus, it can be assumed that the sediment in seagrass
meadows shares a common OC sequestration mechanism with pelagic continental
shelf sediments.
The above empirical relationships imply that (i) OC can be preserved in sediments via adsorption to mineral surfaces even if it is not inherently recalcitrant, and
(ii) the maximal amount of adsorbed OC per unit area of mineral surface is limited
based on the mineralogy of the sediment matrix. As a corollary of (ii), it may be
expected that when the rate of OC supply to the sediment exceeds the supply rate of
T. Miyajima and M. Hamaguchi