Environmental Engineering Reference
In-Depth Information
can't be metabolized, so they tend to be stored to varying
degrees. Essential metals, for example copper (Cu) and zinc
(Zn), tend to be regulated to some optimum concentration,
above which the animal will excrete the excess. Potentially
toxic metals must either be excreted or stored in a nontoxic
form if they are not to cause damage. Toxicity occurs when the
concentration exceeds the amount that can be stored in these
nontoxic forms or excreted. Metals tend to be stored in spe-
cific tissues such as the liver, which generally has the highest
concentration of Cd, Zn, Cu, Pb, and Cr. However, significant
levels may also be found in muscle, which has implications
for human consumption of edible species. For example, some
edible crustaceans, such as blue crabs, store Hg largely in
muscle. Since crustaceans must periodically molt their shell,
depositing metals in the shell prior to molting it is a useful
way to get rid of contaminants. In corals, the symbiotic algae
that live in the coral tissue (zooxanthellae) accumulate greater
metal concentrations than the skeleton or living coral tissue
itself. The loss of zooxanthellae during stress (bleaching) may
help reduce metal levels in the corals. Mollusks secrete a lot of
mucus when stressed, which is also a way to get rid of metals.
Fishes take up metals from water or food, but the type of food
influences the degree of trophic transfer to the fish. Most of the
Hg in fish tissues is in the form of meHg, which biomagnifies
through the food chain and accumulates over time, reaching
highest levels in old large carnivorous fish.
Where and how are metals stored in organisms?
The location inside a cell where metals are placed strongly
affects their toxicity since metals associated with sensitive
organelles and enzymes can impair cell functioning. Metals
tend to bind to proteins and may prevent the protein from
functioning normally. For example, metals can bind to active
sites of essential enzymes, which are proteins, changing their
shapes and inhibiting their activities. However, there are
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