Biochemical Differentiation of Freshly Isolated and Free-Living Cultured Symbiodinium microadriaticum: Fatty Acid Signatures and Proximate Composition

Coral-associated dinoflagellates undergo substantial physiological changes when transferred from the host environment to laboratory culture, yet direct compositional comparisons remain limited. We compared the proximate composition and fatty acid profiles of Symbiodinium microadriaticum (ITS2 type A1) freshly isolated from Stylophora pistillata with free-living cultures derived from the same coral-associated symbiont material and maintained under controlled conditions. Total lipid and crude protein contents did not differ significantly between states, whereas fatty acid composition differed markedly. Sixteen individual fatty acids remained significantly different after Benjamini-Hochberg false discovery rate correction. Freshly isolated samples showed higher relative abundances of several saturated, monounsaturated, and n-6 polyunsaturated fatty acids, including 16:0, 18:0, 16:1n-7, arachidonic acid, and docosatetraenoic acid, together with a higher proportion of total n-6 fatty acids. Free-living cells showed higher relative abundances of 14:0, 16:3n-4, α-linolenic acid, and 18:5n-3, together with higher proportions of total PUFA, n-3, and n-9 fatty acids and a higher docosahexaenoic acid/arachidonic acid (DHA/ARA) ratio. These contrasting signatures indicate marked state-associated biochemical differentiation despite similar bulk lipid and protein contents. The results provide a direct compositional reference for interpreting cultured Symbiodiniaceae used in algal physiology, coral-algal symbiosis research, and experimental host acquisition.