The biology of heavy metals in the sea

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Abstract
Heavy metals are toxic to marine organisms above a threshold availability yet many are essential to metabolism at lower concentrations. Heavy metal concentrations are raised locally in coastal waters but levels in open oceans have stabilised during the earth's history and oceanic dissolved concentrations are typically controlled not by the rate of entry of metals from land but by the rate of their removal from solution via geochemical and more usually biological processes, the metals being accumulated by marine organisms. Uptake of heavy metals from solution by marine organisms depends on the availability of free metal ions, often a low proportion of total metal present as a result of complexation with organic and inorganic ligands. Nevertheless hydrophobic metal compounds are taken up relatively quickly and metal uptake may involve the binding of the hydrophilic free metal ion with a ligand for passage across the cell membrane. Metals accumulated by marine organisms may either be maintained in a metabolically available form with the concurrent possibility of toxic effects or may be detoxified. Possible detoxification mechanisms include the formation of insoluble metal‐rich granules to be stored or excreted, and the binding of some heavy metals with metallothioneins.

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