Chemistry Reference
In-Depth Information
(mainly hard class A ions that do not bind to carriers
and are transported against concentration gradients)
(Philips and Rainbow, 1993; Simkiss and Taylor, 1989).
In biological fl uids, ligand exchange to form thermo-
dynamically stable complexes with multidentate bio-
logical molecules (chelation) prevents back diffusion.
Apparently, the soft class B metals can be transported
against a concentration gradient transferring the ion
along a series of ligands with increasing affi nity along
a thermodynamic gradient, the metals fi nally being
intracellularly sequestered by the ligand with the
highest affi nity. This can lead to storage or subsequent
transport, as, for example, seen for metallothionein.
Some metals are absorbed from solution as hydrated
ions, explaining the increasing bioavailability and tox-
icity of, for example, Cu and Cd, at decreasing salinity
(Mclusky
et al
., 1986). Besides the uptake of hydrated
metal species, inorganic and organic neutral metal
complexes with increased lipophilicity (e.g., CdCl
2
,
HgCl
2
, Hg(CH
3
)
2
) may be absorbed by direct diffusion
(Sanders and Riedel, 1998) ).
Metals with hydrated ionic diameters similar to
those of essential metals can be taken up by the ATP-
driven pumps and calcium channels, thus Pb, Cd, Mn,
and Co can be absorbed as metabolic analogs by means
of the Ca pumps (Markich and Jeffree, 1994). The rele-
vance of this mechanism depends on the organism, for
molluscs and malacostracan crustaceans with exten-
sive calcium uptake for shell and exoskeleton forma-
tion (Bondgaard and Bjerregaard, 2005; Norum
et al
.,
2005), this route may determine total uptake.
uptake from water seems to be the dominant route for
accumulation of cadmium for example (Fischer, 1988;
Riisgard
et al
., 1987).
For most predators, food seems to be the dominant
source for metal uptake (Luoma and Rainbow, 2005),
that is, for cadmium in fi sh (Pentreath, 1977) and crabs
(Bjerregaard
et al
., 2005).
Except for mercury (Section 7.10), there is no con-
sistent tendency that metal concentrations increase
along aquatic food chains.
4 DEFENSE AGAINST AND STORAGE
OF METALS
Various mechanisms protecting against metal tox-
icity exist among different species, several acting by
accumulating metals in inert granules that either are
excreted or deposited for the lifetime of the organism.
Other strategies involve heat-stable metal chelating pro-
teins like metallothioneins (MTs) and metallothionein-
like proteins that occur in eucaryotes (invertebrates,
vertebrates, and plants) and have been characterized
in a variety of species (Vijver
et al
., 2004).
Speciation of metals, homeostatic strategies for
essential metals, and protective strategies for toxic met-
als at the cellular level in organisms may be as follows:
1. Free or complexed ionic species: Mixed aquo,
chloride, phosphate, or carbonate complexes.
Complexes with metabolic compounds (amino
acids, carbohydrate, metabolites, etc.).
2. Bound in active centers of LMW peptides or
functional proteins: Hemoglobin, hemocyanin,
zinc fi nger proteins, cytochromes, carbonic
anhydrase, superoxide dismutase.
3. Bound to transport or sequestration proteins:
Metallothionein, ferritin, transferrin.
4. Bound in lysosomal vesicles as metal granules:
Precipitated in extracellular deposits. Hair,
feathers, exoskeleton, mineral deposits, residual
bodies.
5. Bound to cellular constituents potentially caus-
ing harm: Enzyme poisoning, binding to DNA,
or ion channels.
3.2 Metal Transport in Aquatic
Food Chains
Aquatic organisms may take up metals either
directly from solution or from ingested food. Phyto-
plankton organisms do, of course, accumulate metals
directly from solution (Whitfi eld, 2001), but for most
pelagic fi lter-feeding herbivores (such as, e.g., copep-
ods) metals in the ingested food seem to be the major
source (Mason
et al
., 1996; Reinfelder and Fisher, 1991);
differences between metals and species do, however,
occur (Wang, 2002). It has been shown that metals taken
up into the cytosol of the unicellular algae and subse-
quently becoming associated with proteins (Reinfelder
and Fisher, 1991) are assimilated to a higher degree in
copepods than metal bound to structural elements in
the algae (i.e., cell walls). Recent investigations (Ng
et
al
., 2005), however, show that this phenomenon can-
not be generalized across all species (phytoplankton
as well as herbivores) and metals. In some benthic
fi lter feeders such as the blue mussel
Myrtillus edulis,
There is high sequence homology between MTs from
different species, and they apparently all have similar
functions as mammalian metallothioneins (i.e., stor-
age, transport, and compartmentalization of essential
metals and detoxifi cation of toxic metals). Also, their
metal-binding profi les and gene expression regulation
are similar to that of mammalian metallothionein (i.e.,
binding Zn, Cu, Cd, Hg, and induction by these and
