Chemically mediated plankton interactions : Prevalence uptake and targets of polyunsaturated aldehydes Diatoms are key players in the aquatic ecosystem as they significantly contribute to photosynthetic carbon assimilation and transfer of energy to higher trophic levels. Some of these single celled algae produce polyunsaturated aldehydes (PUAs) which are derived from enzymatic oxidation of fatty acids. These aldehydes are known to inter¬fere with the reproduction of their predators and have been suggested to act as info and allelochemicals. Compared to the biological functions of PUAs their mechanisms of action have been less investigated. Here I present PUA quantification data for plankton communities and provide experimen¬tal results for an enhanced mechanistic understanding of these oxylipins. To perform the studies I estab¬lished fluorescent probes according to the activity based protein profiling (ABPP) strategy on the basis of PUAs and bioinactive saturated aldehydes. I developed key methodological approaches (feeding and two step incubation protocols procedures for fluoescence microscopy) and a novel probe reporter that is a useful and universal tool in chemical biology. Uptake studies showed that the PUA derived probe entered algal cells and accumulated in the gonads of a predator. Moreover investigation of targets based on covalent reactions with PUAs revealed several labeled proteins in a diatom. This first proteomic approach suggests that key metabolic pathways are affected by PUAs.