Physico-chemical form of trace metals accumulated by phytoplankton and their assimilation by filter-feeding invertebrates

Abstract

This study investigated whether the nature of the binding of the trace metals cadmium, silver and zinc accumulated by phytoplankton can affect their subsequent assimilation efficiencies (AE) in 3 filter-feeding benthic invertebrates, the green mussel Perna viridis, the clam Ruditapes philippinarum and the barnacle Balanus amphitrite. Seven phytoplankton species were chosen from a wide systematic range to ensure large differences in the partitioning of their accumulated trace metals into 3 fractions: (1) exchangeable metal adsorbed on the outside of the cells, as defined by extraction with the chelating agent 8-hydroxyquinoline-5-sulphonate; (2) incorporated metal that is in a soluble form; and (3) insoluble incorporated metal. There were few significant correlations between AE and the percentage of phytoplankton metal incorporated into any 1 fraction or combination of fractions. There is no support, therefore, for a generalised conclusion that any of the 3 fractions isolated represents the sole form of phytoplankton metal that is bioavailable for trophic transfer to a herbivore. Even trace metals bound to the insoluble fraction in phytoplankton may be bioavailable to herbivores. Furthermore, there were no consistent effects of phytoplankton concentration on metal AEs in 1 of the herbivores -P. viridis. There was no evidence that the AE of any of the 3 trace metals was changed when the herbivores were feeding on the phytoplankton species (Thalassiosira weissflogii) on which they were fed during acclimation.