Metal cation toxicity in the alga Gracilaria domingensis as evaluated by the daily growth rates in synthetic seawater
Macroalgae of the genus Gracilaria have considerable economic importance as raw material for agar production and belong to an important group of organisms that are tolerant of high concentrations of metal. The median inhibitory concentration (IC50) values obtained by measuring the ratio of fresh mass variation (i.e., daily growth rates) of the red macroalga Gracilaria domingensis during a 48-h aquatic toxicity assay are reported here. The alga was exposed to 14 different metal cations as well as the molybdate anion in synthetic seawater. The actual concentrations of these ionic species (at IC50 values) and the proportion of free ions (aqueous complexes) were determined by inductively coupled plasma atomic emission spectroscopy and the Environmental Protection Agency-recommended software, MINTEQA2, respectively. Based on the free IC50 values (IC50F), the ions were ranked in terms of toxicity: Cd2+ ≫ Cu2+ ≫ Pb2+ ≫ Zn2+ ≫ Ni2+ > Co2+ > La3+ ≫ Mn2+ > Ca2+ ~ Li+ ≫ MoO42− ≫ Sr2+ > Mg2+ ≫ K+ > Na+.