Transcriptomics Reveals an Energy-Saving Metabolic Switch in an Extremophilic Red Microalga Cyanidioschyzon merolae Under Nickel Stress

The red microalga <i>Cyanidioschyzon merolae</i> inhabits extreme environments with high temperatures (40-56 °C), high acidity (pH 0.05-4), and high concentrations of heavy metals that are lethal to most forms of life. However, information is scarce on the precise adaptation mechanisms of this extremophile to such hostile conditions. Gaining such knowledge is important for understanding the evolution of microorganisms in the early stages of life on Earth characterized by such extreme environments. Through an analysis of the re-programming of the global transcriptome upon the long-term (up to 15 days) exposure of <i>C. merolae</i> to extremely high concentrations of nickel (1 and 3 mM), the key adaptive metabolic pathways and associated molecular components were identified. Our work shows that the long-term Ni exposure of <i>C. merolae</i> leads to the lagged metabolic switch demonstrated via the transcriptional upregulation of the metabolic pathways critical for cell survival. DNA replication, cell cycle, and protein quality control processes were upregulated, while downregulation occurred with energetically costly processes, including the assembly of the photosynthetic apparatus and lipid biosynthesis. This study paves the way for future multi-omic studies of the molecular mechanisms of abiotic stress adaptation in phototrophs, as well as the future development of rational approaches to the bioremediation of contaminated aquatic environments.