Measuring Metal Uptake and Loss in Individual Organisms: A Novel Double Stable Isotope Method and its Application in Explaining Body Size Effects on Cadmium Concentration in Mussels.

Interindividual variabilities in metal bioaccumulation confound our interpretation of the biomonitoring data. Measuring metal toxicokinetics in organism "individuals" may provide insights into the processes underlying the variabilities. Therefore, we developed a double stable isotope method that can simultaneously measure uptake and elimination of metals in individual organisms and thus the distribution of the toxicokinetic parameters. Specifically, we exposed organisms to both isotopes (113Cd and 114Cd; Cd = cadmium) during the first stage and to only one isotope (114Cd) during the second stage. Metal uptake and elimination rate constants (i.e., ku and ke) were simultaneously estimated from the content of the two isotopes measured in each organism at the end of the second stage. We applied the method to investigate the interindividual variability in Cd concentrations caused by body size in two marine mussel species. Cd concentrations are higher in larger Xenostrobus atratus but lower in smaller Perna viridis. Size-dependent Cd uptake is found to be responsible for size effects on Cd concentrations in the mussels and the interspecies differences in the relationship between Cd concentration and body size. Specifically, Cd ku increases with size in X. atratus (0.057-0.297 L g-1 d-1) but decreases with size in P. viridis (0.155-0.351 L g-1 d-1). In contrast, Cd ke is not influenced by body size (X. atratus: 0.002-0.060 d-1; P. viridis: 0.008-0.060 d-1). Overall, we extended the applicability of the stable isotope methods to measure metal toxicokinetics in "individual" organisms, providing a readily available tool for investigating problems related to metal bioaccumulation.