No abstract

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.

Contaminated sediments are a possible source of stress to the benthic biota. In order to examine this way of transfer concurrently with direct exposure, oysters were exposed to As dissolved in natural seawater or loaded to particles. The sediment used as a vector of transfer was a mud from a coastal area devoted to oyster culture, the finest particles of which have been selected. It was submitted to experimental contamination then to in vitro desorption tests, in which enzymes and pH changes were used to mimic the digestive processes in Molluscs. Although different enzymes or pH induced the desorption of 3 to 24% of sediment-bound arsenic, the accumulation of this element in the soft tissues of oysters remained low after exposure to contaminated particles. The uptake of soluble arsenic was also limited although checking the level of arsenic in seawater every day revealed no significant decrease of the contaminant in the experimental medium. However, cytological effects were noted in oysters exposed to sediment-bound arsenic and moreover to soluble arsenic. They consisted of structural alterations of mitochondria and nuclei, suggesting a disturbance of both the cellular respiratory metabolism and nucleotid incorporation.

H. Ettajani, L. Pirastru oysters (Crassostrea gigas) exposed through sea water or via particulate matter (natural sediment, diatoms). The physico-chernical forms of silver stored in oysters suggests a moderate transfer to consumers. In the case of copper, transfer through feeding is not a negligible factor, through the associated risk is not doubt limited because of the key biological role played by this metal, and it therefore merits careful evaluation.