Deciphering selenium tolerance in a halotolerant microalga

• Higher salinity (2 M NaCl) acclimated halotolerant D. salina tolerated 50 mg L −1 Se • Salt acclimation attributed to osmoticum and secondary metabolites • Increased glutathione metabolism rewired the metabolic flux led Se tolerance • Distinct metabolic adaptations and redox regulation at different salinity • Se accumulation in D. salina paved the way for biotechnological applications Microorganisms, including microalgae, are sensitive at low doses of selenium (Se). The halotolerant microalga Dunaliella salina, cultivated under high salinity conditions, efficiently accumulated Se at 50 mg L⁻¹ without adversely affecting its growth. Therefore, it was aimed at investigating photoautotrophic growth behaviour, macromolecular content in terms of carbohydrate content and compatible solute glycerol, and secondary metabolites such as flavonoids and phenol in managing the reactive oxygen species (ROS). The elevated levels of antioxidative enzymes ascorbate peroxidase and glutathione peroxidase showed reduced H 2 0 2 levels at 50 mg L −1 Se to that of the control. Gene expression analyses showed heightened expression of genes related to catalase peroxidase, glutaredoxin-dependent peroxiredoxin, thioredoxin reductase and S-adenosylhomocysteine, while genes for autophagy, selenocysteine synthase, membrane ABC transporter and methanethiol oxidase were downregulated. The metabolome profiling of Se exposed target organism revealed significant upregulation of phenylpropanoid biosynthesis, TCA cycle, sterol, carotenoid and N-metabolism. Status of the oxidative stress of the cell significantly decreased the metabolites related with the cellular energy, indicating carbon rewiring towards maintaining antioxidative defence of the cells in the presence of Se. Mitochondria based regulation played a vital role in maintaining redox balance, with elevated NADH ubiquinone oxidoreductase activity offering enhanced protection and catering to its energy demand, as reflected by the accumulation of the TCA cycle intermediate cis-aconitic acid under 50 mg L −1 Se. Therefore, the present study highlights the potential of D. salina in tolerating Se under elevated salinity condition, underscoring its promising application in both environmental bioremediation and nutritional biofortification.