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.