Some useful definitions

It is obvious to anyone that life has evolved in the presence of metals, some of which - the essential trace metals - have become incorporated into metabolic processes crucial to the survival, growth and reproduction of organisms. As a consequence, organisms have developed various mechanisms for the uptake and excretion, regulation and detoxification of essential metals, mechanisms which - in many cases - are functioning also for non-essential metals. In their summary statement on the "Essentiality of Metals", Janssen and Muyssen (2001) point out that an element is considered essential when:

• it is consistently present in all healthy living tissues within a biological family, whereby tissue concentrations from species to species should not vary widely;

• deficiency symptoms are noted as a result of depletion or removal of the element, and dissapear when the element is returned to the tissue; and

• the deficiency symptoms are attributed to a distinct biochemical effect at the molecular level.

For essential metals, each species has an optimal range of concentrations required for normal metabolic functioning. This "Optimal Concentration Range for Essential Elements" (OCEE), termed by Van Assche et al. (1997), is determined by both the natural (bioavailable) concentrations of the essential metal in the species' habitat and the homeostatic capacity of the species, allowing it to regulate its internal metal concentration to an optimal level. Homeostasis is a concept describing the phenomenon that organisms try to maintain their activities by controlling the concentrations of essential and non-essential elements inside their tissues (see, e.g., McGrath, 2001).

An organism's homeostatic capacity has limits, however, and when the external concentrations of an essential metal becomes too high or too low, regulation will fail, and toxicity or deficiency, respectively, will occur (Janssen and Muyssen, 2001). Organisms have developed a variety of homeostatic control mechanisms to regulate internal metal concentrations, which may vary considerably between species or groups of species. Brix and DeForest (2000) listed the following main categories of regulation:

• active regulation, where stable tissue concentrations are maintained by reduced uptake or by the excretion of metal at rates similar to the intake rate;

• storage, i.e. binding of metals in various types of complexes for long-term storage in a detoxified form;

Simultaneously with the organism's own control mechanisms, the environment also contributes to the apparent "no-change" internal metal concentration through physical and chemical processes known as "buffering" (McGrath, 2001). As the concentration of metals increase, especially in soils and sediments, various solids or dissolved materials bind the metals chemically or even physically inside their structure.

Organisms can also try to resist toxicity by mechanisms known as acclimation, increased tolerance or genetic adaptation to increased metal exposure in the environment. These concepts are defined below, according to McGrath (2001):

Acclimation is a non-heritable trait and is seen as the response of an individual to stress. Even without genetic change, an individual (or even a population) may be resistant enough to continue to function normally upon increased metal exposure, a phenomenon referred to as "phenotypic plasticity". This type of non-genetic resistance can easily be lost when the exposure to metals decreases again.

Tolerance is an organism's ability to maintain homeostasis when exposed to a particular array of environmental factors.

Adaptation is a special kind of (metal) tolerance, meaning that the traits have evolved through natural selection in response to high metal exposure in the environment and can therefore be passed on through the genes to subsequent generations. Genetic tolerance or adaptation cannot be lost in the individual. However, it may render the individual less fit to survive in non-contaminated environments.

Some examples of acclimation and adaptation are given by Janssen and Muyssen (2001):

It has been demonstrated that test organisms (e.g., water fleas) cultured in media with low essential metal concentrations (such as Cu and Zn) exhibit an overall decreased fitness (cf. Caffrey and Keating, 1997). Moreover, organisms cultured at these low metal concentrations acclimate to these conditions and become more sensitive to stress, including metal exposure (Muyssen and Janssen, 2001). Conversely, organisms cultured in media with elevated metal concentrations may become less sensitive. Considering that laboratory toxicity test data are used for derivation of water quality criteria and the establishment of a PNEC (Predicted-No-Effect-Concentration) in risk assessments, these acclimation-induced sensitivity shifts may affect the ecological relevance and effectiveness of the test procedures. It has been stressed several times in this report that metals exhibit naturally varying background concentrations in different habitat types and depending on the metal background concentration, biological communities in these different systems may have adapted differently to the natural presence of metal concentrations, resulting in varying community sensitivities.

Rainbow (2002) reviewed the current literature with regard to metal accumulation in aquatic invertebrates. Aquatic invertebrates take up and accumulate metals whether essential or not, resulting in a great variability of subsequent body concentrations across metals and taxa. Accumulated metals may refer to metabolically available and stored detoxified metals, whereas toxicity is related to a threshold concentration of a metabolically available metal and not to the total accumulated metal.

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