Influence of Ligand Complexation on Nickel Toxicity, Speciation and Bioavailability in Marine Waters

Currently there are no site-specific bioavailability-based prediction models for assessing the impacts of nickel (Ni) in marine environments although there are indications that these may be warranted. The aim of this research was to characterize the complexation of Ni in relation to toxicity and speciation. Various complexing ligands were used, and it was predicted that the binding affinity (logKf) of ligands would be inversely correlated to toxicity based on dissolved Ni concentrations ([NiD]) but that on a free ion concentration ([Ni2+]) basis, toxicity would not vary. A two-phased approach was used; the first was a proof of principle where synthetic ligands with known logKf values [EDTA, NTA, tryptophan (TRP), glutamic acid (GA), histidine (HD) and citric acid (CA)] were tested and the second, natural waters were characterized for binding capacity and Ni toxicity. Chronic Ni toxicity assays were performed using two marine species sensitive to Ni, the purple sea urchin (Strongylocentrotus purpuratus; where EC50 and EC20 values were determined) and the mysid (Americamysis bahia; LC50 and LC20). Embryological development and mortality (respectively) were used as the toxicity endpoints. Ni was measured by graphite furnace atomic absorption spectroscopy (GFAAS). The [NiD] E/LC50 values in unmodified artificial seawater (ASW) were 3.6 μM (95% CI 3.0-4.5 μM) for S. purpuratus and 2.6 μM (2.3-2.8 μM) for A. bahia. Tests with synthetic ligands provided significant protection based on [NiD], particularly for those with strong complexation such as EDTA and NTA. However, when considered on the basis of [Ni2+], E/LC50 values were either similar or less than those in ASW. In natural seawater the E/LC50 values ranged from 2.0 to 7.0 mM based on [NiD] and variability was reduced when expressed on a [Ni2+] basis. There were no significant differences in Ni toxicity between natural waters and ASW. Overall, this study supports the theory that free Ni concentration is the best predictor of toxicity and confirms the applicability of a marine BLM for Ni. It also provides insight into the understanding of relationships between aquatic geochemistry, Ni speciation, complexation and toxicity from a biological perspective