with sulfur bonded in organo-sulfur compounds (such as
some proteins and amino acids, thiocarboxylic acids,
thiols, thioethers), to soluble sulfate and sulfide ions
(SO 4
2À , S
2À
). The bacterial reduction of sulfate to sulfide
in sediments, seawater, and pore water is viewed to be
a key process in the oceanic sulfur cycle and is responsible
for the oxidation and mineralization of the organic matter
buried below the oxic and suboxic zones of sediments
(Jørgensen, 1982). The process of transformation of
inorganic forms of sulfur radicles and ions to organically
bound sulfur, and their conversion after death and decay
of the organisms to the environment as inorganic ions or
even as precipitated minerals (such as sulfides), is part of
the complex sulfur cycle (Canfield and Raiswell, 1999).
Formation of minerals as exoskeletons or
endoskeletons (biomineralization)
Many organisms inhabiting estuaries, in environments
from the riverine extreme to the marine mouth, precipitate
and use inorganic minerals such as calcite, Mg calcite, aragonite, amorphous silica, magnetite, and various phosphatic minerals and compounds as their exoskeletons
and as endoskeletons for protection, for strengthening,
and as an aid to feeding, orientation, and navigation. Such
mineralization is commonly termed by some authors as
“biomineralization,” i.e., the process by which living
organisms produce mineral products (Simkiss and Wilbur,
1989). In the arena of biologically mediated mineral
growth, and its utilization internally or externally by an
organism, the distinction between “mineral” and
“organic” becomes blurred. For instance, calcite precipitated from mineral-enriched waters is considered to be
an inorganic mineral, but calcite is also formed by an
organism as shell which is chemically, mineralogically,
and crystallographically similar to the inorganic form.
The problem is particularly complex where hard organic
structures and skeletons formed by organisms are
comprised of interlayered organic material and crystalline
substances. For example, chitin, a polysaccharide, is a
long-chain polymer of N-acetyl-D-glucosamine and,
though organic in composition and origin, has
a crystallographic structure (Carlstrom, 1957; Raabe
et al., 2007), occurring as nanocrystallites. At the nanoscale it is a crystal and essentially an organic “mineral”
(or a biomineral). Further, as an endoskeleton and exoskeleton, chitin commonly is structurally compounded/
interlaced or interlayered with proteins and inorganic
minerals such as calcite and phosphates. Chitin comprises
the exoskeletons of arthropods such as crustaceans
(e.g., crabs) and insects, the radulae of molluscs, and the
beaks and internal shells of cephalopods.
Probably the best known development of minerals by
aquatic organisms, including estuarine organisms, at the
macroscopic scale, is the precipitation of exoskeletons of
calcite and/or aragonite by molluscs, evident as the familiar shells of bivalves and gastropods (Bathurst, 1975;
Rhoads and Lutz, 1980). Here, the molluscan organisms,
by use of their mantle, facilitate precipitation of an
exoskeleton of calcite (e.g., oysters), or of aragonite
(e.g., Paphies, the pipi shell, or Donax, the bean clam),
or mixed layers of calcite and aragonite. The precipitation
of the carbonate minerals can be complex: it involves
a sequential relationship and interactions between the
animal, soft-tissue organic compounds, hard organic
structures, an early amorphous stage of carbonate
development, and the final stage of calcium carbonate
crystal development nucleated on earlier stage crystals
(Addadi et al., 2006).
Grazing molluscs, such as chitons, also can precipitate
the mineral magnetite (Fe 3 O 4 , as a mineral hard enough
to score or scratch some common rock minerals,
especially calcite) in their radulae as teeth for use in scraping algae, diatoms, and biofilms off rocks. Molluscs also
use apatitic calcium phosphate, or limonite, or amorphous
silica in their radulae (Lowenstam and Weiner, 1989;
Macey et al., 1994; Liddiard et al., 2004). More recent
studies have shown magnetite, lepidocrocite, goethite,
apatite, and amorphous precursor phases to be also present
in some species (Saunders et al., 2009). Predatory
molluscs such as Naticidae and Conidae use a chitinous
radula for drilling into the carbonate exoskeletons of other
invertebrate fauna (Carriker, 1961).
Plants also precipitate calcite or aragonite as crystallites
in their tissue. The best known are the green algae
Acetabularia and Halimeda and Charophytes (Bathurst,
1975). They precipitate these carbonate minerals as finegrained crystals, which after death of the plant may be
released into the environment as mud-sized free crystallites,
contributing to the mud fraction of estuarine sediments.
At the microscopic scale, the best known development
of minerals by estuarine organisms is the precipitation of
crystalline calcite, Mg calcite, and aragonite by foraminifera (Bathurst, 1975) and the precipitation of amorphous
silica (SiO 2 ) by diatoms (Kröger et al., 1999). Diatom biogenic silica is mainly amorphous, hydrated SiO 2
containing a small proportion of organic macromolecules,
which are thought to control silica deposition and
nanopatterning (Sumper and Kroger, 2004).
A variety of other estuarine invertebrate fauna, besides
the molluscs, precipitate minerals or hard crystalline or
amorphous substances, as endoskeletons or as exoskeletons, in the form of carbonate minerals (usually as
CaCO 3 ), chitin, complex carbonate/phosphate minerals,
or amorphous silica. These include the crustacea (chitin,
calcite, and calcium phosphate; Dennell, 1960; Warner,
1977; Rhoads and Lutz, 1980; Raabe et al., 2007), other
arthropods (chitin), sponges (biogenic silica), bryozoans
(calcite and aragonite), and echinoderms (calcite).
Outside the realm of invertebrate fauna, vertebrate fauna
in estuaries, ranging from sharks, to fish, to reptiles (e.g.,
crocodilians), to marine mammals, and seabirds produce
endoskeletons, mineral feeding structures, and other structures of bone and teeth which, in terms of mineral content,
range from calcium carbonate to calcium phosphate to magnetite (Lees, 1989; Glimcher, 2006; Pasteris et al., 2008).
440
MINERALIZATION
some proteins and amino acids, thiocarboxylic acids,
thiols, thioethers), to soluble sulfate and sulfide ions
(SO 4
2À , S
2À
). The bacterial reduction of sulfate to sulfide
in sediments, seawater, and pore water is viewed to be
a key process in the oceanic sulfur cycle and is responsible
for the oxidation and mineralization of the organic matter
buried below the oxic and suboxic zones of sediments
(Jørgensen, 1982). The process of transformation of
inorganic forms of sulfur radicles and ions to organically
bound sulfur, and their conversion after death and decay
of the organisms to the environment as inorganic ions or
even as precipitated minerals (such as sulfides), is part of
the complex sulfur cycle (Canfield and Raiswell, 1999).
Formation of minerals as exoskeletons or
endoskeletons (biomineralization)
Many organisms inhabiting estuaries, in environments
from the riverine extreme to the marine mouth, precipitate
and use inorganic minerals such as calcite, Mg calcite, aragonite, amorphous silica, magnetite, and various phosphatic minerals and compounds as their exoskeletons
and as endoskeletons for protection, for strengthening,
and as an aid to feeding, orientation, and navigation. Such
mineralization is commonly termed by some authors as
“biomineralization,” i.e., the process by which living
organisms produce mineral products (Simkiss and Wilbur,
1989). In the arena of biologically mediated mineral
growth, and its utilization internally or externally by an
organism, the distinction between “mineral” and
“organic” becomes blurred. For instance, calcite precipitated from mineral-enriched waters is considered to be
an inorganic mineral, but calcite is also formed by an
organism as shell which is chemically, mineralogically,
and crystallographically similar to the inorganic form.
The problem is particularly complex where hard organic
structures and skeletons formed by organisms are
comprised of interlayered organic material and crystalline
substances. For example, chitin, a polysaccharide, is a
long-chain polymer of N-acetyl-D-glucosamine and,
though organic in composition and origin, has
a crystallographic structure (Carlstrom, 1957; Raabe
et al., 2007), occurring as nanocrystallites. At the nanoscale it is a crystal and essentially an organic “mineral”
(or a biomineral). Further, as an endoskeleton and exoskeleton, chitin commonly is structurally compounded/
interlaced or interlayered with proteins and inorganic
minerals such as calcite and phosphates. Chitin comprises
the exoskeletons of arthropods such as crustaceans
(e.g., crabs) and insects, the radulae of molluscs, and the
beaks and internal shells of cephalopods.
Probably the best known development of minerals by
aquatic organisms, including estuarine organisms, at the
macroscopic scale, is the precipitation of exoskeletons of
calcite and/or aragonite by molluscs, evident as the familiar shells of bivalves and gastropods (Bathurst, 1975;
Rhoads and Lutz, 1980). Here, the molluscan organisms,
by use of their mantle, facilitate precipitation of an
exoskeleton of calcite (e.g., oysters), or of aragonite
(e.g., Paphies, the pipi shell, or Donax, the bean clam),
or mixed layers of calcite and aragonite. The precipitation
of the carbonate minerals can be complex: it involves
a sequential relationship and interactions between the
animal, soft-tissue organic compounds, hard organic
structures, an early amorphous stage of carbonate
development, and the final stage of calcium carbonate
crystal development nucleated on earlier stage crystals
(Addadi et al., 2006).
Grazing molluscs, such as chitons, also can precipitate
the mineral magnetite (Fe 3 O 4 , as a mineral hard enough
to score or scratch some common rock minerals,
especially calcite) in their radulae as teeth for use in scraping algae, diatoms, and biofilms off rocks. Molluscs also
use apatitic calcium phosphate, or limonite, or amorphous
silica in their radulae (Lowenstam and Weiner, 1989;
Macey et al., 1994; Liddiard et al., 2004). More recent
studies have shown magnetite, lepidocrocite, goethite,
apatite, and amorphous precursor phases to be also present
in some species (Saunders et al., 2009). Predatory
molluscs such as Naticidae and Conidae use a chitinous
radula for drilling into the carbonate exoskeletons of other
invertebrate fauna (Carriker, 1961).
Plants also precipitate calcite or aragonite as crystallites
in their tissue. The best known are the green algae
Acetabularia and Halimeda and Charophytes (Bathurst,
1975). They precipitate these carbonate minerals as finegrained crystals, which after death of the plant may be
released into the environment as mud-sized free crystallites,
contributing to the mud fraction of estuarine sediments.
At the microscopic scale, the best known development
of minerals by estuarine organisms is the precipitation of
crystalline calcite, Mg calcite, and aragonite by foraminifera (Bathurst, 1975) and the precipitation of amorphous
silica (SiO 2 ) by diatoms (Kröger et al., 1999). Diatom biogenic silica is mainly amorphous, hydrated SiO 2
containing a small proportion of organic macromolecules,
which are thought to control silica deposition and
nanopatterning (Sumper and Kroger, 2004).
A variety of other estuarine invertebrate fauna, besides
the molluscs, precipitate minerals or hard crystalline or
amorphous substances, as endoskeletons or as exoskeletons, in the form of carbonate minerals (usually as
CaCO 3 ), chitin, complex carbonate/phosphate minerals,
or amorphous silica. These include the crustacea (chitin,
calcite, and calcium phosphate; Dennell, 1960; Warner,
1977; Rhoads and Lutz, 1980; Raabe et al., 2007), other
arthropods (chitin), sponges (biogenic silica), bryozoans
(calcite and aragonite), and echinoderms (calcite).
Outside the realm of invertebrate fauna, vertebrate fauna
in estuaries, ranging from sharks, to fish, to reptiles (e.g.,
crocodilians), to marine mammals, and seabirds produce
endoskeletons, mineral feeding structures, and other structures of bone and teeth which, in terms of mineral content,
range from calcium carbonate to calcium phosphate to magnetite (Lees, 1989; Glimcher, 2006; Pasteris et al., 2008).
440
MINERALIZATION