Since the early 2000s, biotic ligand models and related constructs have been a dominant paradigm for risk assessment of aqueous metals in the environment. We critically review 1) the evidence for the mechanistic approach underlying metal bioavailability models; 2) considerations for the use and refinement of bioavailability-based toxicity models; 3) considerations for the incorporation of metal bioavailability models into environmental quality standards; and 4) some consensus recommendations for developing or applying metal bioavailability models. We note that models developed to date have been particularly challenged to accurately incorporate pH effects because they are unique with multiple possible mechanisms. As such, we doubt it is ever appropriate to lump algae/plant and animal bioavailability models; however, it is often reasonable to lump bioavailability models for animals, although aquatic insects may be an exception. Other recommendations include that data generated for model development should consider equilibrium conditions in exposure designs, including food items in combined waterborne-dietary matched chronic exposures. Some potentially important toxicity-modifying factors are currently not represented in bioavailability models and have received insufficient attention in toxicity testing. Temperature is probably of foremost importance; phosphate is likely important in plant and algae models. Acclimation may result in predictions that err on the side of protection. Striking a balance between comprehensive, mechanistically sound models and simplified approaches is a challenge. If empirical bioavailability tools such as multiple-linear regression models and look-up tables are employed in criteria, they should always be informed qualitatively and quantitatively by mechanistic models. If bioavailability models are to be used in environmental regulation, ongoing support and availability for use of the models in the public domain are essential.

Abstract-The uptake of cadmium and zinc by the common carp, Cyprinus carpio, was studied in chemically defined freshwater in the presence of different organic ligands (i.e., citrate, glycine, histidine, ethylenediaminetetraacetic acid, and nitrilotriacetic acid). In most cases, metal complexation decreased Cd and Zn uptake by reducing the free Cd and Zn ion activity. However, Cd and Zn uptake did not increase linearly with the free Cd and Zn ion activity in the solution. A good fit to the data was obtained when the observations were fitted to a Michaelis-Menten-like model for carrier-mediated transport of the metal ions across the biological interface. In addition, the uptake of Cd in the presence of citrate, glycine, and histidine was markedly higher than expected on the basis of the free Cd ion activity. It was concluded that cadmium complexes of these low molecular weight, hydrophilic ligands contributed to the Cd bioavailability, probably by direct uptake of these complexes. Zinc uptake in the presence of the complexing agents could be predicted on the basis of the ambient free Zn ion activity, although uptake in the presence of citrate was lower than expected on the basis of the free Zn ion activity. These results provide a challenging test for the free ion activity model.

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Rainbow trout were exposed to a sublethal concentration of waterborne Cd (0 or 3 microg/L) or dietary Cd (0 or 500 mg/kg dry wt) for 30 days to induce acclimation, and tissue Cd and metallothionein (MT) levels were examined. The greatest Cd concentrations were observed in the kidney followed by the gills and liver of the fish exposed to Cd via water, but in the gut tissues followed by the kidney, liver, and gills for dietary-exposed fish, reflecting a variation depending on the route of Cd exposure. Some MT was found in the nonacclimated naïve fish with no experience of elevated Cd exposure, and these background MT levels were quite high in the posterior intestine (480 microg/g), cecae (257 microg/g), and liver (248 microg/g) relative to other tissues (7-50 microg/g). With exposure to both waterborne and dietary Cd, MT levels rose significantly in all observed tissues. The increases relative to the control levels of MT in naïve fish were in the order: kidney (5.4 times) > gills (4.6) > liver (1.3) for the waterborne exposure group, and in the order kidney (19.3 times) >> cecae and posterior intestine (approximately 6.5 times) > liver and stomach (approximately 5 times) > midintestine (4.3 times) > gills (2.1 times) for the dietary exposure group. At 24 hours after an acute gastrointestinal dose of Cd (276 microg/kg) infused into the stomach of dietary exposure groups, large increases of total Cd but not MT levels were found in the gut tissues of nonacclimated fish; in the Cd-acclimated fish, the posterior intestine was greatly affected with decreases in Cd (71%), Zn (33%), Cu (70%) and MT (46%) levels, suggesting an enhanced sloughing of tissue materials after infusion. Exposure to Cd did not cause any notable decrease of Zn or Cu in any tissue, except that found in the posterior intestine. However, a molar analysis indicated that although Cd levels remained less than MT binding capacity in both waterborne and dietary exposure groups, the total metal levels (Cd + Zn + Cu) greatly exceeded MT binding capacity in all tissues of Cd-exposed fish, suggesting a potential competition of Cd with other metals for binding sites on MT and non-MT proteins in the tissues.

The release of radioactive strontium to the environment is of concern due to the strong accumulation of this calcium resembling element in the bone and other tissues. To predict the effects of changes in environmental conditions on the uptake of Sr2+ and Ca2+ by freshwater fish, a Michaelis-Menten type model is introduced that accounts for the effects of chemical speciation, hydrogen ion activity, and metal ion competition. The uptake kinetics were characterized in vivo from short-term exposure experiments using the common carp (Cyprinus carpio) as the model organism. Fish were exposed to a wide range of waterborne Sr2+ (0.2-10,000 microM) and Ca2+ (10-10,000 microM) concentrations and water pH (5.0-8.5). Strontium uptake by the whole body of fish increased with increasing Sr2+ activity, displaying saturation kinetics, but decreased significantly with increasing Ca2+ and H+ activities in the water. Likewise, calcium uptake by the fish decreased with increasing Sr2+ and H+ activities in the water. The model fitted to the pooled data explains 97.5% of the variation in Sr2+ uptake and 86% in Ca2+ uptake over the wide range of exposure conditions and reveals that Sr2+ and Ca2+ inhibit each other completely competitively, while H+ inhibits the uptake of both metal ions in a partially noncompetitive way. This model can be used as a mechanistic tool to predict the uptake of these metals in carp under variable conditions.