Air Quality Criteria for Lead Volume II of II - (NEPIS)(EPA) - US ...

of Pb from solution to the extracellular and intracellular compartments of 3 species of aquatic
bryophytes. Relative to the 6 metals tested, Pb was found to accumulate to the largest degree in
the extracellular compartments of all 3 bryophytes. The extracellular metals were defined as
those that are incorporated into the cell wall or are found on the outer surface of the plasma
membrane (i.e., adsorbed) (Vázquez et al., 1999). Intracellular metals were defined as metals
introduced into the cell through a metabolically controlled process.
Arai et al. (2002) examined the effect of growth on the uptake and elimination of trace
metals in the abalone Haliotos. They reported that older abalones had generally lower whole
body concentrations of heavy metals than did younger, rapidly growing individuals. During the
rapid growth of juveniles, the organism is less able to distinguish between essential (e.g., Zn),
and nonessential metals (e.g., Pb). Once they reach maturity, they develop the ability to
differentiate these metals. Li et al. (2004) reported a similar response in zebra fish embryolarvae.
Li et al. (2004) suggested that mature physiological systems are not developed in the
embryo-larvae to handle elevated concentrations of metals. Therefore, metals are transported
into the body by facilitated diffusion. Both the zebra fish and juvenile abalone demonstrate a
rapid accumulation strategy followed by a low rate of loss as described above. There are
insufficient data available to determine whether this phenomenon is true for other aquatic
organisms.
Growth rates are generally thought to be an important consideration in the comparison of
Pb levels in individuals of the same species. The larger the individual the more the metal content
is diluted by body tissue (Rainbow, 1996).
Once Pb is absorbed, it may sequester into varying parts of the organism. Calcium
appears to have an important influence on Pb transfer. For example, Pb uptake and retention in
the skin and skeleton of coho salmon was reduced when dietary Ca was increased (Varanasi and
Gmur, 1978). Organic Pb compounds tend to accumulate in lipids, and are taken up and
accumulated in fish more readily than inorganic Pb compounds (Pattee and Pain, 2003).
Given the complexities of metal uptake in natural systems, a model incorporating some of
the factors mentioned above is desirable. The EPA’s Environmental Research Laboratory in
Duluth, Minnesota developed a thermodynamic equilibrium model, MINTEQ that predicts
aqueous speciation, adsorption, precipitation, and/or dissolution of solids for a defined set of
environmental conditions (MacDonald et al., 2002; Playle, 2004). Although not specifically
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