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

Biological essential and non-essential heavy metals

Heavy metals consists both biological essential as not biological essential metals. Biological essential heavy metals include copper (Cu), iron (Fe) and zinc (Zn). Iron for instance forms an essential part of hemoglobin, a protein in our blood which transports oxygen 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. The order of toxicity (from low to high) has been suggested as follows: cobalt, aluminum, chrome, lead, nickel, zinc, copper, cadmium and mercury. In marine environments however, three metals are of primary concern: lead, mercury and cadmium.

Marine input

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 add 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 in the environment.

Toxicity

When present above threshold concentrations, all heavy metals can be toxic. This threshold concentration depends on the metal, the animal species, but also on the environment, which determines their availability. 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 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. However, these abilities tend to differ between species, making some species more tolerant than others. It should also be considered that although detoxifying mechanisms exist, when environmental concentrations are to high, these can be unsufficient and the organisms will start showing effects. However, areas which experienced very high heavy metal pollution for centuries, like Restronguet Creek in England has for copper, may contain populations which are more tolerant to the pollution than populations of the same species in other regions. There appears to have been selection towards copper resistance in this region.

Behavior in the ecosystem

Heavy metals introduced in the marine ecosystem are mostly concentrated in coastal areas, near densely populated and industrialized regions. Heavy metals are usually associated to particles. These particles are often very small, and can therefore stay in solution for a very long time. Nevertheless they will end up in the sediments, therefore concentrations in the sediments are often 10 to 100 times higher than those in solution. In the sediments, these particles may form an important secondary source of contamination, even when the primary source has disappeared.

Very few studies have shown biomagnification of heavy metals. Only for mercury (as methylmercury) and arsenicum because they have high affinity for organic tissues.