Acute dietary pre-exposure and trace metal bioavailability to the barnacle Balanus amphitrite

Rates of uptake from solution and assimilation efficiencies of the trace metals Ag, Cd and Zn were investigated in the barnacle Balanus amphitrite after exposure in the laboratory for 19 days to low and high doses of added Ag and Cd in a diatom (Thalassiosira weissflogii) diet, the major route of metal uptake in barnacles. The hypothesis under test was that acute metal pre-exposure would affect the assimilation efficiency (AE) of that and other metals and their rate of uptake from solution. It was found that pre-exposure of the barnacles to atypically high dietary challenges of Cd and Ag did not cause changes in the rates of uptake of Cd, Ag or Zn from solution. Similarly, there was no clear consistent effect of dietary pre-exposure to Cd or Ag on the assimilation efficiency of Cd, Ag or Zn. The efflux rates of the metals were also comparable following the acute dietary exposure. Subcellular fractionation data indicated that the majority of the three metals were partitioned in the insoluble fraction, with very little in the soluble fraction consisting of metallothionein-like proteins and other (heat-sensitive) proteins. The lack of induction of increased Cd or Ag AE after pre-exposure in barnacles contrasts with results for mussels; this inconsistency is interpreted to result from differences in physiological accumulation patterns, the barnacles relying to an extreme extent on insoluble detoxification.