A comparative screening of freshwater and marine microalgae for salinity-induced intracellular mannitol accumulation

The growing demand for sustainable, low-glycaemic sugar alternatives has increased interest in biological polyol production; however, comparative evaluations of microalgae for salinity-induced intracellular polyol accumulation remain limited.

This study systematically screened two freshwater species (Chlorella vulgaris and Scenedesmus obliquus) and two marine species (Dunaliella tertiolecta and Nannochloropsis oculata) using a two-stage cultivation strategy to evaluate intracellular polyol accumulation and associated physiological responses under salinity stress. Cultures were grown under optimal conditions before exposure to moderate and high salinity levels (16 and 32 g L^-1 for freshwater species; 60 and 80 g L^-1 for marine species). Growth, nitrate assimilation, intracellular polyols, carbohydrates, proteins, chlorophylls, and carotenoids were quantified.

No detectable intracellular polyols were observed in the freshwater species under any treatment. In contrast, intracellular mannitol accumulated exclusively in the marine species, reaching maximum concentrations of 28 mg g^-1 DW in D. tertiolecta and 34 mg g^-1 DW in N. oculata under moderate salinity. Moderate salinity promoted the highest intracellular mannitol accumulation in both marine species. Although biomass-specific mannitol yields decreased under high salinity, volumetric mannitol concentration remained relatively stable in N. oculata but declined markedly in D. tertiolecta. Salinity stress also induced distinct species-specific changes in carbohydrate, protein, and pigment composition, as well as nitrogen assimilation.

These findings demonstrate that intracellular mannitol accumulation is a species-specific response to salinity stress rather than a universal microalgal adaptation. The comparative screening identifies D. tertiolecta and N. oculata as promising candidates for future optimization of stress-induced intracellular mannitol accumulation and provides a framework for strain selection in sustainable microalgal biotechnology.