Ascorbic acid-induced metabolic responses and antioxidant enzyme activity in the green microalga Dunaliella salina under long-term salinity

Abstract

Stimulation of antioxidant systems with metabolic changes by signaling molecules such as ascorbic acid (AA) is a common stress tolerance strategy in plants and algae. The effect of AA on tolerance to long-term salinity of the green microalga Dunaliella salina was investigated by studying biochemical changes and enzymatic antioxidant responses. Algal cells adapted to three salinity levels of 1, 2, and 3 M NaCl were treated with or without 1 mM AA in exponential growth phase. AA-untreated suspensions at 3 M showed lower cell number and protein carbonylation but highly accumulated levels of pigments, soluble sugars, starch, hydrogen peroxide, proteins, proline, and free amino acid and enhanced the activity of antioxidant enzymes, proteolysis, and lipid peroxidation. AA-treated suspensions at 3 M increased cell number, while decreased the activities of proteolytic and antioxidant enzymes relative to untreated controls. Protein carbonylation showed a mutual correlation with lipid peroxidation levels in both AA-treated and untreated cultures, indicating a unique pattern of protein modification in D. salina. However, AA was ineffective in the accumulation of metabolites in 1 and 2 M suspensions and showed metabolite-reducing effects in algae grown in 3 M NaCl. These data suggest that exogenous AA can improve the tolerance of D. salina to 3 M salinity by regulating the activity of enzymes involved in the metabolism of carbon sources and diverting its metabolites to growth processes instead of the synthesis of osmoprotectant substances. Therefore, despite its prominent ecological role, AA is of little biotechnological importance due to its negative impact on accumulation of desired metabolites.