Heavy metals

Definition of heavy metals: A heavy metal is a member of an ill-defined subset of elements that exhibit metallic properties, which would mainly include the transition metals, some metalloids, lanthanides, and actinides. One definition is metals with a density greater than 5 g/cm3. This is the common definition for heavy metals, other definitions can be discussed in the article

Notes

Heavy metals consists both biological essential as not biological essential metals. Biological essential heavy metals include copper (Cu), iron (Fe) and zinc (Zn). Irons for instance form an essential part of hemoglobine, a protein in our blood which transports oxigen from the longs to other tissues. Although biological essential heavy metals are necessary, they become toxic at high concentrations. Not biological essential heavy metals include lead (Pb), mercury (Hg), cadmium (Cd) and tin (Sn). They can be tolerated at low levels, but become toxic as well at higher concentrations. In marine environments three metals are of primary concern: lead, mercury and cadmium.

Heavy metals are a natural part of the earth crust. Heavy metals enter the sea usually through riverine influx (after weathering and erosion of rocks), atmospheric deposition (dust particles e.g. from volcano's)and anthropogenic activities. Humans at both to the riverine disposion (waste water of factories) and atmosferic depostion (cars, factories,...). Heavy metals are stable and can't be broken down, which makes it easy for them to accumulate.

When present above threshold concentrations, all heavy metals can be toxic. The exposure of marine organisms to toxic levels of metal contaminants can cause damage to tissue, inability to regenerate damaged tissue, growth inhibition, damage to DNA. Although most marine organisms marine organisms tend to accumulate heavy metals from the environment, they are capable to store, remove (trough feces, eggs, or molting) or detoxify (through metallothioneins) many of them.

Heavy metals introduced in the marine ecosystem are mostly concentrated in coastal areas, near densely populated and industrialized regions. They are ofthen associated with the sediments, which form an important secondary source of contamination, even when the primary source has disappeared.