15S-Lipoxygenase Metabolism in the Marine Diatom Pseudonitczha delicatissima

• 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.