Max-Planck Institute, Hans-Knoll-Str. 8, D-07745 Jena, Germany. The cytotoxicity of several saturated and unsaturated marine diatom-derived aldehydes and an oxo-acid have been screened in vitro and in vivo against different organisms, such as bacteria, algae, fungi, echinoderms, molluscs and crustaceans. Conjugated unsaturated aldehydes like 2E,4E-decadienal, 2E,4E-octadienal, 5E,7E-9-oxo-nonadienoic acid and 2E-decenal were active against bacteria and fungi and showed weak algicidal activity. By contrast, the saturated aldehyde decanal and the non-conjugated aldehyde 4Z-decenal had either low or no significant biological activity. In assays with oyster haemocytes, 2E,4E-decadienal exhibited a dose-dependent inhibition of cytoskeleton organisation, rate of phagocytosis and oxidative burst and a dose-dependent promotion of apoptosis. A maternal diatom diet that was rich in unsaturated aldehydes induced arrest of cell division and apoptotic cell degradation in copepod embryos and larvae, respectively. This wide spectrum of physiological pathologies reflects the potent cell toxicity of diatom-derived oxylipins, in relation to their non-specific chemical reactivity towards nucleophilic biomolecules. The cytotoxic activity is conserved across six phyla, from bacteria to crustaceans. Deregulation of cell homeostasis is supposed to induce the elimination of damaged cells through apoptosis. However, efficient protection mechanisms possibly exist in unicellular organisms. Experiments with a genetically modified yeast species exhibiting elevated membrane and/or cell wall permeability suggest that this protection can be related to the inability of the oxylipin compounds to enter the cell.