Some marine diatoms negatively affect the reproduction of dominant zooplankton grazers such as copepods, thus compromising the transfer of energy through the marine food chains. In this paper, the metabolic mechanism that leads to diatom-induced toxicity is investigated in three bloom-forming microalgae. We show that copepod dysfunctions can be induced by highly reactive oxygen species (hROS) and a blended mixture of diatom products, including fatty acid hydroperoxides (FAHs); these compounds display teratogenic and proapoptotic properties. The process is triggered by the early onset of lipoxygenase activities that elicit the synthesis of species-specific products, the basic structures of which were established (1-20); these compounds boost oxidative stress by massive lipid peroxidation. Our study might explain past laboratory and field results showing how diatoms damage zooplankton grazers even in the absence of polyunsaturated aldehydes, a class of molecules that has been formerly implicated in mediating the toxic activity of diatoms on copepods.

Summary In recent years, oxylipins (lipoxygenase‐derived oxygenated fatty acid products) have been reported in several bloom‐forming marine diatoms. Despite increasing attention on the ecophysiological role of these molecules in marine environments, their biosynthesis is largely unknown in these microalgae. Biochemical methods, including tandem mass spectrometry, nuclear magnetic resonance and radioactive probes were used to identify structures, enzymatic activities and growth‐dependent modulation of oxylipin biosynthesis in the pennate diatom Pseudo‐nitzschia delicatissima. Three major compounds, 15S‐hydroxy‐(5Z,8Z,11Z,13E,17Z)‐eicosapentaenoic acid (15S‐HEPE), 15‐oxo‐5Z,9E,11E,13E‐pentadecatetraenoic acid and 13,14‐threo‐13R‐hydroxy‐14S,15S‐trans‐epoxyeicosa‐5Z,8Z,11Z,17Z‐tetraenoic acid (13,14‐HEpETE), were produced by three putative biochemical pathways triggered by eicosapentaenoic acid‐dependent 15S lipoxygenase. Oxylipin production increases along the growth curve, with remarkable changes that precede the demise of the culture. At least one of the compounds, namely 15‐oxoacid, is formed only in the stationary phase immediately before the collapse of the culture. Synthesis and regulation of phyco‐oxylipins seem to correspond to a signaling mechanism that governs adaptation of diatoms along the growth curve until bloom termination. Factors triggering the process are unknown but synthesis of 15‐oxoacid, constrained within a time‐window of a few days just before the collapse of the culture, implies the involvement of a physiological control not directly dependent on distress or death of diatom cells.

Oxylipins are oxygenated derivatives of polyunsaturated fatty acids (PUFAs) that act as chemical mediators in many ecological and physiological processes in marine and freshwater diatoms. The occurrence and distribution of these molecules are relatively widespread within the lineage with considerable species-specific differences due to the variability of both the fatty acids recognized as substrates and the enzymatic transformations. The present review provides a general introduction to recent studies on diatom oxylipins and describes an analytical method for the detection and assessment of these elusive molecules in laboratory and field samples. This methodology is based on selective enrichment of the oxylipin fraction by solvent extraction, followed by parallel acquisition of full-scan UV and tandem mass spectra on reverse phase liquid chromatography (LC) peaks. The analytical procedure enables identification of potential genetic differences, enzymatic regulation, and ecophysiological conditions that result in different oxylipin signatures, thus providing an effective tool for probing the functional relevance of this class of lipids in plankton communities. Examples of oxylipin measurements in field samples are also provided as a demonstration of the analytical potential of the methodology.

Enzymatic preparations and specialized analytical tools have shown that chloroplast-derived glycolipids are the main substrates for the biosynthetic pathway that produces antiproliferative polyunsaturated aldehydes in broken cells of the marine diatom Thalassiosira rotula. This process, which is associated with the formation of free fatty acids and lyso compounds from polar lipids but not triglycerides, is largely dependent on glycolipid hydrolytic activity, rather than phospholipase A(2) as previously suggested. Preliminary characterization of lipolytic enzymes has revealed protein bands of 40-45 kDa. Under native conditions these proteins seem to be associated with soluble aggregates that have an apparent molecular weight of approximately 200 kDa. The biochemical process, which is similar to that described in the algal-bloom forming diatom Skeletonema costatum, suggests a mechanism based on decompartmentalization and mixing of preexisting enzymes and substrates.

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