Evidence of physiological adaptation of Chlorella vulgaris under extreme salinity – new insights into a potential halotolerance strategy

Over the last century, anthropogenic activities have increased global carbon emissions and freshwater use, intensifying critical environmental problems such as global warming and water scarcity. This study unveils the metabolic and physiological adaptation of Chlorella vulgaris under extreme NaCl and CO₂ concentrations, suggesting a possible halotolerance mechanism for this freshwater species. Under hypertonic conditions (up to 3.5% NaCl - 600 mM), C. vulgaris controllably induces Na⁺ uptake and sharply increases intracellular carbohydrate content. This combination, along with an unexpected cell volume plasticity, effectively regulates osmotic equilibrium, ensuring cell viability and functionality. After the initial salinity shock, osmoadaptation mechanisms tend to actively eliminate Na⁺ influx and restore cell turgidity. Interestingly, photosynthetic activity and fluorescence induction measurements, revealed that exposure to a 30% CO₂ atmosphere could partially alleviate photosystem II excitation pressure and increase photosynthetic efficiency. Therefore, enrichment of readily available energy reserves significantly reduces salinity stress while simultaneously, changes in lipid composition result in high-added value biomass. This euryhaline bioenergetic strategy observed in C. vulgaris could pave the way for numerous sustainable biotechnological applications while dealing with major environmental issues.