Are volatile unsaturated aldehydes from diatoms the main line of chemical defence against copepods?

New experiments comparing the effects of 3 species of phytoplankton Prorocentrum minimum Schiller 1933 (PM), Thalassiosira rotula Meunier 1910 strains (TR1) and (TR2), and Skeletonema pseudocostatum (SPC) on the fecundity and egg-hatching rates of Calanus helgolandicus females are described. To further determine the chemical factors causing the inhibitory process related to diatoms, the biological findings were linked with an analysis of aldehydic oxylipins (metabolites derived from the oxidative transformation of fatty acids). Members of this compound class have previously been reported to inhibit copepod egg development. Comparison of the inhibitory properties with results from the chemical analysis, as well as bioassays with synthetic samples using sea urchin eggs as a model system, showed that the observed effect is not restricted to the previously identified volatile aldehydes decadienal and decatrienal but, in fact, depends on a reactive structural element (alpha,beta,gamma,delta-unsaturated aldehyde). In addition, the fatty acid content of the algae was verified with special emphasis on eicosapentaenoic acid (EPA), which is currently under discussion for its metabolic value and defensive potential. Our results demonstrate that only certain diatoms inhibit hatching in copepods and that this effect is independent of the EPA content of the diet. Moreover, saturated aldehydes, like the widely distributed tridecanal, did not affect our assay with sea urchin eggs. Since the observed hatching inhibition, as well as the capability for the formation of reactive aldehydes, is highly species and even isolate dependent, it would appear that no prediction of food quality of certain species can be given without a detailed analysis.