High levels of n−3 polyunsaturated fatty acids in Euphausia pacifica and its role as a source of docosahexaenoic and icosapentaenoic acids for higher trophic levels

Abstract

In Euphausia pacifica specimens, phospholipids (PL, 44.1–81.2%) such as phosphatidylethanolamine (PE, mean±standard error, 26.1±2.1%) and phosphatidylcholine (PC, 29.0±2.3%) were found to be the dominant lipids. Compared to many other marine organisms, the level of PL was markedly high. The major fatty acids in the TAG of E. pacifica were saturated fatty acids (SFA, 14:0 and 16:0), monounsaturated fatty acids (MUFA, 16:1 and 18:1), and polyunsaturated fatty acids (PUFA, 18:4 n−3 and 20:5 n−3, icosapentaenoic acid: EPA). The only fatty acids found in the tissue PL were SFA (16:0), MUFA (18:1), and PUFA (20:4 n−6, arachidonic acid: AA, EPA, and 22:6 n−3, docosahexaenoic acids: DHA). The comparatively high levels of 14:0, 16:0, 16:1 n−7, 16:2 n−4, 16:4 n−1, 18:1 n−7, 18:1 n−9, 18:4 n−3, and EPA in the TAG may be affected by the lipid composition of its phytoplankton prey, such as diatoms, which generally contain high amounts of these fatty acids. The levels of short chain 14:0, 16:1 n−7, 16:2 n−4, 16:4 n−1, 18:1 n−9, and 18:4 n−3 decreased in PL, while AA, EPA, and DHA increased. In particular, the levels of n−3 PUFA in PL were markedly high in all specimens. The mean proportion of EPA plus DHA accounted for more than 45% (55.0% in PE and 47.7% in PC) of the total fatty acids. These results suggest that E. pacifica, which contains markedly high levels of EPA and DHA, may biosynthesize these PUFA by carbon chain elongation and desaturation or selectively accumulate n−3 PUFA. In addition, their lipids may be an important source of n−3 PUFA for higher marine animals, such as the pelagic marine fishes (sardines, anchovies, and other small fishes), which mainly prey on these euphausiids. Marine fish accumulate high levels of these n−3 PUFA because they are unable to synthesize DHA.