Abstract. In the marine environment, the range of values of carbon isotope fractionation between particulate tissue of phytoplankton and inorganic carbon can be more than 20‰ (− 35‰ < δ13C < − 14‰). This review considers the influence of seawater temperature, lipid content of phytoplanktonic cells, kinetic fractionation, and carbon pathway on δ13C values observed at sea. In order to study the contribution of carboxylases (RUBISCO and the β‐carboxylases phosphoenolpyruvate carboxylase, phosphoenoplpyruvate carboxykinase and pyruvate carboxylase) to variations of particulate δ13C values at sea, we present results obtained simultenously on carboxylase activities and δ13C in various environmental conditions. The lowest δ13C values are clearly associated with predominance of ribulose‐1.5‐bisphosphate carboxylase activity, but it was more difficult to explain the high δ13C values. Different hypotheses are discussed.

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Summary• Measurements of δ 13 C, δ 15 N and C : N ratios on modern pollen grains from temperate plants, including whole grains as well as extracted sporopollenin, were analysed in order to characterize physiological plant types at the pollen level and to determine the variation of these parameters in modern pollen grains of the same climatic area.• Measurements are presented for 95 batches of whole modern pollen from 58 temperate species and on the stable fraction of modern pollen grains, chemically extracted sporopollenin, for two modern species. Fourier transform infrared (FTIR) and cross-polarization and magic-angle spinning (CP/MAS) sporopollenin spectra were conducted in parallel.• C 3 and C 4 plants can be separated by δ 13 C measurements based on pollen. Probabilistic assignments to plant functional groups (herbaceous, deciduous woody, evergreen woody) of C 3 plants by the means of a discriminant analysis can be made for C : N ratios and for δ 13 C. The results are related to other studies on sporopollenin in order to use this method in future work on fossil samples.• Stable isotope measurements on pollen allow improved pollen diagrams, including forms that cannot be differentiated at species level, increasing the accuracy and resolution of plant physiological type distribution in quaternary and older fossil sediments.

Inorganic carbon may be assimilated through the Calvin-Benson cycle via the enzyme ribulose-1.5-bisphosphate carboxylase (Rubisco) andlor by 0-carboxylation [via the enzymes phosphoenolpyruvate carboxylase (PEPC), phosphoenolpyruvate carboxykinase (PEPCK) or pyruvate carboxylase]. Here, carboxylase activity measurements for marine phytoplankton are described. Two indices measuring carboxylase activity in marine phytoplankton were used. The first measures Rubisco activity per unit chlorophyll [R/Chl; nmol CO2 (pg chl a+b+c)-' h-'] while the second is the ratio of P-carboxylase activity to Rubisco activity, expressed as % (PC/R), which reflects the proportion of inorganic carbon fixed by these 2 groups of carboxylases. These ratios were studied in (1) different algal species in culture, (2) dunng the different growth phases of a culture, and (3) after a light-dark transition to measure the time response of carboxylase activities. These indices were different from one species to another at the same stage of growth. In autotrophic cells, P-carboxylation remained low (PC/R<40). The PCIR ratio increased significantly when R/Chl began to decrease at the end of the growth phase of a culture of Skeletonerna costaturn. The heterotrophic dinoflagellate Crypthecodinjurn cohnii, grown on an organic medium, incorporated inorganic carbon in the dark through PEPCK activity. The wlde range in PC/R ratio observed among the species confirm that in phytoplankton there may exist a continuum between autotrophy and heterotrophy. From a carbon budget point of view the 2 mechanisms are not equivalent. Rubisco fixation uses light as an energy source and results in gross production; 0-carboxylation also fixes inorganic carbon but as energy source uses metabolites synthesized by other pathways in the cell or from the external medium.

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Two nitrogen-limited continuous cultures of Dunaliella tertiolecta were grown on light/dark cycles. One was submitted to limiting photon flux density (PFD) and the other to nonlimiting PFD. The growth rate was identical in the two cultures despite the difference in the PFD conditions. Once equilibria were reached in both cultures, the PFD were reversed to simulate a decrease and an increase in light conditions. A large suite of variables was measured to characterize the response of the cells, mainly through the interactions of carbon and nitrogen assimilation pathways. A decrease in irradiance led to a rapid decrease in algal biovolume; the biovolume-based growth rate (p,) descended to levels lower than those estimated for cells of the light-limited culture. An increase in irradiance rapidly led to an increase in lo, which attained values greater than those observed in high-light cells of the other culture before the shift, For the two cultures before and after the shift, cell carbon and cell volume were strongly correlated, showing the same pattern of diel variations.The specific C fixation rates (~(2) of the two cultures before and after the light shift declined significantly during the light periods. Before the light shift, p,C was paradoxically higher in the low light culture and could not be predicted only from the light levels. This suggests that some compensatory phenomena may occur during light and nitrogen limitations, Time variations of cell Chl a due to photoacclimation in both cultures were correlated with their N status. Similarly, the regulation of carboxylase activities (Rubisco) by the light levels was sensitive to the degree of N limitation. We found that nitrogen limitation has an overriding effect compared to light for the regulation of cell volume, C fixation and respiration rates, Chl a synthesis, electron transport system, and Rubisco activities. In cultures subjected to low irradiance, NO,-uptake rate decreased several hours into the dark phase, suggesting a time lag between the end of photosynthate production and the exhaustion of compounds necessary for dissolved inorganic nitrogen assimilation. The implications of these results concerning phytoplankton growth modeling in a variable environment are important because there are no existing models that correctly integrate the simultaneous effects of light and nitrogen on primary production. It is shown that the effects of these limitations were not additive in the range of light and N limitations tested in this experiment.It is now evident that the understanding of physiological processes involved in the acclimation of phytoplanktonic organisms subjected to a variable light or nutrient environment AcknowledgmentsWe thank Gilbert Malara for technical help, Michele Etienne and Olivier Bernard for assistance in programming, Serge Dallot for the statistical analysis, Jean-Francois Berthon for the P vs. I curves, Valerie Martin for the electron transport system analysis, and Patrick Chang for improving the English. The manuscript was imp...

The influence of dissolved inorganic nitrogen (DIN) enrichments on cell-normalized carbon uptake rate, chlorophyll a content, and apparent cell size of a picoeukaryote (<1 microm) ( Ostreococcus tauri, the smallest eukaryotic cell) from a natural summer phytoplanktonic assemblage (<200 microm) in a northern Mediterranean Lagoon (Thau Lagoon) was studied in 20-L enclosures in June 1995. The natural planktonic community was incubated in situ for 24 h with initial ammonium and nitrate enrichments and compared to a control without enrichment. O. tauri cell-normalized productivity was estimated from the combination of flow cytometric (FCM) enumeration and 2-h (radioactive) carbonate incorporation measured on post-incubation size fractions (<1microm). No difference between the effects of the two DIN sources of enrichment on the studied biological parameters was measured during this experiment. Growth of natural O. tauri was perturbed by the low DIN availability in the control with drastic changes in cell productivity, chlorophyll content, and cell cycle (from the variations in apparent cell size) as compared to the DIN sufficiency conditions. On the other hand, a very high specific growth rate for natural O. tauri, up to 8 day(-1) under DIN enrichments, has been estimated from production and abundance data obtained during this experiment. This supports values measured in culture and suggests that the yearly high contribution of picophytoplankton to the total primary production in Thau Lagoon is likely to be due to their high growth rate rather than the previously suggested lack of grazing pressure.