Bioavailability of biologically sequestered cadmium and the implications of metal detoxification

Abstract

The deposit-feeding oligochaete Limnodrilus hollmelsteri possesses metallothionein-like proteins and metal-rich granules for storing and detoxifying cadmium (Cd). In this study we investigated the bioavailability of Cd sequestered within this oligochaete by conducting feeding experiments with '""Cd-labeled oligochaetes and the omnivorous grass shnmp Palaemonetes puglo. We also make predictions on Cd trophic transfer based on oligochaete subcellular Cd distnbutions and absorption efficiencies of Cd by shrimp. Cytosol (~ncluding metallothionein-like proteins and other proteins) and a debris fraction (including metal-rich granules and tissue fragments) isolated from homogenized '""Cdlabeled oligochaetes were embedded in gelatln and fed to shrimp. The '""Cd absorption efficiencies of shrimp fed these subcellular fractions were 84.8 and 48.6'!.), respectively, and were significantly different (p < 0.001), indicating that ""d bound in these fractions was not equally available to a predator. Mass balance equations demonstrate thdt shrimp fed whole worms absorb 61.5% of the ingested In9Cd, an absorption efficiency similar to that obtained experimentally (57.1 % ) . Furthermore, the majority of the absorbed In"Cd comes from the fraction containing metallothionein-like proteins (1 e. cytosol). ' O v d absorbed from the debris fraction probably comes from the digestion of tissue fragments, rather than metal-rich granules. The ecological significance of these f ~n d ~n g s is that prey detox~fication mechanisms may mediate the bioreduction or bioaccumulation of toxic metals along food chains by altering metal bioavailability. Another important finding is that trophic transfer of metal can be predicted based on the subcellular metal distribution of prey.