Effects of heavy metals on Tetraselmis suecica: Ultrastructural and energy‐dispersive X‐ray spectroscopic studies

Summry— The influence of metal contamination on the marine alga Tetraselmis suecica was investigated at physiological and ultrastructural levels. For this analysis, the growth response of this microalga was studied after the addition of various concentrations of heavy metals (Cd, Cu). The concentration corresponding to 50% growth inhibition (IC 50 ) and the number of days per cell cycle (Td) studied, revealed that the toxic effects of copper are heavier than those of cadmium. In the case of copper contamination, the Td grows with increasing metal concentration in the culture medium, while it remains unchanged during the cadmium contamination. The toxicity of cadmium, only observed in the latency phase of growth, suggests an adaptation phenomenon of T suecica to this metal. Ultrastructural changes in response to pollutants were investigated; copper induced cytoplasmic vacuolisation, organelle changes, appearance of cells with multilayered cell walls and excretion of organic matter. In the case of cadmium contamination, ultrastructural changes mainly affected the osmiophilic vesicles, of which both number and volume increased with increasing metal concentration in the culture medium. The results of X‐ray microanalysis revealed that Cd and Cu were strongly present in excreted organic matter and osmiophilic vesicles. The latter can be excreted during cell division, thus participating in detoxification processes. Intracellular cadmium incorporation proved that some toxic effects of this metal are a result of interaction with endogenous cellular constituents. In the case of copper contamination, the presence of copper in walls of a multilayered cell suggests that these structures constitute an additionnal adsorbing area for this element, reducing metal free concentration in the medium. Mechanisms of metal detoxification of Tetraselmis suecica are discussed.