Role of oxidative stress and thiol antioxidant enzymes in nickel toxicity and resistance in strains of the green alga Scenedesmus acutus f. alternans

Treatment with Ni(NO3)(2) leads to the formation of reactive oxygen species (ROS) in the green alga Scenedesmus acutus f. alternans, causing lipid per oxidation. This effect was stronger in a Ni-sensitive strain, UTEX72, than in a Ni-resistant strain, B4. In the resistant strain, Ni induced an increa sed ratio of reduced to oxidized glutathione (GSH:GSSG), whereas it caused a lowered ratio in the sensitive strain. Enzymes involved in the control of ROS were studied in these strains as well as two others that have shown di fferent degrees of nickel resistance. The resistant strain, B4, which grows while containing large amounts of internal Ni, had much higher levels of g lutathione reductase and catalase than the other strains. The sensitive str ain, UTEX72, had higher levels of glutathione peroxidase, superoxide dismut ase, and glucose-6-phosphate dehydrogenase than did strain B4. The resistan t strains, Ni-Tol and Cu-Tol, derived from strain UTEX72, which are partly able to exclude Ni, had enzyme profiles that resembled that of UTEX72 more closely than that of B4. Treatment with 10 and 100 muM Ni for 4 or 22 h had complex effects on enzyme levels in all four strains. Ni decreased glutath ione reductase in B4, slightly increased it in Ni-Tol and Cu-Tol, and did n ot affect the low levels of this enzyme in UTEX72. Ni lowered glutathione p eroxidase in B4 and either did not affect it or slightly raised it in the o ther strains. Ni lowered catalase in B4 and did not affect the other strain s. Superoxide dismutase was raised in B4 and Ni-Tol and lowered in Cu-Tol a nd UTEX72, and glucose-6-phosphate dehydrogenase was lowered in all four st rains. These results suggest that one major mechanism of Ni resistance, esp ecially in strain B4, may be the ability to combat the formation of ROS whe n exposed to this metal, likely by maintaining a high GSH:GSSG ratio.