Ecophysiology of trace elements in marine vertebrates

Metals occur naturally in ecosystems: they are released into soils by weathering on rocks, or released into the air by volcanic eruptions. To survive, living things have evolved systems to keep concentrations of free metal ions in their cells below certain limits: an elaborate system of transporters to ensure proper distribution between internal organs, cells and intracellular compartments, and detoxification systems to remove metals in concentrations greater than those required by the organism. Large marine vertebrates in the higher levels of the trophic network have developed physiological strategies to prevent high concentrations of metals resulting in sometimes lethal damage to the organism itself. The mechanisms may be regulated excretion , chemical transformation, accumulation in particular compartments, and the ability to form bonds with compounds such as proteins, polysaccharides, and amino acids.
Research on these organisms provides important information on the behavior of trace elements, offers a wealth of information regarding the diffusion of heavy metals, in the environment and in biological systems and along the food chain, as well as regarding the physiological mechanisms that limit their excessive accumulation. For example, with regard to mercury, an element that by biomagnification is found at high concentrations in marine predators, both the total levels and the proportion of organic to inorganic mercury vary according to the different tissues and zoological groups examined (Mammals, Fish, Reptiles). In marine mammals, the decrease in the relative proportions of organic mercury from prey to predator was considered as evidence of the existence of other detoxification processes that are based on the biotransformation of MeHg into less toxic inorganic "storage" forms with the formation of Hg-Se crystals in livers of Odontocete Cetaceans and Pinnipeds. This process of compartmentalization in the liver with the formation of tiemmannite (Hg-Se) granules is considered to be the physiological adaptation to a naturally mercury- and selenium-rich diet as an alternative to normal excretion systems.
Research on this topic began in the 1980s with investigations of tissue levels of Hg and other trace elements in mammals and seabirds in the Mediterranean Sea and continued with comparative studies aimed at identifying ecophysiological differences among different classes of marine vertebrates that might explain the different storage and excretion capacity. Attention has been directed both to histological observation for the evaluation of
ultrastructural differences at the level of storage organs such as the liver, but also to the identification of a different the in vitro biochemical behavior of methylmercury in the main compartment of transport and exchange between the various organs and tissues namely the blood.