A Biotic Ligand Model for Acute Copper Toxicity to the Zebrafish, the Most Studied Fish in the World: Back to Basics

The Biotic Ligand Model (BLM) integrates geochemistry, physiology, and toxicology to predict the site-specific toxicity of metals relative to water chemistry. We provide a brief history of BLM development, noting that (i) modern versions are done by data-fitting, not by experimentation; (ii) no BLM has yet been developed to predict the toxicity of the most studied metal (Cu) to the most studied fish (zebrafish); and (iii) no BLM has been based on a tropical species. We have returned to experimental basics: 96-h LC50 and 3-h gill binding assays (with radiolabeled ⁶⁴Cu) in soft water, where individual components were manipulated separately. These yielded Log K_tox values for Cu toxicity, Log K_gill values for gill Cu binding, and LA50 (3-h gill accumulation predictive of 96-h LC50). Key novel findings in zebrafish included (i) a different mechanism of Cu toxicity; (ii) high LA50; (iii) high protection against both gill binding and Cu toxicity by dissolved organic carbon (DOC) and strong cations (potency order Mg²⁺ > Ca²⁺ > Na⁺ > K⁺); (iv) the BLM constructed using Log K_tox values predicted acute Cu toxicity to adult zebrafish better than one based on Log K_gill values, but the performance of both was superior to generic BLMs for other fish which are in widespread use.