Coping with variations naturally
All life needs zinc, Human beings have more than three hundred enzymes which can not function without zinc. Specifically, zinc is needed for the transmission of nerve signals. Hereditary characteristics could not be passed on without zinc - as it supports the structure of DNA. What applies to human beings also applies to all other forms of life. In spite of the extremely wide variations in the external concentration, the con-centration of zinc within all organisms is generally constant, at approximately 20 mg Zn/kg. Organisms themselves have mechanisms which maintain these zinc concentrations at the required Ievel. To achieve this, phytoplankton in the deep oceans must concentrate zinc obtained from water more than one million times. For algae in the surface water of a country such as the Netherlands, this factor is only around 1,000. An aquatic organism can there-fore overcome certain variations in external zinc concentrations in its environment. If the external concentration changes by a factor of 100, the organisms’ internal con-centration only changes by a factor of just 2 to 3. Significantly larger changes in the external concentration might not be accommodated by an organism without adverse effects. These larger variations may be so great that the organisms would have to adapt to them over several generations. So, there is a specific range of variations in zinc concentrations within which organisms flourish best.

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Fig. 1: Optimal range of zinc concentrations for aquatic life. Natural concen-trations of zinc vary very widely. In the deep oceans, zinc levels are less than one-thousandth of that in surface waters. Surface waters in an urea such as the Netherlands have a more or less constant zinc constant, and life has adapted to these conditions during the course of their evolution. Too little zinc causes deficiency.

Above this range, damage might be caused and below it damage can be caused through a lack of zinc -as a “deficiency” arises. It follows that the boundaries below which zinc deficiency occurs vary with the external zinc concentration. For zinc-resistant organisms, zinc deficiency arises at zinc concentrations of less than 50 ug Zn / litre; for freshwater organisms below 5 ug Zn / litre; for marine organisms below 0.5 ug Zn/ litre and for organisms in the deep oceans, below 001 ug Zn /litre. These zinc deficiency boundaries form the Iower limits of an optimal range of living conditions for aquatic life. The upper limits are approximately 100 times hig-her than the deficiency boundaries (in all cases). This means that all organisms have a natural range of existence within which the organisms can best flourish.

For life in surface waters, the deficiency boundary lies at approximately 2 ug Zn/litre and the upper boundary lies at 2OOug Zn/litre. The midpoint of this concentration band should coincide with the mean value of the natural background zinc toncentration. The organisms which are at home in this environment are already genetically adapted to this toncentration. For example, the mean natural back-ground toncentration for surface waters in the Netherlands lies at approximately 20 ug Zn/litre which is the mean logarithmic value between 2 and 200 pg Zn/Iitre. This determines the optimal range of living conditions for freshwater organisms in those waters.

Summary
Despite considerable variations in the natural zinc levels in the aquatic environment, organisms have adjusted and are naturally able to cape with such variations. This, along with the obvious environmental benelits of protecting against corrosion, has provided fut-ther assurance to users of zinc of the environmental compatibility of processes and products involving zinc.