Mixture toxicity of copper and zinc to barley at low level effects can be described by the Biotic Ligand Model

Abstract

Background and aims: The biotic ligand model (BLM) is a bioavailability model for metals based on the concept that toxicity depends on the concentration of metal bound to a biological binding site; the biotic ligand. Here, we evaluated the BLM to interpret and explain mixture toxicity of metals (Cu and Zn). Methods: The mixture toxicity of Cu and Zn to barley (Hordeum vulgare L.) was tested with a 4 days root elongation test in resin buffered nutrient solutions. Toxicity of one toxicant was tested in presence or absence of a low effect level of the other toxicant or in a ray design with constant toxicant ratios. All treatments ran at three different Ca concentrations (0.3, 2.2 and 10 mM) to reveal ion interaction effects. Results: The 50 % effect level (EC50) of one metal, expressed as the free ion in solution, significantly (p < 0.05) increased by adding a low level effect of the other metal at low Ca. Such antagonistic interactions were smaller or became insignificant at higher Ca levels. The Cu EC10 was unaffected by Zn whereas the Zn EC10 increased by Cu at low Ca. These effects obeyed the BLM combined with the independent action model for toxicants. Conclusions: The BLM model explains the observed interactions by accounting for competition between both metals free ions and Ca2+ at the Cu and Zn biotic ligands. The implications of these findings for Cu/Zn interactions in soil are discussed. © 2014 Springer International Publishing Switzerland.