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

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 a-linolenic acid (a-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 a-LA, which indicates that a-LA is the limiting PUFA and that EPA cannot be converted into a-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 a-LA into EPA, both PUFAs are substitutable resources under EPA limitation, but because EPA cannot be converted into a-LA, both PUFAs are nonsubstitutable resources under a-LA limitation. In aquatic food webs, the factors that regulate energy transfer between primary producers and consumers are crucial in understanding the transfer of energy across the plant‐ herbivore interface. It has been clear for many years that variation in the carbon transfer efficiency from primary to secondary production is quite large. This variation can be attributed to variation in food quality, but the determinants of food quality might be of a different nature, such as morphology, digestive resistance, toxicity, and nutritional inadequacy. From a nutritional point of view, not all units of carbon are equal. Nutrient-limited algae (in freshwater systems mostly P-limited) are widely accepted to be a food source of low quality (Sterner and Schulz 1998). However, at C : P ratios ,300, food quality for Daphnia might become constrained by factors others than P (Sundbom and Vrede 1997). Unless mineral limitation, toxins, or algal morphology constrain the utilization of algal biomass, the quality of algal carbon determines carbon transfer efficiency. Low quality of carbon can be due to a shortage of essential biochemicals in the diet, since such nutrients cannot be synthesized or are synthesized by a consumer in amounts inadequate to sustain growth. Polyunsaturated fatty acids (PUFAs, fatty acids with two or more double bonds) are essential for many vertebrates and invertebrates (Stanley-Samuelson et al. 1988), and the importance of PUFAs in freshwater zooplankton nutrition has recently been articulated (Gulati and DeMott 1997). Re1