Modeling acute toxicity of metal mixtures to wheat (Triticum aestivum L.) using the biotic ligand model-based toxic units method

Mingyan Wu,Xiaoxiu Li,Zhiguo Jia,Xiaolin Chen,Xuedong Wang,Karel De Schamphelaere,Dongxue Ji

doi:10.1038/s41598-017-09940-5

Mingyan Wu, Xiaoxiu Li + Show 5 more

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https://doi.org/10.1038/s41598-017-09940-5

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Abstract

The combined toxic effects of copper (Cu) and cobalt (Co) were predicted using the biotic ligand model (BLM) for different concentrations of magnesium (Mg2+) and pH levels, with parameters derived from Cu-only and Co-only toxicity data. The BLM-based toxic unit (TU) approach was used for prediction. Higher activities of Mg2+ linearly increased the EC50 of Cu and Co, supporting the concept of competitive binding of Mg2+ and metal ions in toxic action. The effects of pH on Cu and Co toxicity were related not only to free Cu2+ and Co2+ activity, respectively, but also to inorganic metal complexes. Stability constants for the binding of Cu2+, CuHCO3+, CuCO3(aq), CuOH+, Mg2+, Co2+, CoHCO3+ and Mg2+ with biotic ligands were logKCuBL 5.87, mathrm{log},{K}_{{{rm{CuHCO}}}_{3}{rm{BL}}} 5.67, mathrm{log},{K}_{{{rm{CuCO}}}_{3}{rm{BL}}} 5.44, logKCuOHBL 5.07, logKMgBL 2.93, logKCoBL 4.72, mathrm{log},{K}_{{{rm{CoHCO}}}_{3}{rm{BL}}} 5.81 and logKMgBL 3.84, respectively. The combinations of Cu and Co showed additive effects under different conditions. When compared with the FIAM-based TU model (root mean square error [RMSE = 16.31, R2 = 0.84]), the BLM-based TU model fitted the observed effects better (RMSE = 6.70, R2 = 0.97). The present study supports the BLM principles, which indicate that metal speciation and major cations competition need to be accounted for when predicting toxicity of both single metals and mixtures of metals.

Highlights

IntroductionOne of the models most commonly used to assess the toxicity of chemical mixtures is the toxic unit (TU) approach[5], which is employed to calculate the sum of the concentrations of individual chemicals divided by their median effective concentrations (EC50)[7]
Jho et al found that data describing the toxicity of a single metal can be used in the biotic ligand model (BLM)-based toxic unit (TU) method to predict the joint toxicity of Cd–Pb mixtures to Vibrio fischeri[1]
The present study indicates that incorporating inorganic metal complexes when assessing the joint toxicity of Cu–Co improves the predictive capacity of the BLM

Summary

Introduction

One of the models most commonly used to assess the toxicity of chemical mixtures is the toxic unit (TU) approach[5], which is employed to calculate the sum of the concentrations of individual chemicals divided by their median effective concentrations (EC50)[7]. Conventional studies using the TU approach to examine the combined toxicity of multiple metals do not consider the speciation and phytotoxicity of metals under different environmental conditions. The critical assumption of the BLM is that metal toxicity depends on free metal ions (or other reactive metal species), which can react with biological binding sites and form a metal–biotic ligand (BL) complex. Jho et al found that data describing the toxicity of a single metal can be used in the BLM-based TU method to predict the joint toxicity of Cd–Pb mixtures to Vibrio fischeri[1]. To the best of our knowledge, only three reports have employed the BLM to estimate the combined toxicity of metals on joint toxicity[10,11,12]

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