Determination of limiting polyunsaturated fatty acids in Daphnia galeata using a new method to enrich food algae with single fatty acids

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A new method that allows the highly reproducible supplementation of free fatty acids to planktonic microalgae was used to investigate the role of particular highly unsaturated fatty acids (PUFAs) in somatic growth limitation of Daphnia galeata feeding on Scenedesmus obliquus or Stephanodiscus hantzschii. No evidence for biotransformation of the supplemented fatty acids into other fatty acids by the algae was found. Using the algal cell itself as a transfer vehicle, the supplemented fatty acids were incorporated by D. galeata. In standardized growth experiments with juvenile D. galeata, growth on S. obliquus was improved by supplementation with the PUFAs α‐linolenic acid (α‐LA, 18:3n–3), eicosapentaenoic acid (EPA, 20:5n–3), and docosahexaenoic acid (DHA, 22:6n–3), but not by arachidonic acid (ARA, 20:4n–6), which illustrates that PUFAs should not be regarded as a single resource. Corresponding changes in the fatty acid pattern of D. galeata indicated that EPA is the limiting PUFA during growth on S. obliquus and that D. galeata converts DHA and C18‐PUFAs into EPA. Growth on S. hantzschii was not improved by supplementation with EPA and ARA but was with α‐LA, which indicates that α‐LA is the limiting PUFA and that EPA cannot be converted into α‐LA. These results suggest that the availability of EPA determines which PUFA is limiting for growth. Because of the ability of the daphnids to convert α‐LA into EPA, both PUFAs are substitutable resources under EPA limitation, but because EPA cannot be converted into α–LA, both PUFAs are nonsubstitutable resources under α–LA limitation.