Silver uptake by a marine diatom and its transfer to the coastal copepod Acartia spinicauda

Silver (Ag) is an important metal contaminant in many coastal waters and often is accompanied by high nutrient concentrations in the effluent outfall. The biological uptake of Ag by the marine diatom Thalassiosira pseudonana at various levels of nitrate, ammonium, and phosphate was examined under different growth conditions of the cells. The biological uptake of Ag increased significantly with increasing concentrations of nitrate, ammonium, and phosphate, presumably because of the increase in algal growth rate or increasing sulfur-containing ligands available for Ag transport. The calculated Ag uptake rate constants increased by 3 to 16 times with increasing nitrate and ammonium concentration from 5.88 microM to 176 microM. The assimilation efficiency (AE) of Ag by the coastal copepod Acartia spinicauda was quantified under different ecological and chemical conditions of the diatom prey. The Ag AEs were in the range of 3 to 23%, and increased with decreasing diatom food concentrations or when the diatoms were grown at a lower nutrient level. Significant correlations were demonstrated between the AE and the distribution of Ag in the diatom's cytoplasm, the Ag concentration factor in the diatoms, Ag retention in the particles during the feeding period, and the Ag gut passage time in the copepods. Desorption within the copepod's gut appears to play a critical role in Ag assimilation and partially accounts for the variability of Ag AEs under different food and geochemical conditions. Our study highlights that several geochemical and physiological processes all significantly affect Ag trophic transfer in marine copepods. Given the dependence of Ag transfer on its concentration in ingested particles and food concentration, dietary uptake of Ag is probably variable in natural environments. The influence of eutrophication on Ag trophic transfer is dependent on the degree to which each kinetic parameter is affected by nutrient enrichments.