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

in sufficient amounts to chelate the metal ion, the enzyme is deactivated (Morelli and Scarano,
2001).
Morelli and Scarano (2001) studied phytochelatin synthesis and stability in the marine
diatom Phaeodactylum tricornutum in the presence of Pb. They found that when metal exposure
was alleviated, significant cellular Pb-phytochelatin complex content was lost. Their findings
support a hypothesis of vacuolarization proposed for higher plants (Zenk, 1996), in which metal-
phytochelatin complexes are actively transported from the cytosol to the vacuole, where they
undergo rapid turnover. Zenk (1996) suggested that the complex dissociates, and the metal-free
peptide is subsequently degraded. Morelli and Scarano (2001) proposed concomitant occurrence
of phytochelatin synthesis and release during metal exposure, as a coincident detoxification
mechanism in P. tricornutum.
Aquatic Invertebrates
Like plants and protists, aquatic animals detoxify Pb by preventing it from being
metabolically available, though their mechanisms for doing so vary. Invertebrates use
lysosomal-vacuolar systems to sequester and process Pb within glandular cells (Giamberini and
Pihan, 1996). They also accumulate Pb as deposits on and within skeletal tissue (Knowlton
et al., 1983; Anderson et al., 1997; Boisson et al., 2002), and some can efficiently excrete Pb
(Vogt and Quinitio, 1994; Prasuna et al., 1996).
Boisson et al. (2002) used radiotracers to evaluate the transfer of Pb into the food pathway
of the starfish Asterias rubens as well as its distribution and retention in various body
compartments. Boisson et al. (2002) monitored Pb elimination after a single feeding of Pbcontaminated
molluscs and found that Pb was sequestered and retained in the skeleton of the
starfish, preventing it from being metabolically available in other tissues. Elimination (as
percent retention in the skeleton) was found to follow an exponential time course. Elimination
was rapid at first, but slowed after 1 week, and eventually stabilized, implying an infinite
biological half-life for firmly bound Pb. Results of radiotracer tracking suggest that Pb migrates
within the body wall from the organic matrix to the calcified skeleton. From there, the metal is
either absorbed directly or adsorbed on newly produced ossicles (small calcareous skeletal
structures), where it is efficiently retained as mineral deposition and is not metabolically active
(Boisson et al., 2002).
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