Biotic ligand model development predicting Zn toxicity to the alga Pseudokirchneriella subcapitata: possibilities and limitations.

Biotic ligand models have been developed for various metals (e.g. Cu, Ag, Zn) and different aquatic species. These models incorporate the effect of physico-chemical water characteristics (major cations, pH, dissolved organic carbon) on the bioavailability and toxicity of the metal. In this study, the individual effects of calcium, magnesium, potassium, sodium and pH on zinc toxicity to the green alga Pseudokirchneriella subcapitata (formerly and better known as Selenastrum capricornutum and Raphidocelis subcapitata) were investigated. Stability constants for binding to algal cells (K(BL)) were derived for those cations affecting zinc toxicity, using the mathematical approach proposed by De Schamphelaere and Janssen [Environ. Sci. Technol. 63, (2002) 48-54]. Potassium proved to be the only cation tested that did not alter zinc toxicity to algae significantly. Log (K(BL)) values for Ca, Mg and Na, derived at pH 7.5, were 3.2, 3.9 and 2.8, respectively. Toxicity tests performed at different pH values (5.5-8.0) indicated that competition between H(+) and Zn(2+) reduces zinc toxicity. However, the observed relationship between (H(+)) and the 72h-EbC(50) [expressed as microM (Zn(2+))] is not linear and suggests that pH affects the physiology of the biotic ligand. Although, in general, our findings seem to suggest that zinc toxicity to algae can be modelled as a function of key water characteristics, the results also demonstrate that the part of the conventional BLM-hypothesis-i.e. that the binding characteristics of the biotic ligand are independent of the test medium characteristics-is not valid for algae. The observed pH-dependent change of stability constants should therefore be further investigated and incorporated in future BL-modelling efforts with algae.