Isolation, identification and biological activity of grazing resistance promoting kairomones in the Scenedesmus (phytoplankton) -Daphnia (zooplankton) system.
01 / 1998 - 06 / 2004
Plants have evolved a set of defense mechanisms against herbivores, involving structural, behavioral and, direct and indirect chemical mechanisms. This is well known for terrestrial systems but there is very restricted knowledge about this subject in aquatic ecology. In aquatic systems, numerous studies have been devoted to the grazing pressure of herbivorous zooplankton on phytoplankton, and it is well known that zooplankton species feed with highly different success on various phytoplankton species. Fish feeds on water flea and the water flea grazes on green algae. The fish exudes a compound that as a signal on water flea. Similarly the Daphnia produces a compound that alerts algae to their presence. In some cases this is due to the production of endo- or extra cellular toxins by primary producers. These toxins serve as a highly efficient predator defense (Lampert, 1981). Another defense mechanism is a change in size, shape and cell wall structure of the phytoplankton (De Bernardi and Guissani, 1990). While it is notorious that grazing resistance may even vary among different strains of the same algal species, there is hardly any knowledge on the factors inducing these morphological changes. A possible cause might be chemicals released into the aquatic environment by either phyto- or zooplankton. Chemical communication in aquatic systems has gained interest recently, but has mainly focused on the response of zooplankton to the presence of potential predators (e.g. Fish-water flea interaction). There is little information regarding grazer-mediated anti-predator responses in phytoplankton. Recently, Hessen and van Donk (1993) discovered Daphnia-induced plasticity in the green alga. Scenedesmus subspicatus Chod. They demonstrated that the unicellular cells started to form eight-cell coenobia, heavily armed with spines, within 3-5 days after adding water filtered through a 0.45 µm membrane from an algal culture with Daphnia present. More detailed grazing experiments with daphnids and species from other algal groups were carried out with exudates from Daphnia. Morphological and structural changes were investigated and a bioassay for monitoring the effects of exudates was developed (Lürling, 1999). The next obvious step would be elucidation of the structures of the substances causing the effect. The group of Prof. Francke (University of Hamburg) is working on the isolation of the fish compound and at our department research is done on the isolation of the Daphnia compound. At the moment virtually nothing is known about chemicals serving as chemical messengers in the communication between plants and animals in the aquatic environment. Therefore everything which is found in the course of this project will be new and lead to more insight in aquatic chemical communication. Once the structures are known and the compounds available in larger quantities more detailed biological tests will be carried out. Questions about type of compounds, stability, origin, specificity, activity and occurrence could then be solved. AimsThe aims of the project are: Elucidation of the structure(s) of the kairomone in the Daphnia- Scenedesmus system Making available larger quantities of the kairomone for further experiments Development of a sensitive quantitative assay for the kairomone Determination when, how and by which organisms the compounds are produced Preparing a hypothesis on the biogenetic origin of the compound(s) Elucidation of the structure of the kairomone(s) in other Daphnia- phytoplankton systems