Ascorbate-glutathione cycle alleviates low-temperature-induced oxidative stress for augmented growth of Nannochloropsis oceanica Rose Bengal mutants

Studying the adaptive mechanisms of algae to abiotic stresses, such as low temperature and high light intensities, can help facilitate large-scale outdoor production. Consequently, the role played by the antioxidant defense system in the tolerance of Nannochloropsis oceanica Rose Bengal (RB) mutants, with a truncated PSII complex, to low-temperature (LT, 18°C) under high-light (HL, 250 μmol⋅m⁻²⋅s⁻¹) conditions was explored. The wild type (WT) exhibited O₂^•- and H₂O₂ accumulation, lipid peroxidation, and cell death upon LT treatment, which was exacerbated by LT-HL. The RB mutants showed no oxidative stress during LT or LT-HL. Ascorbate peroxidase (APX; EC 1.11.1.11), dehydroascorbate reductase (DHAR; EC 1.8.5.1), and glutathione reductase (GR; E.C. 1.6.4.2) activity and transcript abundance increased by LT and LT-HL conditions in the RB mutants but not in the WT. In the RB mutants, the ascorbate (AsA) pool size stayed low, but the AsA/DHA ratio increased under LT and LT-HT conditions, while the glutathione (GSH) pool size and GSH/GSSG ratio increased. The RB mutants treated with buthionine sulfoximine (BSO), an inhibitor of GSH biosynthesis, became susceptible to LT-HL stress. The expression of GLUTAMYLCYSTEINE SYNTHETASE (NoGSH1) and GLUTATHIONE SYNTHASE (NoGSH2A and NoGSH2B), responsible for GSH biosynthesis, was upregulated in two RB mutants by LT and LT-HL stress, while that of GULONOLACTONE OXIDASE (NoGulo), involved in AsA biosynthesis, remained constant. Beyond reduced light energy absorption, increased the Ascorbate-Glutathione cycle enzyme expression and AsA and GSH regeneration in Nannochloropsis RB mutants enable adaptations to prevent oxidative damage caused by high-intensity illumination at low temperatures.