EFFECTS OF HEAVY-METALS ON TETRASELMIS-SUECICA - ULTRASTRUCTURAL AND ENERGY-DISPERSIVE X-RAY SPECTROSCOPIC STUDIES

The influence of metal contamination on the marine alga Tetraselmis su ecica was investigated at physiological and ultrastructural levels. Fo r this analysis, the growth response of this microalga was studied aft er the addition of various concentrations of heavy metals (Cd, Cu). Th e concentration corresponding to 50% growth inhibition (IC50) and the number of days per cell cycle (Td) studied, revealed that the toxic ef fects of copper are heavier than those of cadmium. In the case of copp er contamination, the Td grows with increasing metal concentration in the culture medium, while it remains unchanged during the cadmium cont amination. The toxicity of cadmium, only observed in the latency phase of growth, suggests an adaptation phenomenon of T suecica to this met al. Ultrastructural changes in response to pollutants were investigate d; copper induced cytoplasmic vacuolisation, organelle changes, appear ance of cells with multilayered cell walls and excretion of organic ma tter. In the case of cadmium contamination, ultrastructural changes ma inly affected the osmiophilic vesicles, of which both number and volum e increased with increasing metal concentration in the culture medium. The results of X-ray microanalysis revealed that Cd and Cu were stron gly present in excreted organic matter and osmiophilic vesicles. The l atter can be excreted during cell division, thus participating in deto xification processes. Intracellular cadmium incorporation proved that some toxic effects of this metal are a result of interaction with endo genous cellular constituents. In the case of copper contamination, the presence of copper in walls of a multilayered cell suggests that thes e structures constitute an additional adsorbing area for this element, reducing metal free concentration in the medium. Mechanisms of metal detoxification of Tetraselmis suecica are discussed.