COMPARISON OF PHYSIOLOGICAL-CHANGES IN EUGLENA-GRACILIS DURING EXPOSURE TO HEAVY-METALS OF HETEROTROPHIC AND AUTOTROPHIC CELLS

The effect of different concentrations of Hg2+, Cd2+, and Pb2+ on ultr astructure, growth, respiration, photosynthesis, chlorophyll content, and metal accumulation in Euglena gracilis was examined. The toxicity of the heavy metals was dependent on the culture medium used and wheth er cells were grown in the dark or under illumination. Hg2+ was the mo st toxic metal, which showed effects at. a concentration as low as 1.5 mu M; Cd2+ showed an intermediate toxicity (effects observed above 50 mu M); and Pb2+ was almost ineffective up to 1 mM. Cells grown for se veral weeks in the dark, in the presence of 1.5 mu M Hg2+ showed a red uced sensitivity to subsequent exposure to Cd2+ or Pb2+. The Hg2+-pret reated cells also presented an enhanced capacity to accumulate other m etals. In comparison, light-grown cells showed a greater Cd2+ accumula tion, but a lower Pb2+ uptake than Hg2+-pretreated dark-grown cells. P retreatment of light-grown cells with Hg2+ did not enhance the accumul ation of Cd2+. These results suggest chat the capacity to tolerate hea vy metals by Euglena may have mechanistic differences when cells are g rown in the dark or under illumination. (C) 1997 Elsevier Science Inc.