Bioavailability and toxicity of sediment-bound lead to a filter-feeder bivalve Crassostrea gigas (Thunberg)

Two different approaches were used to study the bioavailability of sediment-bound lead. In vitro techniques simulating the potential metal desorption under conditions prevailing in the digestive tract were assayed on a contaminated sediment. An experimental model of a food chain was designed to determine the retention of lead in the soft tissues of oysters according to the environmental source of the metal (water or sediment). Neither enzymatic action nor leaching at low pH (both aspects of digestion) induce the release of important lead amounts from particles. Therefore, after 3 weeks of exposure, the retention of lead from the trophic source is lower (1%) compared with direct contamination (5%). Lysosomes are the major intracellular structures responsible for lead storage in the gills, digestive tract and digestive gland. The abundance of lysosomes and their lead amount vary according to the gross concentrations in the soft tissues. The cytopathological data are in agreement with the results about lead accumulation: in oysters exposed to sediment-bound lead, impairments are not so marked as in individuals contaminated directly from water but the same organelles are concerned. Mitochondrial impairments may be related to the effect of lead on cellular respiration processes and changes involving the granular endoplasmic reticulum may have an effect on the level of protein synthesis. Cellular extrusions carrying away numerous lysosomes loaded with lead could account for the balancing of lead incorporation between 2 and 3 weeks of exposure.