The bioavailability of sediment-bound Cd, Co, and Ag to the mussel Mytilus edulis

Marine sediments, long thought to be a final repository of toxic contaminants in marine ecosystems, may also serve as a source of toxic contaminants for marine food chains. The influence of physical and chemical properties of oxidized sediments on the bioavailability of metals from sediments to marine filter-feeding invertebrates is largely unknown. We examined the relative importance of specific sedimentary components that may affect the uptake of Cd, Co, and Ag by the mussel Mytilus edulis. Iron and manganese oxides, montmorillonite clay, silica, and natural sediment particles were radiolabeled with Cd-109, Co-57, and Ag-110m; some particles were also coated with marine fulvic acids to simulate the influence of organic coating on metal bioavailability. Mussels were fed radiolabeled particles and then allowed to depurate by feeding on nonradioactive particles, following a pulse-chase protocol developed to determine assimilation efficiencies of ingested metals. Metals were absorbed by mussels from all sediment particles, but metals associated with organic-coated particles were generally absorbed to a greater extent than metals associated with uncoated particles; absorption efficiencies were as high as 34% (for Co from organic-coated silica particles). Desorption experiments with radiolabeled particles at pH 5 were performed in parallel to simulate the behavior of food-bound metals in the acidic gut of bivalves. High correlations (r > 0.97) between the amount of metal desorbed under these conditions and the assimilation efficiency for metal bound to particles were noted among organic-coated particles but not uncoated particles (r < 0.6). Differences between coated and uncoated particles were principally due to differences in gut transit times of the ingested particles. These results suggest that suspended sediment particles may be a source of metals for suspension-feeding animals, but the relationship between metal partitioning to sediments and biological availability is not obvious, because organic coatings and the acidic gut environment influence metal absorption in mussels.