Bioavailability of sediment-bound methyl and inorganic mercury to a marine bivalve

The extent to which ingested suspended sediment can be a source of mercury for suspension feeders is largely unknown, yet this information is required to evaluate the biological significance of sediment contamination. We used radiotracer methodology to experimentally compare the bioavailability of dissolved and sediment-bound mercury for a marine bivalve. We examined the relative importance of specific sedimentary components that may exert control on the uptake of Hg(II) and CH₃Hg(II) from sediments in the mussel Mytilus edulis, which is widely used in national monitoring programs as a bioindicator organism for detecting coastal contamination. Iron and manganese oxides, montmorillonite clay, and silica, with and without organic coatings, as well as natural sediment particles were radiolabeled with either ²⁰³Hg(II) or CH₃²⁰³Hg using nanomolar levels of Hg. Fulvic acid coatings enhanced the sorption of both Hg(II) and CH₃Hg(II) onto every type of particle, and partition coefficients were as high as 10⁵. The assimilation efficiencies (AE) in mussels were very low for Hg(II) (1-9%) but were typically >30% for CH₃Hg(II) and as high as 87%. The organic coatings increased AE for CH₃Hg(II) bound to all types of particles, but had no impact on Hg(II). Unlike Cd, Co, and Ag, there was little (<10%) desorption of ²⁰³Hg from particles at pH 5 (to simulate the behavior of food-bound metals in the acidic gut of bivalves), and it did not influence Hg assimilation in mussels. The absorption of CH₃Hg(II) by mussels is rapid and efficient even from uncoated inorganic particles that were rapidly egested out of the animal's gut due to their low nutritional value. The concentration factor (10³) of dissolved Hg(11) in mussels was half that of dissolved CH₃Hg(II). These results suggest that contaminated sediments can be a significant source of Hg, especially organic mercury, to marine suspension feeders and that CH₃Hg(II) is assimilable from all major sedimentary components.