Bioaccumulation of Metals in Tissues of Marine Animals, Part I: the Role and Impact of Heavy Metals on Organisms

Heavy metals contribute to the anthropogenic contamination of marine ecosystems. Some of them are essential to the life processes of organisms; others are toxic, even at low concentrations. They penetrate organisms via food, respiratory pathways or the skin. The extent to which metals penetrate organisms is measured by bioconcentration and bioaccumulation factors and also by their transport between organisms at different trophic levels of an ecosystem. These factors define the course of metal bioaccumulation in the environment or in organisms, their organs, and tissues. Our paper discusses the role of heavy metals in organisms at different levels of the trophic pyramid (food web) and their influence on life processes. The levels of some elements, like Zn and Cu, are regulated by metabolic processes and are important constituents of enzymes and other compounds. Other such elements, e.g. Hg, Pb, and Cd, are toxic and may adversely affect DNA and enzymatic processes, hence interfere with life processes, even though organisms possess mechanisms for the detoxification and excretion of metals. An important role in metal detoxification is performed by metallothionein (MT), which binds to toxic metals, thereby preventing organisms from harmful effects. Information about the increasing level of a metal is transmitted by the MT gene as it initiates expression regulated by zinc in order to bind MT with the metal. Elements like cadmium, copper, or mercury have a greater affinity for ligands than zinc, and will tend to displace it at MT binding sites. Structures from which zinc has been displaced take part in detoxification, thereby limiting the toxicity of such metals as Cd, Cu, or Hg.