Comparison of Physiological Changes in Euglena gracilis During Exposure to Heavy Metals of Heterotrophic and Autotrophic Cells

The effect of different concentrations of Hg 2+, Cd 2+, and Pb 2+ on ultrastructure, 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 whether cells were grown in the dark or under illumination. Hg 2+ was the most toxic metal, which showed effects at a concentration as low as 1.5 μM; Cd 2+ showed an intermediate toxicity (effects observed above 50 μM); and Pb 2+ was almost ineffective up to 1 mM. Cells grown for several weeks in the dark, in the presence of 1.5 μM Hg 2+ showed a reduced sensitivity to subsequent exposure to Cd 2+ or Pb 2+. The Hg 2+-pretreated cells also presented an enhanced capacity to accumulate other metals. In comparison, light-grown cells showed a greater Cd 2+ accumulation, but a lower Pb 2+ uptake than Hg 2+-pretreated dark-grown cells. Pretreatment of light-grown cells with Hg 2+ did not enhance the accumulation of Cd 2+. These results suggest that the capacity to tolerate heavy metals by Euglena may have mechanistic differences when cells are grown in the dark or under illumination.