Metal uptake and toxicity in rainbow trout (Oncorhynchus mykiss) when exposed to metal mixtures plus natural organic matter

Multiple metal-gill modelling based on the toxic unit concept suggests that metals with the same toxic actions (e.g. Pb and Cd interacting at Ca-gill channels) will exhibit strictly additive binding at the gills of fish if the concentrations of the two metals sum to one toxic unit. Due to the non-linear nature in which metals bind to fish gills, the metal mixture will be more than additive below one toxic unit and less than additive above one toxic unit. This research tested the models by exposing rainbow trout to mixtures of Cd and Pb to investigate the metal-gill binding of these two metals when present in mixtures with fish. The relationship between Cd and Pb in mixtures was also investigated when natural organic matter (NOM) was added to the treatment solutions.;The bioaccumulation and partitioning of Cd and Pb within the body of 50 g rainbow trout was investigated when these fish were exposed to mixtures of Cd and Pb. Trout were exposed to 3 muM Pb, 3 muM Cd and a mixture of 3 muM Pb and Cd for 1, 3 and 7 d. The accumulation of metals within the trout body did not change between the single metal and mixture exposures. Trout accumulated the greatest amount of Pb on the gills with some accumulation also in the stomach. The greatest Cd accumulation was in the liver and trunk kidney. Accumulation of Pb was not limited to the gills of trout resulting in the rejection of the hypothesis that the gills were sequestering Pb while Cd was being transported into the body of the fish.;To investigate how Pb affects Cd binding to fish gills, and how these metals exert toxicity when present in mixtures, trout (2 g) were exposed to a range of Cd concentrations (0.75, 1.5, 2.25 and 3.0 6M) while maintaining constant Pb concentrations over the same concentration range. At the lowest metal concentrations (0.75 6M Cd and Pb), gill-Pb and gill-Cd were about equal, suggesting strict additivity. Also, gill-Cd exceeded that of gill-Pb when 0.75 6M Pb was added to higher concentrations of Cd. In the presence of 10 mg C/L natural organic matter (NOM) collected from Luther Marsh, gill-Cd always exceeded gill-Pb, regardless of the mixture concentrations used.;LT50 results for fish exposed to metal mixtures for 96 h revealed that without NOM, the mixture toxicity was dependent on Pb concentration. As Pb concentration increased, the LT50 for the mixtures decreased. With the addition of NOM, mixture toxicity decreased with increasing Pb. Toxicity results did not always agree with metal-gill binding, especially in experiments with added NOM.;Juvenile rainbow trout (5 g) were exposed to equal mixtures of Cd and Pb at 0.75, 1.5, 2.25 and 3.0 muM each. Metal accumulation by trout gills was non-linear as suggested by the model and approximated additivity at 1.5 muM Cd + 1.5 muM Pb. There was some evidence of competitive effects at the higher metal concentrations (3.0 muM Cd + 3.0 muM Pb) because gill Cd for trout exposed to mixture treatments were lower than that for trout exposed to Cd alone, while gill-Pb remained high. 96 h toxicity experiments revealed that Cd and Pb in mixtures were more toxic than either Cd or Pb alone. The high gill-Pb accumulation in relation to gill-Cd was unexpected and suggested an additional gill-binding site for Pb aside from the apical Ca channels on the gill.;It is concluded that metal mixtures of Cd and Pb interact in a far more complex manner than that suggested by the multiple-metal model mentioned above. The addition of NOM further complicates the interactions of Cd and Pb in mixtures with fish gills by possibly increasing the bioavailability of Cd.