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It is widely accepted that in many food webs, the trophic transfer efficiency among primary producers and herbivores is determined by the nutritional value of primary producers. In pelagic freshwater and marine ecosystems, secondary production by herbivorous crustacean zooplankton is often limited by the seston's content of essential ω3 polyunsaturated fatty acids (ω3 PUFAs). However, little is known about the genetic network behind the positive relationship between phytoplankton ω3 PUFA content and zooplankton growth and reproduction. In our experimental study, we analysed gene expression changes of the freshwater cladoceran Daphnia magna under different food regimes differing in their ω3 PUFA composition. To disentangle ω3 PUFA effects from other factors, we fed D. magna with different pure phytoplankton cultures (i.e., algal and cyanobacterial diets) with or without supplementing the essential ω3 PUFA eicosapentaenoic acid (EPA). As hypothesized, we observed enhanced growth on diets supplemented with EPA. We applied an Illumina RNA-seq approach to D. magna from different diet treatments to find and monitor genes that are regulated dependent on EPA availability. Of 26,646 potential protein products (mapped to the D. magna genome), we identified transcriptomic signatures driven by the different food sources. Further analyses revealed specific candidate genes involved in EPA metabolism, irrespective of the basal food source. This allows a first functional annotation of previously uncharacterized genes involved in the EPA-specific response of D. magna and may finally provide a link to molecular processes connected to ω3 PUFA metabolism and conversion and thus trophic transfer efficiency in pelagic food webs.

Research on the thermal biology of Antarctic marine organisms has increased awareness of their vulnerability to climate change, as a flipside of their adaptation to life in the permanent cold and their limited capacity to acclimate to variable temperatures. Here, we employed a species–specific microarray of the Antarctic eelpout, Pachycara brachycephalum to identify long-term shifts in gene expression after 2 months of acclimation to six temperatures between -1°C and 9°C. Changes in cellular processes comprised signalling, post-translational modification, cytoskeleton remodelling, metabolic shifts and alterations in the transcription as well as translation machinery. The magnitude of transcriptomic responses paralleled the change in whole animal performance. Optimal growth at 3°C occurred at a minimum in gene expression changes indicative of a balanced steady state. The up–regulation of ribosomal transcripts at 5°C and above was accompanied by the transcriptomic activation of differential protein degradation pathways, from proteasome-based degradation in the cold towards lysosomal protein degradation in the warmth. From 7°C upwards increasing transcript levels representing heat shock proteins and an acute inflammatory response indicate cellular stress. Such patterns may contribute to a warm-induced energy deficit and a strong weight loss at temperatures above 6°C. Together, cold or warm acclimation led to specific cellular rearrangements and the progressive development of functional imbalances beyond the optimum temperature. The observed temperature–specific expression profiles reveal the molecular basis of thermal plasticity and refine present understanding of the shape and positioning of the thermal performance curve of ectotherms on the temperature scale.