Photoautotrophic Growth and the Transcriptome of the Halotolerant Model Microalga Dunaliella salina MCC43 Under Salt Stress

Dunaliella salina is a geographically distinct, green alga without a rigid cell wall recognized as a model system for halotolerance. In this study, D. salina strains of the Indian origin isolated from the Sambhar Lake, Rajasthan, were initially cultured optimally at a 0.5 M NaCl level, and these cultures were exposed to 1-2 M NaCl for up to 2 h for transcriptomic analyses. The cells did not accumulate substantial levels of intracellular Na⁺. Reduced photosynthetic efficiency was observed at 2 h with an Fv/Fm ratio (~0.2) and thereafter reached a maximum on the 10th day. Concurrently, elevated levels of proline and malondialdehyde (MDA) were also detected at higher salt concentrations, suggesting the initiation of a stress response. The transcriptomic study revealed that amino acid biosynthesis pathways, along with pyruvate and carbohydrate metabolism-related genes, were significantly enriched in 0.5 M versus 2 M NaCl-grown D. salina cells at 2 h. Downregulated genes were mainly related to cytoskeletal and microtubule proteins as owing to the loss of flagellar structures at 2 h. There was a change in the carbon flux because of the upregulation of key genes concerned with glyoxylate and TCA cycles, along with fatty acid metabolism, contributing to the remodeling of membrane lipids, thereby supporting membrane integrity. The upregulated differentially expressed genes (DEGs) were validated through qRT-PCR for the photosynthetic apparatus, antioxidative defense, ion homeostasis, and protein translocation and folding. The metabolomic profile under long-term (10th day) acclimation showed the upregulation of secondary metabolites and sterols. Therefore, the altered expression of genes indicated that D. salina strains tolerated elevated salt levels in a specified adaptive process to attain metabolic readjustments.