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Calcifying haptophytes (coccolithophores) sequester carbon in the form of organic and inorganic cellular components (coccoliths). We examined the effect of phosphorus (P) limitation and heat stress on particulate organic and inorganic carbon (calcite) production in the coccolithophore Emiliania huxleyi. Both environmental stressors are related to rising CO2 levels and affect carbon production in marine microalgae, which in turn impacts biogeochemical cycling. Using semi-continuous cultures, we show that P limitation and heat stress decrease the calcification rate in E. huxleyi. However, using batch cultures, we show that different culturing approaches (batch versus semi-continuous) induce different physiologies. This affects the ratio of particulate inorganic (PIC) to organic carbon (POC) and complicates general predictions on the effect of P limitation on the PIC ∕ POC ratio. We found heat stress to increase P requirements in E. huxleyi, possibly leading to lower standing stocks in a warmer ocean, especially if this is linked to lower nutrient input. In summary, the predicted rise in global temperature and resulting decrease in nutrient availability may decrease CO2 sequestration by E. huxleyi through lower overall carbon production. Additionally, the export of carbon may be diminished by a decrease in calcification and a weaker coccolith ballasting effect.

Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) is a fundamental enzyme in CO2-fixation in photoautotrophic organisms. Nonetheless, it has been recently suggested that the contribution of this enzyme to total cellular protein is low in phytoplankton, including diatoms (< 6%). Here we show that RuBisCO content is high in some diatom species isolated from northern waters (> 69°N). Two species contained the highest RuBisCO levels ever reported for phytoplankton (36% of total protein). These high RuBisCO requirements do not increase these species’ requirements for nitrogen and do not impart a fitness disadvantage in terms of growth rate. On the contrary, high RuBisCO levels in psychrophilic diatoms may be a necessary mechanism to maintain high growth rates at low temperature at which enzymatic rates are low.

Numerous biological activities such as grazer defense and intraspecific signaling have been described for diatom oxylipins, fatty acid derived secondary metabolites produced by some diatom species. As the function and importance of these compounds are still controversial, the production of a subclass of these molecules, nonvolatile oxylipins, was studied during an induced bloom of Skeletonema marinoi (Samo et Zingone) in a mesocosm setup. Reproductive parameters of one of the main grazers, Calanus finmarchicus, were also examined during the bloom. Oxylipins detected during the bloom were the same as those previously described for S. marinoi and were detected predominantly in the mesocosm inoculated with this diatom. Reproductive success of C. finmarchicus remained unaffected during the course of the bloom. This may have been due to a dilution effect by the availability of alternative suitable prey or to the limited exposure of the copepods to the oxylipins generated during the short bloom. Follow up laboratory studies showed that oxylipin composition changed both when the S. marinoi clone used for inoculation was grown in the laboratory and in comparison to the well-studied Adriatic clone of S. marina These results highlight the necessity of quantitatively measuring oxylipin concentrations during diatom blooms at sea to be able to correctly evaluate their ecological significance. (C) 2013 Elsevier B.V. All rights reserved.

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.

The steady increase in global ocean temperature will most likely lead to nutrient limitation in the photic zone. This will impact the physiology of marine algae, including the globally important calcifying coccolithophores. Understanding their adaptive patterns is essential for modelling carbon production in a low-nutrient ocean. We investigated the physiology of Helicosphaera carteri, a representative of the abundant but under-investigated flagellated functional group of coccolithophores. Two strains isolated from contrasting nutrient regimes (South Atlantic and Mediterranean Sea) were grown in phosphorus-replete and phosphorus-limited batch cultures. While growing exponentially in a phosphorus-replete medium, the Mediterranean strain exhibited on average 24% lower growth rate, 36% larger coccosphere volume and 21% lower particulate inorganic carbon (PIC) production than the Atlantic strain. Under phosphorus limitation, the same strain was capable of reaching a 2.6 times higher cell density than the Atlantic strain due to lower phosphorus requirements. These results suggest that local physiological adaptation can define the performance of this species under nutrient limitation.

Most diatom species produce at least two classes of oxylipins, polyunsaturated aldehydes (PUAs) and non-volatile oxylipins (NVOs). These compounds can act as grazer deterrents by reducing the reproductive success of copepod grazers. The aim of the present study was to better understand the impact of late-winter diatom blooms and of diatom-derived PUAs and NVOs on the reproduction of two dominant copepod species in the northern Adriatic Sea, in two consecutive years. Higher diatom abundance in 2005 compared to 2004 coincided with higher egg-production rates in both Acartia clausi and Calanus helgolandicus. The increased contribution of diatoms to the phytoplankton assemblage in 2005 also led to a higher phytoplankton production of NVOs, whereas PUA levels were low in both years. These three-times-higher oxylipin levels correlated with a decrease in average hatching success in both A. clausi and C helgolandicus from similar to 80% in 2004 to similar to 60% in 2005, indicating that the observed negative effects on copepod recruitment in the latter year were related to the greater production of diatom NVOs. Our findings reveal the importance of these metabolites and offer an explanation as to why diatom blooms may be more toxic for copepods in some years compared to others. (C) 2015 Elsevier B.V. All rights reserved.

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 CO2 into particulate organic carbon through photosynthesis (POC production) and into particulate inorganic carbon through calcification (PIC production). As calcification produces a net release of CO2, 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 CO2. 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 nutrient-replete 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 CO2 sequestration over release. It seems unlikely that P-limitation by itself causes changes in the PIC / POC ratio in this species.

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.

International audience; 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.

The coccolithophore Coccolithus pelagicus (Wallich) Schiller fixes CO2 into particulate organic carbon (POC) through photosynthesis and into particulate inorganic carbon (PIC) in the form of calcite. To examine the role of phosphorus (P) availability in the production of POC and PIC, C. pelagicus subsp. braarudii (Gaarder) Geisen et al. was grown in semi-continuous cultures at three initial phosphate concentrations (P-replete, 1, and 0.5 μM [P]). Reduced P-availability (1 and 0.5 μM [P]) decreased POC production, while PIC production only decreased when phosphate concentrations became growth limiting (0.5 μM [P]). This decrease has not been observed previously in batch cultures, highlighting the inadequacy of the batch culture approach with regard to determining carbon production. The reduction in growth rate by 50% at 0.5 μM [P] was accompanied by a doubling in cell volume (and POC). PIC production was halved, resulting in a lowered PIC to POC ratio. The average number of coccoliths per cell (and PIC content) remained the same among treatments, despite the significant change in cell size. Our data suggest that POC production in C. pelagicus is more sensitive towards a moderate reduction in phosphorus availability than PIC production. Once phosphorus availability limits cell division, however, phosphorus resources are invested into POC rather than PIC production. This reduces cell density and sinking rates, indicating that coccoliths do not act as ballast for reaching deeper nutrient-rich layers under nutrient limitation.