Sublethal Cytotoxic Effects of Mercuric Chloride on the Ciliate Tetrahymena pyriformis*

SYNOPSIS. The behavior and ultrastructure of Tetrahymena pyriformis was assessed after exposure to dosages of 8 and 16% of the lethal concentration of HgCl² (TLm 96 hr). The lower dosage caused no abnormal changes in cell motility, activity of the water explusion vesicles, or cell shape; the higher dosage caused deleterious changes in these parameters. The higher sublethal HgCl² concentration (0.50 mg/liter) elicited damage of several cell structures. This damage persisted and accumulated with time up to 24 hr. At the lower HgCl² dosage (0.25 mg liter) there were extensive changes after 1-hr exposure involving primarily mitochondria; however, all major changes were repaired after 24 hr of constant exposure to the HgCl², indicating adaptation to the toxicant. Based solely on cytotoxic evidence an attempt is made to apply the findings defining what constitutes a “safe'’concentration of HgCl₂ in the cell's environment.