“Unequal Twins”: The Unique Structural Composition of Triacylglycerols in Oleaginous Ochrophytes from the IPPAS Collection

Microalgae are widely regarded as promising sustainable sources of edible oils and nutraceuticals. However, the evaluation of their biotechnological potential is often limited to the fatty acid (FA) composition of total lipids (TL), overlooking the regiospecific distribution of acyl groups within triacylglycerols (TAGs). In this study, we investigated the FA composition and positional distribution in TAGs of three ochrophyte oleaginous microalgal strains (Vischeria sp. IPPAS C-70, Gedaniella sp. IPPAS H-1564, and Tribonema sp. IPPAS H-150) cultivated under nutrient limitation in the stationary phase. Our primary objective was to demonstrate that species with nearly identical FA profiles in TL and native TAGs can exhibit fundamentally different regiospecific TAG structures. While all three strains accumulated comparable amounts of palmitic (16:0), palmitoleic (16:1), and eicosapentaenoic (EPA, an n-3 FA) acids in their TL and TAGs, positional analysis via enzymatic hydrolysis and 13C NMR spectroscopy revealed substantial differences in their regiospecific distribution, particularly within the sn-2-monoacylglycerol (MAG) fraction. In the eustigmatophycean strain Vischeria sp. IPPAS C-70, palmitic acid was highly concentrated at the sn-2 position (60.4%), making its oil structurally similar to human breast milk fat and highly valuable for functional nutrition. Conversely, in the xanthophycean strain Tribonema sp. IPPAS H-150, palmitic acid was almost entirely excluded from the sn-2 position (2.9%) and directed to the sn-1,3 positions, mimicking the structure of common vegetable oils. The diatom Gedaniella sp. IPPAS H-1564 demonstrated an intermediate chemotype, with palmitic acid relatively evenly distributed, showing only a slight preference for the sn-2 position. These findings underscore that TL and overall TAG FA profiles are insufficient for predicting the nutraceutical value and physical properties of microalgal oils. Regiospecific analysis is a crucial step in identifying true sustainable substitutes for specific dietary fats.