Enhanced Tolerance to Mercury in a Streptomycin-Resistant Strain of <i>Euglena gracilis</i>

Toxicity of Hg 2+ was determined in two strains of Euglena gracilis var. bacillaris : the wild-type (B) and a streptomycin-resistant strain (Sm r ). Cells were cultured under 12-h dark/12-h light regime or under continuous darkness. In the dark/light cultures, cellular growth was severely diminished in the B strain by Hg 2+ , whereas only a slight decrease was attained in the Sm r strain; Hg 2+ also affected the photosynthetic and respiratory activities of the B strain, but not those of Sm r strain. Under continuous darkness, cellular growth of both strains was lower than under dark/light cycles, but it was inhibited by Hg 2+ to a much lesser extent. Cell culture by 5 days under continuous dark or by 8 days of dark/light cycles resulted in a higher intracellular content of mercury in B strain than in Sm r strain. In contrast, in both culture conditions, the fraction of mercury removed from medium by B strain was lower than that attained by Sm r strain, whereas the ability to bio-transform (reduce) Hg 2+ was two times higher in the mutant strain. The results suggested that Sm r strain cells acquired an ability to remove Hg 2+ from the medium, which was not associated to accumulation and which conferred protection against mercury.