Modelling metal interactions at fish gills

Waterborne metals can bind to gills of freshwater fish and disrupt the ionoregulatory and respiratory functions of the gills. Competing cations, such as Ca2+ and complexing ligands, such as dissolved organic matter (DOM) reduce these effects by preventing metals from binding to the gills. The gill membrane can also be considered as a complexing ligand, and a conditional equilibrium stability constant can be calculated for the strength of metal binding to the gill membrane (log Kmetal–gill). Once a conditional stability constant is calculated, it can be inserted into aquatic chemistry programs, producing a powerful predictive tool which includes changes in metal speciation as water pH is varied, includes competitive effects due to cations, such as Ca2+, and includes complexation by DOM, all of which affect metal binding to fish gills and therefore metal toxicity to fish. Fish release ammonia and carbon dioxide at their gills, which tend to make the gill micro-environment more basic and more acidic, respectively, depending on the inspired water pH and the buffer capacity of the water. These pH changes near the gills can alter metal speciation or solubility, so may need to be considered when modelling the physiological and toxicological effects of some metals. The concentration of DOM is most important when assessing the protective effects of DOM against metal binding to fish gills, but especially at low DOM concentrations it is apparent that different sources of DOM keep some metals off fish gills better than others. Overall, metal–gill modelling is a good framework for understanding and predicting metal toxicity to fish, although more work is certainly needed to assess the applicability of this approach for chronic metal exposures and for organisms besides freshwater fish.