Effect of salt stress on antioxidant system and the metabolism of the reactive oxygen species in <i>Dunaliella salina</i> and <i>Dunaliella tertiolecta</i>

Abstract

The physiological and biochemical adaptations of two chlorophytes Dunaliella salina and Dunaliella tertiolecta developed to extreme saline environment were assessed in the present study. Both Dunaliella cultures were treated with a range of NaCl concentrations ranging from 0.05 to 4 M NaCl and the influence of salinity on growth and antioxidant parameters were determined. Biomass yields, chlorophyll and carbohydrate contents were reduced at salinity extremes. Protein contents were elevated under low salinities. No evidence or for large change was found in soluble amino acids during salinity stress. Osmoregulation is mediated by glycerol as compatible solute in both Dunaliella species. The maximum glycerol production was observed at high growth salinities. Under hyposaline conditions, a low content of β-carotene was noticed, whereas hypersaline conditions induced an increase in this product, about 1.4 and 1.1-fold more than its value at optimum salinities for D. salina and D. tertiolecta, respectively. An exposure to 0.5, 0.1, and 4 M NaCl increased H₂O₂ contents were positively correlated with the level of thiobarbituric acid reactive substances. The levels of six antioxidant enzymes (superoxide dismutase, catalase, peroxidase, ascorbate peroidase, glutathione reductase and glutathione S-transferase) and two antioxidant substrates (glutathione and ascorbic acid) were quantified. The data revealed a differential response between D. salina and D. tertiolecta in response to different salinity levels. The involvement of oxidative stress at various salinity levels is implied by the alterations in antioxidant enzymes and substrates, but the specific changes are very different between hypo and hypersaline stress conditions.

Key words: Antioxidant system, β-carotene, Dunaliella, glycerol, reactive oxygen species, salt stress.