Accumulation and toxicity ol Cd, Zn, Ag, and Hg in four marine phytopiankters

Abstract

Accumulation and toxicity of Cd, Zn, Ag, and Hg were measured in the diatom Thalassiosira pseudonana, the chlorophyte Dunaliella tertiolecta, the coccolithophore Emiliania huxleyi, and the cyanophyte Oscillatoria woronichinii. Bioaccumulation of the metals was measured over a wide (up to lo5) range of metal concentrations, using gamma-emitting radioisotopes of each metal. Metal content of cells was related to total external metal concentration in all cases, in accordance with Freundlich adsorption isotherms. Dead cells accumulated metals comparably to living cells, indicating that the initial association of metal with the cell is governed by adsorption. Volume/volume concentration factors computed at equilibrium ranged from 3 X 10' for Cd in T. pseudonana to 9.5 X lo4 for Hg in E. huxleyi. Metal toxicity, as measured by depression of cell division rate, could generally be described as an exponential function of the log of the external (or cellular) metal concentration, consistent with the concept of cell thresholds of safe metal accommodation. Regression analyses of the data were used to calculate EC5,, and ECo metal concentrations for each alga/metal combination. The general order of accumulation and metal toxicity vs. external metal concentration was Hg > Ag > Zn 2 Cd. Expressed on a cellular metal basis, Hg was usually the most toxic, but there was no clear trend for the other metals for all species. Upon normalizing cellular metal data on the basis of cellular weight, volume, and surface area, interspecific comparisons revealed that these algal species can have markedly different surface affinities for metals and that cellular ECso values ranged from 7.9 X l o 5 M for Hg in 0. woronichinii cells to 0.2 M for Zn in D. tertiolecta cells. The most resistant species to Zn, Ag and Hg was D. tertiolecta. Enhanced metal tolerance in D. tertiolecta was partly attributable to metal exclusion, however these cells were able to tolerate cellular metal concentrations of up to 10 (Zn) or 100 (Ag and Hg) times those in the other species. Since the toxicity of any metal was correlated with its concentration factor in a given cell, these data may provide a basis for predicting the toxicity of other metals to algae from metal concentration factors in the cells.