Biotic ligand models
António J.A. Nogueira
The molecular form of metals and metalloids, particularly the uncomplexed (free) form, controls their bioavailability and toxicity in solution. The relative amounts of the various hardness cations has also been shown to play an important role. The hardness-based approach does not account for the presence of dissolved ligands, such as organic matter, which strongly influence metal speciation and bioavailability. A better approach would be to establish a procedure whereby toxicity could be accurately predicted from simple water quality measurements, potentially those that are already part of a site monitoring program (e.g. in situ feeding bioassays or post-exposure feeding inhibition). The biotic ligand model (BLM) approach takes into account not only metal complexation by ligands in solution, but also competitive binding of hardness cations and metal ions to biological receptor sites. The BLM provides a means of both interpreting the results of acute and chronic toxicity tests and of incorporating these results into a regulatory framework. The traditional BLM approach as several weaknesses like inability to predict non-lethal life-history parameters or to deal with varying temperatures.
The BLM is an efficient alternative to actually conducting bioassays, and may lead to more technically defensible ecological risk assessments, although the BLM is basically a chemical equilibrium model used to calculate the distribution of a metal among various species. One of the components is the site of toxic action on the organism, usually the gill in fish and aquatic invertebrates. This site corresponds to the “biotic ligand”.
Figure 1. Adaptation of the BLM approach to Daphnia sp.
The BLM-computed toxicological response will systematically vary with the characteristics of the site water, in a predictable way, and in the same manner as would the results of laboratory toxicity tests. The manner in which water chemistry effects the accumulation of metal at the biotic ligand depends on the speciation of the metal. Computational methods for determining the formation of metal complexes with inorganic ligands (HCO3-, Cl-, OH-, SO42-, etc.) and the competition of other cations (Ca2+, Mg2+, Na+, K+) for these ligands are well established.
Figure 2. Modelling framework being implemented
The modelling approach we are developing incorporates the Wimdermere Humic-Aqueous Model (WHAM), version 6, for computing metal-organic matter complexation coupled with a physiological model able to predict several life-history parameters. This we involves collaborations with the Department of Chemistry and Geochemistry, Colorado School of Mines (US) and the Centre for Ecology & Hydrology (UK).
1. Nogueira, A. J. A., D. J. Baird & A. M. V. M. Soares(2004). Testing physiologically-based resource allocation rules in laboratory experiments with Daphnia magna Straus. Annales de Limnologie-International Journal of Limnology 40(4): 257-267.
2. Pedrosa, J.A.M., F.T. Jesus & A.J.A. Nogueira (in prep.). Zinc and copper toxicity in aquatic organisms: relation between lethal and non lethal effects. (To be submitted to Chemosphere)
3. Pedrosa, J.A.M., F.T. Jesus & A.J.A. Nogueira (in prep.). Evaluation of physic-chemical media characteristics in Daphnia magna Straus feeding rates. (To be submitted to Environmental Toxicology and Chemistry)
António Nogueira (born 1964) got his Ph.D. in 1996 from the Institute of Aquaculture, University of Stirling, UK, working on the establishment of a general modelling framework to predict life-history responses of aquatic under contaminant exposure. The model was developed and parameterized for Daphnia magna. From January 1997 till April 2001 he was assistant professor at the Department of Zoology, Coimbra University, Portugal. He has been in Aveiro in the Biology Department since April 2001, as associate professor, and in April 2005 received his habilitation to Full Professor of Biology). His main research interests are related to Ecotoxicology, both terrestrial and aquatic, in particular with aspects dealing with ecotoxicological modelling, the use and validation of in-situ bioassays, in both temperate and tropical conditions and toxicity of complex mixture included metals and organic contaminants is another main area of research interests. Recently, the ecotoxicological aspects associated with nanoparticles and nanomaterials, in collaboration with north American colleagues, also became part of his research interests. António Nogueira published (or has in press) over 35 SCI papers. He has been a member of several Scientific Committees (e.g. CICTA, SETAC) and is a current referee of up to 15 different SCI journals in the field of ecology, biochemistry and environmental toxicology.