Influence of salinity on trace metal (Cu, Zn, Ag) accumulation at the molecular, cellular and organism level in the oysterCrassostrea gigas Thunberg

Abstract

The accumulation and physico-chemical forms of metals were determined by atomic absorption spectrometry in the whole soft tissues of oysters, by histological and microanalytical techniques in tissue sections, by gel permeation chromatography of soft tissue homogenates. Oysters were reared according to four types of experimental conditions: exposed to silver (20 micrograms Ag/l) or unexposed, in sea water (33%) or brackish water (8%). Copper, zinc and silver accumulation in oysters are inversely related to salinity. Amoebocytes, which play a key role in accumulating copper and zinc in natural sea water, are able to sequester an important part of added silver as Ag2S. In brackish water an increase of the number of amoebocytes may be considered as responsible for the enhancement of Cu and Zn concentrations in the whole soft tissues. In such conditions, additional silver is concentrated in these cells rather than in the basement membranes which are target structures for Ag2S accumulation in sea water. If the global fluctuations of metal concentrations in the soft tissue are ascribable mainly to changes at the histological level, the fate of metals in the soluble fraction must not be neglected since the speciation of metals influences their toxicity. The freshening of sea water induced a change in the distribution of cytosolic silver and zinc but in no case were the molecular masses of compounds associated with each metal perfectly identical. From these results, it is concluded that the accumulation of silver by oysters is not mediated by the same mechanisms as those for copper and zinc.