Uptake and toxicity of waterborne uranium and cadmium to Atlantic salmon (Salmo salar L.) depends on life stage and combined exposure

Controlled laboratory experiments were performed to investigate the toxicity of depleted uranium (U) and the frequent co-contaminant cadmium (Cd), individually and in combination, on two life stages (embryos and juveniles) of Atlantic salmon (Salmo salar L.). U and Cd accumulation in both eggs and juveniles were positively correlated with waterborne concentrations and toxic effects. The embryo stage was more sensitive to U (LC₅₀: 3.17 μM U), while the juveniles were more sensitive to Cd (LC₅₀: 19.6 nM Cd). Continuous uptake of U in eggs during the 90-day exposure period was observed to our knowledge for the first time, and could explain why U was more toxic to fish embryos than Cd, taken up only during the early days. The acute toxic mechanisms of U and Cd were different as the U gill and liver concentration correlated with increased blood glucose and reduced plasma Cl concentrations, while no such responses were observed during Cd exposure, although negative invers correlation of Cd uptake in liver to survival rate (LC₅₀: 4.8 nmol g^-1 liver) was also observed for U (LC₅₀: 8.66 nmol g^-1 liver). Combined exposure to juveniles showed an antagonistic interaction between the two elements, where U reduced systematic uptake of Cd in liver and thereby increased the survival of lethal Cd concentrations. Using independent action as reference model, antagonistic effects on both the toxicokinetic and the toxicodynamic level were confirmed. Results demonstrated the importance of integrating life stage dependent toxicokinetic and toxicodynamic effects into risk assessment associated with multiple stressor scenarios.