State of the art
Changes in trophic state can impose major challenges on aquatic organisms and thus are expected to drive micro-evolutionary processes. Cyanobacterial mass developments, frequently observed during eutrophication, represent one important challenge aquatic consumers have to cope with. Cyanobacteria are of poor food quality for aquatic consumers due to morphological properties that hamper ingestion, the production of harmful secondary metabolites, and/or a deficiency in essential lipids, i.e. polyunsaturated fatty acids (PUFA) and especially sterols (von Elert et al. 2003; Wilson et al. 2006; Martin-Creuzburg et al. 2008). Many Daphnia populations experienced long-term changes in food quantity and quality during the last decades due to eutrophication and re-oligotrophication (Hartwich et al. 2012). Resurrection ecology approaches conducted in Upper Lake Constance (ULC) already revealed that these environmental changes were associated with changes in the genetic architecture of Daphnia populations (Brede et al. 2009) and that the mean resistance of Daphnia genotypes to toxic cyanobacteria increased significantly during eutrophication, i.e. when they were exposed to high cyanobacterial biomass (Hairston et al. 1999, 2001).
Preliminary data obtained in our laboratory suggest that Daphnia can also adapt to temporal changes in the availability of essential nutrients. D. pulicaria clones isolated from sediment cores taken from Lower Lake Constance (LLC) (age 1979-2010) were found to differ in sterol requirements and these differences seem to be related to temporal changes in cyanobacterial biomass. The absence of sterols in cyanobacteria has been shown to severely constrain life history traits of single Daphnia clones (Martin-Creuzburg et al. 2009). Our data suggest that progenies of clones that were naturally exposed to high cyanobacterial biomass have lower sterol requirements than progenies of clones that were not exposed to cyanobacteria. However, these preliminary data have to be verified using a larger set of clones representing different time periods and the experiments have to be extended to assess clone-specific differences in dietary PUFA requirements. D. pulicaria invaded LLC in the early 1970s during the height of eutrophication and the hatching success from resting eggs deposited until the early 1980s is very low (Möst et al. 2015). Thus, to increase the accessible timeframe and to allow for comparison between ULC and LLC, additional experiments will be conducted with D. galeata and D. longispina (and their hybrids). Both species were present in ULC and LLC already prior to the height of eutrophication and there is evidence that resting eggs of these species stay viable for a long time in LC sediments.
Proposed project and role within the RTG
The aim of the project is to explore micro-evolutionary changes in the adaptation to cyanobacterial food within Daphnia populations in ULC and LLC. By hatching resting eggs from sediments deposited during different time periods, genotypes from past populations will be recovered (resurrection ecology approach). The isolated Daphnia clones will be used to conduct comparative life history experiments in which potential genotype-specific changes in the ability to cope with cyanobacterial food will be investigated. The focus will be on differences in essential lipid requirements among clones. It is hypothesized that progenies of clones that were exposed to high cyanobacterial biomass during eutrophication are better adapted to cyanobacteria and thus have lower dietary requirements for sterols and PUFA than clones isolated from older or more recent sediment layers. The results obtained will be related to temporal changes in the phytoplankton species composition; especially to changes in cyanobacterial biomass (long-term data sets are available for ULC and LLC). To assess potential dietary deficiencies during different time periods, resting eggs will be isolated and analysed for temporal changes in the essential lipid composition. The aim of this project is to explore the physiological background of resilience to cyanobacteria within Daphnia populations and to improve our understanding of how human-made environmental changes can drive micro-evolutionary processes in aquatic communities.
The proposed project will be closely linked to project A3, in which temporal changes in the genetic architecture of Daphnia populations will be explored. This collaboration will allow also for a genetic characterization of the isolated Daphnia clones. The vertical distribution of ephippia (D. pulicaria and D. longispina/galeata) in sediment cores taken from different locations in LLC will be related to the spatiotemporal changes in phytoplankton investigated in project A4. In collaboration with C3, resting eggs will be analysed for stable isotopes (amino acids) and in conjunction with stable isotope analyses of archived zooplankton samples used to assess temporal changes in the trophic position of the different Daphnia species. Sediment cores and age models will be used jointly with projects A1, A3, A4, and C3. In collaboration with project C4, the food quality of mixotrophs (chrysophytes) for Daphnia will be explored and related to toxic compounds (chlorosulfolipids) potentially produced by chrysophytes (Boenigk 2004; Hiltunen et al. 2012).