Examples of copper essentiality and copper tolerance

The essentiality of copper to living organisms was recognized relatively late. It was not until 1928 when Hart et al. (cited in IPCS, 1998) showed that copper was needed for the formation of red blood cells in rats fed a milk-based diet. The anaemia in the rats could be corrected by the addition to the diet of ash from animal or vegetable sources. It was later demonstrated that the hydrogen sulfide precipitate from the ash, containing copper sulfide, was responsible for the recovery. Copper is also essential for the utilization of iron in the formation of haemoglobin.

The reason for copper's essentiality is its specific incorporation into a large number of enzymatic and structural proteins. Copper plays an important role in oxidation/reduction enzyme activities, and this is a consequence of its ability to function as an electron transfer intermediate. Thus, copper is a constituent of enzymes involved in, for example, cellular respiration, free radical defence, neurotransmitter function, connective tissue biosynthesis, and cellular iron metabolism. Copper also plays an essential role in the activation and repression of gene transcription and has a lot of additional function in the cell (IPCS, 1998).

In the "Environmental Health Criteria" for copper (IPCS, 1998), it is clearly emphasized that the above-mentioned aspects must be taken into consideration when assessing the impact of copper on biological systems. In order to illustrate the complexity of the role of essential metals in natural ecosystems, the following points may be cited with regard to Cu from the IPCS document:

"The adverse effects of copper must be balanced against its essentiality. Copper is an essential element for all biota, and care must be taken to ensure that copper nutritional needs of organisms are met. At least 12 major proteins require copper as an integral part of their structure. It is essential for the utilization of iron in the formulation of haemoglobin, and most crustaceans and molluscs possess the copper-containing haemocyanin as their main oxygen-carrying blood protein. In plants copper is a component of several enzymes involved in carbohydrate, nitrogen and cell wall metabolism."

"Because copper is an essential element, procedures to prevent toxic levels of copper should not incorporate safety factors that result in recommended concentrations being below natural levels."

"Tolerance to copper has been demonstrated in the environment for phytoplankton, aquatic and terrestrial invertebrates, fish and terrestrial plants. Tolerance mechanisms which have been proposed in plants include binding of metal to cell wall material, presence of metal-tolerant enzymes, complex formation with organic acids with subsequent removal to the vacuole, and binding to specialized thiol-rich proteins orphytochelatins."

"Wood (1983) found that naturally occurring marine phytoplankton populations show a tolerance to added cupric ions which far exceeds the physiological limits of phytoplankton cultures grown in chemically defined media. The tolerance appears to be due to regulation of bioavailability of added copper by an abundance of copper-complexing agents. Coastal phytoplankton were less sensitive than continental shelf or oceanic communities. The toxicity of copper correlated more with copper-complexing capacity than with biotic species composition or community structure."

"Copper-rich granules have been reported to occur in a wide variety of invertebrates inhabiting copper-polluted habitats.. . . In addition, copper deposits also appear in (the tissues of bladder-wrack (Fucus sp.) and in copper-tolerant isolates of the green alga Scenedesmus. It was concluded) that the occurrence of these inclusions could be regarded as a detoxifying mechanism because they were absent in the non-tolerant strains."

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