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 (α,β,γ,δ-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.KEY WORDS: Phytoplankton · Oxylipin · Diatom · Copepod · Hatching · Inhibition · Activated defence Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 245: [33][34][35][36][37][38][39][40][41][42][43][44][45] 2002 nature of the factors that have anti-proliferative properties in marine pelagic algae and their ecological relevance. Miralto et al. (1999) identified reactive volatile aldehydes from the diatom Thalassiosira rotula Meunier 1910 (TR1) with the ability to inhibit egg hatching in copepods. As shown in Fig. 1, these aldehydes, namely 2E,4Z-decadienal (hereafter decadienal) and 2E,4Z,7Z-decatrienal (hereafter decatrienal) are derived from arachidonic acid (C20:4 n-6) and eicosapentaenoic acid (EPA, C20:5 n-3), respectively (Pohnert 2000). These metabolites thus belong to the oxylipins, a class of compounds that is by definition derived from the oxidative transformation of fatty acids. Their formation is initialised by the action of lipoxygenases, an enzyme class that has been previously detected to be active in numerous diatoms ( Fig. 1) (for a review see Pohnert & Boland 2002). For example, the freshwater diatom Gomphonema parvulum uses EPA as a precursor for the formation of hormosirene, a volatile C11-hydrocarbon released together with a seco...

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.

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Rapid polymer formation under physiological conditions is triggered upon wounding of the unicellular macroalga C. taxifolia. This process is initiated by esterase‐mediated deacetylation of caulerpenyne (1). The resulting 1,4‐dialdehyde oxytoxin 2 (2) instantaneously reacts with nucleophilic groups from proteins to form a cross‐linked biopolymer.

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