Effects of paraquat on the green alga Dunaliella salina: protection by the mimic of superoxide dismutase, Desferal-Mn(IV).

Paraquat caused a time-, dose-, and light-dependent bleaching of the halophilic green alga Dunaliella salina. Sublethal levels of paraquat elicited increases in cell content of both superoxide dismutase and catalase, with changes in the pattern of electromorphs of these enzymes. Desferal-Mn(IV), which catalyzes the dismutation of O2- in vitro, protected against the toxic effect of paraquat. Desferal (desferoxamine mesylate) alone was toxic to D. salina, and the salts of Mn(II), Mn(III), and Mn(IV), in the absence of Desferal, did not protect. Desferal-Mn(IV) is green, but its absorbance was 15% or less than the peak absorbances due to the chlorophyll in D. salina under the conditions of exposure; hence, masking of incident light could not have been the basis of the protective effect of the complex. Incubation of the cells with Desferal-Mn(IV), for up to 8 h prior to the addition of paraquat, did not increase its protective action, and brief washing, following 30 min incubation with the complex, eliminated its protective effect. Neither catalase nor superoxide dismutase, added to the medium, provided protection against paraquat. These results support the view that Desferal-Mn(IV) gains entry into D. salina and protects against the lethal effect of paraquat by there catalyzing the dismutation of O2- into H2O2 + O2.