DeMott, W.R., D.C. Muller-Navarra
Abstract:
We used laboratory growth and feeding experiments to evaluate the role of omega-3 highly unsaturated fatty acids (HUFA's) in zooplankton nutrition. Polyunsaturated fatty acids comprised 40% of total fatty acids in the green alga Scenedesmus acutus but only 6% in the cyanobacterium Synechococcus elongatus. Scenedesmus contained modest amounts of the omega-3 fish oils eicosapentaenoic acid (EPA, 20:5w-3) and docosahexaenoic acid (DHA, 22:6w-3), whereas Synechococcus contained only a trace of EPA and no DHA.
One-day old Daphnia maintained high growth rates for 2-3 d on a diet of Synechococcus but exhibited reduced growth or even lost weight over the later part of the 6 or 7 d experiments. In contrast, daphnids fed the same concentration (0.5 mgC/L) of green algae or a mixture of green algae and Synechococcus maintained high growth rates. Daphnia magna grew better than D. galeata or D. pulicaria on a diet of pure Synechococcus.
Supplementation of Synechococcus with fish oil emulsions markedly improved the growth and reproduction of all three Daphnia species. Supplementation with an emulsion of monounsaturated oleic acid (18:1w-9), however, had no effect on the growth of D. galeata and caused a decrease in the growth of D. magna. Short-term (7min) and long-term (3h) feeding trials with 14^C-labeled Synechococcus were used to evaluate the effects of acclimation and mixed diets on carbon incorporation. D. galeata which had fed on unlabeled Synechococcus for three days exhibited no decline in clearance rate but a marked decline in carbon incorporation in comparison with animals that had been acclimated with Scenedesmus or were fed a mixture of Synechococcus and Scenedesmus.
Our results support the hypothesis that the poor nutritional quality of a cyanobacterium is due to a deficiency in omega-3 HUFA's. Growth and feeding experiments both suggest that the utilization of Synechococcus is enhanced by HUFA's from lipid reserves or a mixed diet.
(Max Planck Institute of Limnology, Department of Physiological Ecology, P.O. Box 165, D-24302 Plon, Germany)
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