THE IMPORTANCE OF HIGHLY UNSATURATED FATTY-ACIDS IN ZOOPLANKTON NUTRITION - EVIDENCE FROM EXPERIMENTS WITH DAPHNIA, A CYANOBACTERIUM AND LIPID EMULSIONS

1. We used laboratory growth and feeding experiments to evaluate the r ole of omega 3 highly unsaturated fatty acids (HUFA) in zooplankton nu trition. Polyunsaturated fatty acids (PUFA) comprised 40% of total fat ty acids (FA) in the green alga Scenedesmus acutus but only 6% in the cyanobacterium Synechococcus elongatus. Scenedesmus contained modest a mounts of the omega 3 HUFA eicosapentaenoic acid (EPA, 20:5 omega 3) a nd docosahexaenoic acid (DHA, 22:6 omega 3), whereas Synechococcus con tained only a trace of EPA and no DHA. 2. One-day-old Daphnia maintain ed high growth rates for 2-3 days on a diet of Synechococcus but exhib ited reduced growth or even lost weight over the later part of the 6- or 7-day experiments. Daphnia magna grew better than D. galeata or D. pulicaria on a diet of pure Synechococcus. Daphnids fed the same conce ntration (0.5 mgC l(-1)) of green algae or a mixture of green algae an d Synechococcus maintained high growth rates. 3. Supplementing Synecho coccus with fish oil emulsions rich in omega 3 HUFA markedly improved the growth and reproduction of all three Daphnia species. Supplementat ion with an emulsion of monounsaturated oleic add (18: 1 omega 9), how ever, did not affect the growth of D. galeata and caused a decrease in the growth of D. magna. 4. Short-term (7 min) and long-term (3 h) fee ding trials with C-14-labelled Synechococcus were used to evaluate the effects of acclimation and mixed diets on carbon incorporation. D. ga leata that had fed on unlabelled Synechococcus for 3 days exhibited no decline in clearance rate but a marked decline in carbon incorporatio n in comparison with animals acclimated with Scenedesmus or fed a mixt ure of Synechococcus and Scenedesmus. 5. Our results support the hypot hesis that the poor nutritional quality of a cyanobacterium is at leas t partially due to a HUFA deficiency. Growth and feeding experiments b oth suggest that the utilization of Synechococcus is enhanced by HUFA from lipid reserves or a mixed diet.