Prediction of mono-, bi-, and trivalent metal cation relative toxicity to the seaweed Gracilaria domingensis (Gracilariales, Rhodophyta) in synthetic seawater

The present study reports a 48-h aquatic metal-toxicity assay based on daily growth rates of the red seaweed Gracilaria domingensis (Gracilariales, Rhodophyta) in synthetic seawater. The median inhibitory concentration (IC50) for each metal cation was experimentally determined, and the ratios of free ions (aqueous complex) were calculated by software minimization of the total equilibrium activity (MINTEQA2) to determine the free median inhibitory concentration (IC50F). A model for predicting the toxicity of 14 metal cations was developed using the generic function approximation algorithm (GFA) with log IC50F values as the dependent variables and the following properties as independent variables: ionic radius (r), atomic number (AN), electronegativity (Xm ), covalent index (Xm (2) r), first hydrolysis constant (|log KOH |), softness index (σp ), ion charge (Z), ionization potential (ΔIP), electrochemical potential (ΔEo ), atomic number divided by ionization potential (AN/ΔIP), and the cation polarizing power for Z(2) /r and Z/AR. The 3-term independent variables were predicted as the best-fit model (log IC50F: -23.64 + 5.59 Z/AR + 0.99 |log KOH | + 37.05 σp ; adjusted r(2) : 0.88; predicted r(2) : 0.68; Friedman lack-of-fit score: 1.6). This mathematical expression can be used to predict metal-biomolecule interactions, as well as the toxicity of mono-, bi-, and trivalent metal cations, which have not been experimentally tested in seaweed to date. Quantitative ion-character relationships allowed the authors to infer that the mechanism of toxicity might involve an interaction between metals and functional groups of biological species containing sulfur or oxygen.