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.

Lipid and fatty acid composition are considered to be key parameters that determine the nutritive quality of phytoplankton diets for zooplanktonic herbivores. The fitness, reproduction and physiology of the grazers are influenced by these factors. The trophic transfer of lipids and fatty acids from algal cells has been typically studied by using simple extraction and quantification approaches, which, as we argue here, do not reflect the actual situation in the plankton. We show that cell disruption, as it occurs during a predator's grazing on diatoms can drastically change the lipid and fatty acid content of the food. In some algae, a rapid depletion of polyunsaturated fatty acids (PUFAs) is observed within the first minutes after cell disruption. This fatty acid depletion is directly linked to the production of PUFA‐derived polyunsaturated aldehydes (PUA); these are molecules that are thought to be involved in the chemical defence of the algae. PUA‐releasing diatoms are even capable of transforming lipids from other sources if these are available in the vicinity of the wounded cells. Fluorescent staining reveals that the enzymes involved in lipid transformation are active in the foregut of copepods, and therefore link the depletion processes directly to food uptake. Incubation experiments with the calanoid copepod Temora longicornis showed that PUFA depletion in PUA‐producing diatoms is correlated to reduced hatching success, and can be compensated for by externally added single fatty acids.

Abstract. Rising ocean temperatures will likely increase stratification of the water column and reduce nutrient input into the photic zone. This will increase the likelihood of nutrient limitation in marine microalgae, leading to changes in the abundance and composition of phytoplankton communities, which in turn will affect global biogeochemical cycles. Calcifying algae, such as coccolithophores, influence the carbon cycle by fixing CO 2 into particulate organic carbon through photosynthesis (POC production) and into particulate inorganic carbon through calcification (PIC production). As calcification produces a net release of CO 2 , the ratio of PIC to POC production determines whether coccolithophores act as a source (high PIC / POC) or a sink (low PIC / POC) of atmospheric CO 2 . We studied the effect of phosphorus (P-) limitation and high temperature on the physiology and the PIC / POC ratio of two subspecies of Coccolithus pelagicus. This large and heavily calcified species is a major contributor to calcite export from the photic zone into deep-sea reservoirs. Phosphorus limitation did not influence exponential growth rates in either subspecies, but P-limited cells had significantly lower cellular P-content. One of the subspecies was subjected to a 5 • C temperature increase from 10 • C to 15 • C, which did not affect exponential growth rates either, but nearly doubled cellular P-content under both high and low phosphate availability. This temperature increase reduced the PIC / POC ratio by 40-60 %, whereas the PIC / POC ratio did not differ between P-limited and nutrientreplete cultures when the subspecies were grown near their respective isolation temperature. Both P-limitation and elevated temperature significantly increased coccolith malformations. Our results suggest that a temperature increase may intensify P-limitation due to a higher P-requirement to maintain growth and POC production rates, possibly reducing abundances in a warmer ocean. Under such a scenario C. pelagicus may decrease its calcification rate relative to photosynthesis, thus favouring CO 2 sequestration over release. It seems unlikely that P-limitation by itself causes changes in the PIC / POC ratio in this species.

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Diatom oxylipins have been observed to deleteriously impact copepod reproductive success. However, field studies have revealed very variable and case-dependent results. Therefore, the plasticity of diatom oxylipin metabolism was studied among four clones of the marine diatom Skeletonema marinoi Sarno et Zingone. Diatom oxylipin metabolism was studied by two lipoxygenase (LOX) activity assays carried out at different pH values and by oxylipin quantification. The four clones showed no major metabolic differences in terms of protein content or growth rate. However, two of the clones produced significantly higher levels of oxylipins than the other two. LOX activity measurements also indicated clonal variability in fatty acid oxidative metabolism. The presence of clone-specific differences in oxylipin metabolism may play a role in shaping diatom population dynamics by conferring selective advantages to certain clones.