This study investigated the effects of elevated salinity on the growth, morphology, and biochemical composition of two freshwater microalgae species: Chlorella thermophila (a green alga) and Anabaena variabilis (a cyanobacterium). The goal was to understand their adaptive mechanisms under saline stress and evaluate their potential for biofuel production. These species were chosen for their ecological significance and contrasting cellular structures (unicellular versus filamentous), which provide complementary insights into salinity tolerance among freshwater microalgae. Cultures were maintained in both standard BG11 medium and artificial seawater medium (SWM) under controlled light intensity (200 μmol photons m-2 s-1) and pH (7.5). Over a cultivation period of 10-20 days, we quantified key parameters, including cell size, volume, chlorophyll a, protein, lipid content, and fatty acid profiles, using microscopy (ImageJ), spectrophotometry, FTIR, and GC-MS analysis. Both species showed increased cell volume and lipid accumulation in SWM, with C. thermophila experiencing a dramatic volume increase from 5.83 μm3 to 54.76 μm3) and a 6.8% rise in lipid productivity. Fatty acid profiling identified distinct fatty acids, such as palmitoleic and pantetheic acids, in C. thermophila cultivated in SWM. These specific fatty acids may indicate adaptive strategies for osmoregulation or metabolic shifts toward energy storage under salinity-induced stress. These findings highlight the potential of salinity-driven modulation of microalgal metabolism to enhance biofuel precursors, offering a sustainable approach for biomass production in saline environments with reduced reliance on freshwater.

Abstract
1. INTRODUCTION
2. MATERIALS AND METHODS
    2.1. Collection, morphological identification ofalgal species
    2.2. Isolation and purification of algal species
    2.3. Algal species cultivation
    2.4. Cell size and cell volume measurement
    2.5. Estimation of biomass of selected algalspecies
    2.6. Estimation of photosynthetic pigment
    2.7. Determination of protein
    2.8. Extraction of lipids
    2.9. Lipid productivity using FTIR analysis
    2.10. Fatty acid analysis
    2.11. Transesterification of algal oil
    2.12. Fourier transform infrared spectrameasurement
3. RESULTS AND DISCUSSION
    3.1. Isolation, purification, and cultivation ofalgal species
    3.2. Effects of seawater medium on cell size,volume and biomass percentage
    3.3. Photosynthetic pigment content
    3.4. Determination of protein
    3.5. Lipid productivity
    3.6. FTIR analysis
    3.7. Fatty acid composition of the algal lipids
4. CONCLUSION
CRediT authorship contribution statement
Declaration of Competing Interest
ACKNOWLEDGEMENTS
REFERENCES

• Manu Upadhyay(Department of Biotechnology, Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh 484887, India)
• Vijetna Singh(Department of Biotechnology, Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh 484887, India)
• Sana Fatima Ikram(Department of Biotechnology, Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh 484887, India)
• Bhumi Nath Tripathi(Department of Biotechnology, Indira Gandhi National Tribal University, Amarkantak, Madhya Pradesh 484887, India) Corresponding author
• Baik Ho Kim(Department of Life Science, College of Natural Science, Hanyang University, Seoul 04763, Republic of Korea)