High Elimination Rates Limit Thallium Bioaccumulation across Diverse Marine Taxa

Thallium (Tl) is a highly toxic metal, yet its bioaccumulation in marine organisms remains poorly understood. Reference concentration factors (CFs) recommended by the International Atomic Energy Agency (IAEA) imply substantial Tl bioaccumulation potential, whereas Tl concentrations in marine organisms are generally low. Here, we resolved this apparent discrepancy by characterizing Tl toxicokinetics in eleven marine species spanning macroalgae, mollusks, and crustaceans using stable-isotope tracing (²⁰³Tl). Across taxa, Tl was rapidly eliminated (k_e = 0.33-3.67 d^-1), resulting in low kinetic bioconcentration factors (BCFs: 16-336 L kg^-1 dry weight and 0.51-68 L kg^-1 wet weight). Interspecies variation in Tl accumulation was more strongly associated with uptake than with elimination. Dissolved uptake dominated bioaccumulation when particulate Tl was present, with little particulate contribution in most species with the tested SPM; oysters were an exception, showing enhanced accumulation and a particulate Tl assimilation efficiency of 33.5%. These results demonstrate that rapid elimination broadly limits Tl bioaccumulation across marine taxa and support re-evaluation of the high CFs currently used as reference values. Rapid turnover further indicates that tissue Tl concentrations primarily reflect recent exposure rather than long-term retention.