BIOACCUMULATION AND BIOMAGNIFICATION - Marine Pollution: What Everyone Needs to Know - page 144

special proteins (metal-binding proteins, e.g., metallothio-
neins, MT) that can bind metals and make them unavailable.
MTs are low molecular weight, heat-stable proteins that bind
high amounts of metals and prevent the metals from doing
damage. There is a limit to how much the MTs can store, how-
ever. Some animals, such as crustaceans, can also store met-
als in the form of insoluble metal-rich granules or deposits
in tissues. Thus, metals may be toxic and available or may be
unavailable, depending on where they are located and what
they are bound to. In those animals that can accumulate high
concentrations of metals without major effects, most of the
metal is in nonavailable form such as MT and granules, which
may explain their ability to survive in highly contaminated
environments. The site of storage can also affect how much
metal will get trophically transferred to predators, as found
by William Wallace and Samuel Luoma. A predator would be
better off eating prey with its metals tied up in granules rather
than bound to MT, which is more trophically available to the
predator.
How are organic contaminants taken up by organisms?
After taking up foreign hydrocarbons organisms may metab-
olize, store, and/or excrete them. Small zooplankton take up
organic contaminants from the water, while benthic species
accumulate them mostly from sediments, which generally
have higher levels than the water. Concentrations of chlori-
nated organics (e.g., PCBs, DDT) in mollusks can exceed those
in nearby sediments tenfold. Chlorinated organic chemicals
tend to be metabolized slowly if at all, and are bioaccumulated
and stored in the liver (or hepatopancreas, the comparable
organ in invertebrates), or in blubber of marine mammals.
These chemicals accumulate in fatty tissues, including yolk
and liver, and biomagnify up the food web. In general, the
most important factor determining an animal's concentrations
of these chemicals is the trophic level, followed by the lipid