bioavailability concepts

Metals and metal compounds are a specific group of substances in the context of chemical risk assessment and monitoring because they are present in the environment naturally due to geochemical processes. Nevertheless anthropogenic activities causing metal emissions may be a threat to organisms and processes in the environment. However, the actual toxicity of metals toward organisms in waters can strongly depend on local conditions.

Only a portion of the total amount of a metal in water can actually be taken up by organisms. A fraction is considered as bioavailable if it is free for uptake by an organism and can cause adverse effects. The toxic effect of a metal does not only depend on its total concentration in the ambient medium but also on the complex interactions between water constituents as well as physical and biological factors.

One approach to predict metal bioavailability is the biotic ligand model (BLM). The BLM aims to predict how dissolved metals interact with aquatic organisms. The binding sites on the epithelial surface of organisms are considered as the so-called biotic ligand and the biological response after metal uptake is predicted on basis of the extent of metal binding to the biotic ligand. The BLM concept considers further that ions competing for the same biotic ligand may reduce the observed toxicity of a metal. Thus, the same metal concentration may result in a different degree of toxicity under different environmental conditions.

The BLM approach was originally applied to predict short-term (acute) toxicity to aquatic organisms based on a combination of existing chemical, toxicological, biological, and physiological data. More recently the focus was broadened towards predicting chronic toxicity, too.

More information:

Consideration of the bioavailability of metal/metalloid species in freshwaters: experiences regarding the implementation of biotic ligand model-based approaches in risk assessment frameworks
Rüdel H, Díaz Muñiz C, Garelick H, Kandile NG, Miller BW, Pantoja Munoz L, Peijnenburg WJ, Purchase D, Shevah Y, van Sprang P, Vijver M, Vink JP
Environ Sci Pollut Res Int (2015) 22, 7405-7421
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