Gas chromatography for analysis and estimation of C at natural abundance level in fatty acids produced from , a sustainable source of polyunsaturated fatty acid.

is a promising microbial source of polyunsaturated fatty acids (PUFAs), particularly Docosahexaenoic Acid (DHA, C22:6n-3). In this study, we first optimized the culture conditions of ATCC MYA-1381 (strain SR21). Cell growth was monitored optical density, cell counts, and glucose concentration. Cells were harvested at exponential and stationary phases, and lipids were extracted using a green method. Fatty Acid Methyl Esters (FAMEs) were prepared and analyzed using Gas Chromatography-Flame Ionisation Detection (GC-FID). At the exponential phase, DHA was the most abundant (65.6% of total fatty acids) followed by palmitic acid (C16:0) at 34.4%. At the stationary phase, Docosapentaenoic acid (DPA, C22:5n-3) and DHA were the most abundant at 45.4% and 33.9%, before respectively. Myristic acid (C14:0), myristoleic acid (C14:1n-9), palmitic acid (C16:0) were present at 4.6%, 6.2% and 9.9%, respectively. Compound-specific isotope analysis (CSIA) using Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry (GC-C-IRMS) revealed that all FAMEs had negative δC values, indicating depletion in C. At the exponential phase, δC (‰) of C16:0 and DHA were -16.8 ± 0.2 and -18.5‰ ± 0.1‰, respectively. At the stationary phase, δC (‰) of C14:0, C14:1n-9, C16:0, C22:5n-3 and DHA were -10.6 ± 1.1, -11.3 ± 0.1, -11.1 ± 0.2, -8.3 ± 0.2 and -10.6‰ ± 0.1‰, respectively. Overall, our findings emphasized the importance of as a viable microbial platform for environmentally friendly production of PUFA such as DHA. Also, the study reinforced the utility of CSIA in tracking PUFA metabolic fate, which has latent applications in biomedical research, particularly in neurodegenerative disease frameworks where DHA plays a vital role. Finally, these results may also contribute to understanding isotopic fractionation patterns and metabolic flux variations across different microalgal growth phases.