Growth and reproduction of Daphnia galeata in response to changes in fatty acids, phosphorus, and nitrogen in Chlamydomonas reinhardtii

The importance of changes in elemental and fatty acid composition of the algal food for Daphnia galeata was investigated. The green alga Chlamydomonas reinhardtii was grown under nitrogen or phosphorus limitation to modify its elemental and biochemical composition. Both N- and P-Limited algae exhibited similar fatty acid compositions but differed from algae grown under N and P saturation. Nutrient limitation of algae caused the amounts of saturated fatty acids, monounsaturated, and diunsaturated fatty acids to increase, but those of polyunsaturated fatty acids to decrease markedly. Life-history experiments with D. galeata, carried out to examine the effects of the varying N and P regimes to the food quality of Chlamydomonas, revealed that N-limited and N+P- saturated algae were of a comparable quality. In contrast, P- limited alga was a very poor food; that is, both population growth and somatic growth rate were much lower than with N- and N+P-saturated algae. Differences of algal fatty acid composition did not explain the differences in algal food quality as N+P-saturated and N-limited Chlamydomonas were both similar in quality despite differences in fatty acid composition. P limitation of daphnid growth is more consistent with the observed differences in grow-th and reproduction. The low growth rates of the daphnids when fed P-limited alga, however, may also be a result of indirect effects because P limitation may induce changes in algal morphology or biochemical compounds other than fatty acids. [KEYWORDS: Fresh-water zooplankton; nutrient element limitation; planktonic cladocerans; mineral limitation; food quality; p- limitation; phytoplankton; lipids; magna; microalgae]

Effect of the addition of polyunsaturated fatty acids to the diet on the growth and fecundity of Daphnia galeata

1. The importance of long-chain polyunsaturated fatty acids (PUFAs), in particular eicosapentaenoic acid (EPA), for the growth and development of Daphnia galeata (Sars) was tested using food types differing in PUFA and EPA contents. 2. Life history experiments of D. galeata fed with the cryptophyceans Rhodomonas lacustris (Pascher & Ruttner) and Cryptomonas pyrenoidifera (Gentler), and the green alga Scenedesmus acutus (Meyen), showed that both cryptophycean species were higher in quality than S. acutus. 3. Since the cryptomonads contained significant amounts of EPA while no EPA could be detected in Scenedesmus, tests were performed to ascertain whether EPA was responsible for the differences in food quality. Feeding daphnids a mixed diet of Scenedesmus and emulsion particles that were rich in EPA and DHA, resulted in a significant improvement in the intrinsic population growth rate. The initial difference in food quality between Scenedesmus and the cryptomonads was completely compensated for by addition of emulsion to the Scenedesmus food. 4. From the observed stimulatory effect of the addition PUFA to the daphnid diet, this study concludes that the presence of such long-chain PUFA improves food quality for daphnids. [KEYWORDS: Blue-green-algae; lipid-composition; microalgae; phytoplankton; quality; magna; fish]

Is the fatty acid composition of Daphnia galeata determined by the fatty acid composition of the ingested diet?

1. The fatty acid (FA) composition of Daphnia galeata and their algal food was analysed and showed many similarities, however, some significant differences were also found in the relative abundance of the FA C16:4 omega 3 and docosahexaenoic acid (DHA). Their relative abundances were much lower in daphnids than in their algal diet. 2. When daphnids were fed three distinct emulsion particles with DHA: eicosapentaenoic acid (EPA) ratios of c. 0.7, 2 and 4, the final DHA: EPA ratio in the daphnids always favoured EPA. The increase of the food DHA : EPA ratio resulted in a minor increase of DHA (to c. 2%). Feeding the animals on emulsion particles with increasing ratios of DHA : EPA, caused a minor (c. 2%) increase of DHA level but EPA levels remained high (c. 10%). 3. When labelled with [C-14]linoleic acid and [C-14]linolenic acid daphnids showed low conversion of both essential FA into C20 polyunsaturated fatty acids (PUFAs). This low conversion activity might explain the importance of C20 PUFAs as dietary compounds in the food of Daphnia. 4. The results indicate the insignificance of DHA and C16:4 omega 3 for daphnids. As EPA can be derived from C18:3 omega 3 it is not strictly essential, although it might be a significant factor in food quality for Daphnia. [KEYWORDS: Insects; microalgae; metabolism; quality; lipids; magna