Abstract
Metals assimilated by organisms are sequestered in various compartments and some forms are more stable than others. Sequestration mechanisms used by invertebrates to detoxify metals and prevent interaction with important biomolecules include metal binding to proteins and other ligands, and storage in inorganic granules. The rate and extent at which metal concentrations in different compartments respond to metal concentrations in food and food characteristics has not received much attention, despite being of great relevance. We performed an experiment on the carabid beetle Pterostichus oblongopunctatus exposed to Cd via food made of ground mealworm (Tenebrio molitor) larvae, either reared on Cd contaminated medium or artificially spiked after grinding with CdCl2 solution. Thus, in both cases we used the same type of food, differing only in the soluble Cd pool available to the predators, represented by P. oblongopunctatus. Subcellular compartmentalisation of Cd into organelles, heat-sensitive and heat-stable proteins (the first supernatant, S1 fraction), cellular debris (the second supernatant, S2 fraction) and metal-rich granules (G fraction) was checked a few times during the contamination (90 d) and decontamination (24 d) phases in a toxicokinetic experiment by using different centrifugation steps. The results showed no effect of the type of food (naturally, Cd-N, vs. artificially contaminated with Cd, Cd-A) on Cd sequestration kinetics in P. oblongopunctatus, but the amount of Cd sequestered in the S1 and G fractions were in general higher in the Cd-A than the Cd-N treatment, indicating that Cd transfer in the food web depends on the speciation of the metal in the food. The proportional distribution of Cd over different fractions was, however, similar in beetles fed both food types. Most of the accumulated Cd in the beetles existed as fraction S1 (ca. 35%), which is important for the transfer of metals to higher trophic levels in a food web.
Highlights
One of the major challenges in assessing potential effects of toxicants on organisms is predicting the internal active concentration of toxic chemicals in the body and/or target organs
The aim of this study was to investigate the kinetics of cadmium and its compartmentalisation in the ground beetle Pterostichus oblongopunctatus (Coleoptera: Carabidae) exposed to the metal via food made of Tenebrio molitor larvae, either artificially contaminated with metal-salt solution or reared on contaminated medium
Female beetles were not used in this study because P. oblongopunctatus mate during April/May (Brunsting 1981), as such, there was a high probability that some of the caught females were fertilised, and this can influence toxicokinetics
Summary
One of the major challenges in assessing potential effects of toxicants on organisms is predicting the internal active concentration of toxic chemicals in the body and/or target organs. In organisms exposed to high metal concentrations, physiological acclimation, for example through shifts in toxicokinetics (TK), may help to overcome potential chemical stresses, especially during prolonged exposure to metals under sublethal concentrations. Several experiments on terrestrial invertebrates revealed that during prolonged metal exposure, animals are able to maintain metal concentration unchanged in their bodies (Kramarz 1999) or to even decrease it (Bednarska et al 2011). The usefulness of TK parameters (i.e. assimilation and elimination rate constants) derived from singleconcentration laboratory experiments for predicting metal concentrations in organisms in actual polluted environments has been, questioned recently (Bednarska et al 2015). After testing the toxicokinetic parameters for a range of exposure concentrations of Zn and Cd in crickets Gryllus assimilis, Bednarska et al (2015) concluded that there is no metal-specific assimilation or elimination rates. The impact of the bioavailability of a metal in food on TK and sequestration processes, has not been studied so far for orally exposed animals
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