The calcium (Ca) biokinetics and the protective effects of Ca against the bioaccumulation and toxicity of cadmium (Cd) were investigated in the freshwater cladocerans. Water was the dominant source of Ca for cladocerans. Ca uptake from water was not actively regulated in daphnids according to the ambient Ca levels, whereas the efflux rate constant of Ca was lower in low-Ca waters. Daphnia species had higher Ca content than the other cladocerans and thus were more susceptible to Ca limitation. Ca biokinetics could be used to explain the interspecies differences in Ca content and the susceptibility to Ca limitation. High-Ca cladocerans generally had higher dissolved Ca and Cd uptake rates due to the coupling between Ca and Cd in membrane transport. Therefore, high-Ca species were more sen...[ Read more ]

The calcium (Ca) biokinetics and the protective effects of Ca against the bioaccumulation and toxicity of cadmium (Cd) were investigated in the freshwater cladocerans. Water was the dominant source of Ca for cladocerans. Ca uptake from water was not actively regulated in daphnids according to the ambient Ca levels, whereas the efflux rate constant of Ca was lower in low-Ca waters. Daphnia species had higher Ca content than the other cladocerans and thus were more susceptible to Ca limitation. Ca biokinetics could be used to explain the interspecies differences in Ca content and the susceptibility to Ca limitation. High-Ca cladocerans generally had higher dissolved Ca and Cd uptake rates due to the coupling between Ca and Cd in membrane transport. Therefore, high-Ca species were more sensitive to aqueous Cd exposure. Food was the major source of Cd for cladocerans, and the assimilation efficiency of Cd was the principal determinant of their Cd bioaccumulation. There were large interspecies differences in the dietary Cd accumulation capability and sensitivity to internal Cd exposure, which both determined the sensitivity of cladocerans to dietary Cd exposure. The decreased Cd bioaccumulation in high-Ca environment was mainly attributed to the competition of ambient Ca with Cd during bioaccumulation rather than the protective effects of higher body Ca. Elevated Ca concentration significantly decreased the acute toxicity of Cd; however, the sensitivity to internal Cd exposure was not affected. Ca concentration could affect the relative contribution of different Cd transport pathways and thereby affect Cd uptake kinetics. The results obtained in this study provide comprehensive understanding of the roles of Ca in Cd bioaccumulation and toxicity.