Contrasting interactions of Cd, Cu, and Zn during uptake and efflux in Daphnia magna

Metals often coexist as mixtures in aquatic ecosystems, where they bioaccumulate in organisms and induce combined biological effects. However, the interaction mechanisms governing their bioaccumulation remain poorly understood. Using stable isotope tracers (¹¹⁴Cd, ⁶⁵Cu, and ⁶⁸Zn), this study investigated the uptake and efflux dynamics of three common metals (Cd, Cu, Zn) and their binary mixtures in Daphnia magna. Uptake experiments were conducted over 6 h across four concentration levels, while efflux kinetics were monitored over a 7-day depuration period following 3-day exposure. Our results revealed metal-specific interaction patterns during both biokinetic processes. During uptake, Cd, Cu, and Zn displayed significant and primarily inhibitory interactions, indicating potential competition for shared transport pathways. More notably, efflux kinetics were modulated by coexisting metals in a concentration-dependent manner. For Cd-Cu mixtures, mutual efflux inhibition intensified at higher concentrations, with the Cd efflux rate constant (k_e) decreasing from 0.082 d^- 1 to 0.047 d^- 1, and that of Cu from 0.282 d^- 1 to 0.076-0.098 d^- 1. Conversely, Cd and Zn mutually enhanced efflux, with Zn k_e increasing from 0.166 d^- 1 to 0.465-0.583 d^- 1 and Cd k_e from 0.082 d^- 1 to 0.119 d^- 1, though Cd enhancement occurred only under low Zn exposure. In Cu-Zn mixtures, asymmetric interactions occurred: Zn suppressed Cu efflux while Cu accelerated Zn elimination, with Zn k_e reaching 1.222 d^- 1 at elevated Cu concentrations. Collectively, interactions during both uptake and efflux determine the net bioaccumulation of metal mixtures, providing critical insights for environmental risk assessment of mixed-metal pollution.