"How do we understand toxic effects of metal mixture?" 

Masashi Kamo 

National Institute of Advanced Industrial Science and Technology

SETAC North America (Milwaukee 2007/11/11)

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Abstract

According to rapidly developing biotic ligand model (BLM), we can predict the toxicity of heavy metals in various water qualities. The basic assumptions of BLM is that living organisms have positively charged biotic organs (such as gills), which is responsible for the calcium ion uptake and negatively charged heavy metal ions also bind to the organs, and death occurs when the amount binding metals exceeds a certain threshold. Current versions of BLM can predict toxicity of single metal only, and not able to predict toxicities when organisms are exposed to multiple metals. In this paper we aim to understand the toxic effects of metal mixtures. 

Toxic mechanisms of the toxic mechanism are not completely understood yet. However, it is widely known that heavy metals inhibit a calcium uptake resulting hypocalcemia, which is now considered to be the major mechanism of acute toxicity in some metals. If toxic mechanism of heavy metals is hypocalcemia, we can predict toxicity of metals by computing the amount of calcium uptake, and it can be done by using BLM's theoretical framework with a tiny modification. Estimating toxicity by the amount of calcium uptake has two merits. First, toxicity of one metal can be translated into others by searching metal concentrations leading to the same calcium uptake. Second, toxicity of metal mixtures can be estimated by developing a theoretical framework for calcium uptake when multiple metals exist. 

We concentrate on binary mixture of zinc, cadmium and copper in rainbow trout acute toxicity data sets, and find synergism in binary mixture in zinc and cadmium, and antagonism in zinc and copper. We also find that these non-addtive effects are more pronounced in softer water. Our approach is also feasible for the risk reduction strategies in metal polluted area such as mining places. 

Keywords

ecotoxicology, toxicity of metal mixture, biotic ligand model, synergism, antagonism, rainbow trout