Temperature and diet effects on Daphnia lipids and fitness

Aquatic primary producers synthesize and subsequently supply dietary essential lipids and fatty acids required for somatic development, reproduction, and eventually survival of consumers. Poikilotherms, including algae, zooplankton, and fish, have a vital physiological requirement for polyunsaturated membrane lipids to keep their membranes fluid, to stay active and eventually survive at low water temperatures. Although we could lately increase our understanding of taxa-specific fatty acid retention patterns, little is still known about, a) the ability of algae and zooplankton to adapt their membrane lipid composition at different temperatures, b) how temperature and algal diet affect somatic growth condition, reproduction, and survival of zooplankton. We propose a series of laboratory experiments to study effects of water temperatures on lipid composition of organisms at the aquatic plant-animal interface and on life-history traits of the cornerstone freshwater herbivore Daphnia. In the first step, the effect of temperature on lipid composition of different primary producers (cryptophyta, bacillariophyta, chlorophyta) will be tested. Second, laboratory feeding experiments will be used to investigate temperature- dependent regulation of the fatty acid composition of the two main lipid classes (membrane and storage lipids) in Daphnia. Moreover, we test the ability of daphnids to respond adaptively to different temperatures by regulation of enzymes enabling them to form different phospholipids to offset a cold-induced rigidification of cell membranes (`homeoviscous adaptation`). Subsequently, this knowledge will be related to variation of somatic growth rates, reproductive success, and survival of Daphnia at different temperatures. This research project links water temperature (external factor) with physiological adaptation strategies (endogenous regulation) at the base of the aquatic food chain. This proposal will employ state-of-the-art methods, including cell membrane-specific fatty acids analysis, gene expression as well as nucleic acids analysis (for indicating somatic growth conditions). Results of this proposal will contribute to a more detailed and highly required understanding of nutrient and biomass dynamics at the base of the aquatic food chain, and will provide critical information about biochemical quality of dietary lipid supply to higher trophic levels under different water temperature scenarios.

This project explored how zooplankton can develop at different water temperatures and food compositions and how their lipid and fatty acid composition gets changed. These studies showed that zooplankton modify their lipids with water temperature and especially preferably retained important omega-3 fatty acids. We could also show that zooplankton can quickly absorb dietary lipids, thus their lipid profile is changed. This finding has important implications for subsequent consumers at higher trophic levels such as fish. Although terrestrial organic matter in freshwater ecosystems is predicted to increase and will thus become a potentially more available resource for zooplankton, we show that such terrestrial organic matter cannot compensate for algae that make up the most important diet source for zooplankton. Zooplankton fed on terrestrial organic matter grew much less and produced no offspring, thus additional terrestrial matter as potential food for zooplankton cannot support a steady zooplankton population in nature. This project was the first to demonstrate that omega-3 fatty acids preferentially bioaccumulated in cell membranes of freshwater copepods, especially at low water temperatures and were strongly retained even during a period of starvation, while only its storage fats were consumed. This research project provided important insights for aquatic food web research and shows that zooplankton in warmer waters deliver lower omega-3 fatty acids to fish, which in turn can lead to physiological deficits in fish and ultimately in humans.