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International Journal of Marine Science 2013, Vol.3, No.30, 238-243
http://ijms.sophiapublisher.com
239
complexes, hydrated ion, and stable organic or
inorganic colloids particularly hurnics (Bubb et al.,
1991). Levels of Zn in coastal areas and estuaries are
often much higher than oceanic concentrations
(Bruland et al., 1979) in Bryan and Langston, 1992.
The most available and abundant form of Zn is the
free ion Zn
2+
. Due to its comparatively high solubility,
Zn tends to exist in the dissolved forms (Bryan and
Langston, 1992; Bubb et al., 1991).
Though Cu and Zn are essential nutrients, it can cause
toxicity when not present in sufficient concentrations.
Excess amounts of those toxic when they interact with
certain biomolecules in an organism. These features,
along with the fact that metals persist as inorganic
forms in environmental sinks (e.g., soil & sediments)
are cycled through the biotic components of an
ecosystem.
In sediments, metals also partition among different
types of ligands associated with the various
components of the particulate material. Metals are
distributed predominantly among sites on iron oxides,
manganese oxides and various types of organic
materials in oxidized sediments (Jenne, 1968; Jenne,
1977; Luoma and Davis, 1983; Davies-Colley et al.,
1984).
Table 1 shows how availability and mobility of
trace metals in the environment.
Table 1 Availability and mobility of trace metals
Metal species and associations
Mobility
Exchangeable (dissolved) actions
High. Changes in major cationic composition (e.g. estuarine environment) may
cause a release due to ion exchange
Metals associated with Fe-Mn oxides
Medium. Changes in redox conditions may cause a release but some metals
precipitate if sulfide mineral present is insoluble
Metals associated with organic matter
Medium/High. With time, decomposition/oxidation of organic matter occurs
Metals associated with sulfide minerals
Strongly dependent on environmental. Under oxygen-rich conditions,
oxidation of sulfide minerals leads to release of metals
Metals fixed in crystalline phase
Low. Only available after weathering or decomposition.
Deposit/detritus feeding animals are exposed to metals
both in solution and through ingestion of
metal-enriched particulate material (Luoma, 1983).
Thus knowledge of the geochemical reactions of
metals in both water and sediments is critical to
understanding what controls metal bioavailability.
Interactions between water and sediment also are
important. For example, total concentrations of most
metals in sediments are orders of magnitude higher
than concentrations in solution. Thus, proportionally
small changes in metal exchange between water and
sediments may profoundly influence total concentrations
in solution, and affect that route of exposure.
Metals may bind to a wide variety of molecules in the
organism. Metals can bind to biomolecules that are
essential to cellular function (e.g. enzymes, structural
proteins), alter their function, and cause toxicity. In
some cases, metals bind to metallothioneins or
phytochelatins, cysteine-rich compounds, or to other
ligands that can help the organism regulate the metal
within cells and detoxify the metal by preventing the
binding to receptors that may result in toxicity. Metals
may also be precipitated in phosphate or sulfide
bodies within cells, thereby sequestering them and
preventing mobility and subsequent toxicity.
Over the past decade, significant scientific advances
have been made in addressing metals bioavailability in
sediments in relation to their toxicity to benthic
organisms e.g. molluscs. Metals such as Ag, Cd, Zn,
Cu and Hg are generally bioaccumulated to much
higher concentrations and vary very markedly among
different species of marine bivalves. Bivalves species
(mussels, oysters, scallops), especially, have been used
as biological indicator
organisms to monitor marine
environmental pollution by heavy metals and
chemicals due to their own properties of inhabitation.
Heavy metal concentrations in soft bodies have been
discussed in all of the investigations concerned
(Moloukhia and Sleem, 2011). Among the many
bivalve species mentioned, scallops can accumulate
metals to very high concentrations. Belcheva et al
(2006) found that Cd concentrations in the digestive
glands of the Japanese scallop
Patinopecten yessoensis
from the Sea of Japan were as high as 150 μg/g dry
weights, and increased significantly with age.