Metabolic regulation and stress-defense strategies underlying differential responses of multiple microalgal strains to high CO2 stress.

Abstract

High CO2 levels can inhibit microalgal growth, limiting carbon sequestration efficiency. Most studies on microalgal responses to high CO2 have focused on single strains, leaving cross-strain comparisons underexplored. This study integrated physiology and transcriptomics to compare high-CO2 responses among three environmental microalgal strains: two Chlorella sorokiniana strains (SCU-5 and SCU-6) with contrasting CO2-tolerance phenotypes and one Micractinium pusillum strain (SCU-7). High CO2 affected energy and nutrient metabolism, protein synthesis and processing, genetic information maintenance, and stress defense, while strain-specific regulation appeared to determine tolerance. Under CO2 stress, the growth of SCU-5 and SCU-7 was inhibited. SCU-5 may preferentially allocate nitrogen to chlorophyll and photosynthetic components, potentially constraining functional protein synthesis, NADPH consumption, and electron transport, thereby increasing oxidative pressure and resource investment in antioxidant defense. In SCU-7, psbO downregulation likely weakened photosystem II electron donation and increased oxidative pressure, thereby contributing to the suppression of central carbon metabolism. In contrast, SCU-6 showed superior CO2 tolerance, achieving the highest biomass (1.42 g/L) under 15% CO2. Upregulation of ACLY, DGD, UGP2, and GR suggests that fatty acid biosynthesis, membrane remodeling, extracellular carbon metabolism, and glutathione regeneration may have contributed to the lower oxidative pressure and photosynthetic stability. Overall, this study provides insights into shared and strain-specific mechanisms under high CO2 stress and identifies candidate targets for improving microalgal CO2 tolerance.