The influence of metal speciation on the bioavailability and sub-cellular distribution of cadmium to the terrestrial isopod, Porcellio dilatatus

Cadmium is a non-essential toxic metal that is able to bioaccumulate in both flora fauna and has the potential to biomagnify in some food chains. However, the form in which cadmium is presented to consumers can alter the bioavailability and possibly the internal distribution of assimilated Cd. Previous studies in our laboratory highlighted differences in Cd assimilation among isopods when they were provided with a plant-based food with either Cd biologically incorporated into plant tissue or superficially amended with ionic Cd2+. Cd is known for its high affinity for sulphur ligands in cysteine residues which form the basis for metal-binding proteins such as metallothionein. This study compares Cd assimilation efficiency (AE) in Porcellio dilatatus fed with food amended with either cadmium cysteinate or cadmium nitrate in an examination of the influence of Cd speciation on metal bioavailability followed by an examination of the sub-cellular distribution using a centrifugal fractionation protocol. As hypothesized the AE of Cd among isopods fed with Cd(NO3)2 (64%, SE = 5%) was higher than AE for isopods fed with Cd(Cys)2 (20%, SE = 3%). The sub-cellular distribution also depended on the Cd species provided. Those isopods fed Cd(Cys)2 allocated significantly more Cd to the cell debris and organelles fractions at the expense of allocation to metal-rich granules (MRG). The significance of the difference in sub-cellular distribution with regard to toxicity is discussed. This paper demonstrates that the assimilation and internal detoxification of Cd is dependent on the chemical form of Cd presented to the isopod.

Research highlights
► Metal speciation will dictate the bioavailability of Cd to terrestrial isopods.
► Cd(Cys)2 provided a model species for biologically complexed Cd.
► Assimilation of Cd(Cys)2 was significantly lower than ionic Cd2+ in isopods.
► Cd speciation also dictated the sub-cellular distribution of Cd in isopods.
► Cd speciation might dictate the toxicity of Cd after in isopods after assimilation.