Chantal Descolas-Gros

The dynamics of CO2 fixation in the Southern Ocean as indicated by carboxylase activities and organic carbon isotopic ratios

Abstract Recent studies have suggested that a direct relationship exists between the dissolved CO 2 concentration and the carbon isotopic composition of phytoplankton in the surface ocean. Thus, measurement of the δ 13 C of planktonic organic matter in deep-sea cores can potentially yield a record of the past atmospheric CO 2 variations. However, we present here results from three cruises in the Indian and Atlantic sectors of the Southern Ocean (between 40 and 66°S) in which biochemical and physiological factors associated with photosynthetic processes lead to carbon isotopic fractionation by phytoplankton which cannot be directly related to variations within the mineral carbon pool. Simultaneous measurements of the carboxylase activities and the l3 C/ l2 C ratio of particulate organic carbon show that there is a large variability in phytoplankton carbon metabolism, especially on a seasonal scale, in spite of a relative uniformity of the environmental conditions. Phytoplankton carbon metabolism is clearly a main factor governing variations in the stable isotopic composition of organic matter in the euphotic layer. Interrelationships between light, Rubisco activity and δ 13 C are clearly shown by our data. Heterotrophic processes also may influence the carbon isotope mass balance, especially during the break-up of the ice pack. In addition to the influence of photosynthetic metabolism, the effect of the meridional temperature gradient is also verified by our data set.

PHOTORESPIRATION IN CONTINUOUS CULTURE OF DUNALIELLA TERTIOLECTA (CHLOROPHYTA): RELATIONSHIPS BETWEEN SERINE, GLYCINE, AND EXTRACELLULAR GLYCOLATE

The concentrations of extracellular glycolate and intracellular free pools of serine and glycine were monitored in nitrogen‐limited continuous cultures of Dunaliella tertiolecta (Butcher) UTEX LB999, grown at two different irradiances on a light:dark cycle. Under steady‐state conditions, this microalga excreted into the medium a large amount of glycolate during the light phase, up to 100 nmol·(106 cells)−1 for a cell concentration of around 1.5 108 cells·L−1, but glycolate disappeared from the dissolved phase in the dark. Cells grown at 70 and those grown at 430 μmol photons·m−2·s−1 differed in maximal glycolate concentration, intracellular serine and glycine concentrations, and serine:glycine ratio. Reversal of these photon flux densities to which the cultures were exposed caused rapid modification of the extracellular glycolate and intracellular serine and glycine pools. These results suggest that photorespiratory metabolism in D. tertiolecta could be approximately quantified by measuring the changes in dissolved glycolate and intracellular serine and glycine concentrations, extending previous results from cultured phytoplankton and suggesting methods for field studies.

Stimulating effect of light-to-dark transitions on carbon assimilation by a marine diatom

Abstract The marine planktonic diatom Skeletonema costatum (Grev.) Cleve was cultured under three light regimes: constant light, a 12h light:12h dark (12h:12h) cycle and a 2h light:2h dark (2h:2h) regime. In continuous light, the enzymatic system involved in inorganic carbon fixation showed a loss in efficiency compared with that of cells cultured in 12h:12h or 2h:2h. On the other hand, a 2h:2h photoperiod induced larger cells with more chlorophyll a per cell and a higher assimilation rate. Carbon dioxide incorporation in the light, investigated by a study of the photosynthetic products and the measurement of enzyme activities (ribulose-1,5-bisphosphate carboxylase and phosphoenolpyruvate carboxykinase), was shown to occur, in the three regimes, by a C 3 pathway on which were superimposed β-carboxylations. The β-carboxylating reactions in the light represented 12% of the C 3 pathway in CL, 8% in 12h:12h, and 3% in 2h:2h. Dark carbon dioxide incorporation (β-carboxylations in the dark)was ≈15 times higher in 2h:2h than in 12h:12h. The rate of this fixation was nearly constant throughout the day in 2h:2h and represented 12% of the fixation in the light, while in 12h:12h it varied between 0.9 and 1.7%. These results lead to the conclusion that β-carboxylations occurring in the light are different from those occurring in the dark. To explain the great difference in dark fixation between 2h:2h and 12h:12h cultures, two hypotheses are proposed: higher cellular content of dark-activated phosphoenolpyruvate carboxykinase (PEPCKase) in 2h:2h culture and/or higher amount of substrate for β-carboxylations. Finally, we assume that medium-light fluctuations (2h) induced modifications of the total photosynthetic system; these are expressed in an improved efficiency of inorganic carbon fixation per cell, either in the light or in the dark. The ecological implications of these mechanisms might be important in oceans characterized by high turbulence.

Autotrophic carbon assimilation and biomass from size-fractionated phytoplankton in the surface waters across the subtropical frontal zone (Indian Ocean)

Abstract The composition of the phytoplanktonic communities in the surface waters of the La Reunion-Kerguelen transect (from 38°36S to 46°33S) has been investigated under spring conditions (Antarès 3 cruise, France-JGOFS, 28 September–8 November, 1995). The study, conducted at six stations in the subtropical frontal zone, involved size fractionations (threshold: 2 μm). The large variations in the overall biomass and autotrophic carbon fixation, calculated via Rubisco activity measurements and expressed respectively in terms of μg chlorophyll (a + b + c) per liter and nmol fixed carbon dioxide per liter and per hour, were attributable only to phytoplanktonic cells of >2 μm, with a peak observed in the frontal zone. The picophytoplankton (<2 μm) biomass remained constant throughout the transect, but the evolution of the species composition of the picophytoplanktonic population, as calculated from flow cytometry measurements through this frontal zone, changed. This study provides evidence, for the first time in this area, of the disappearance of prochlorophytes from the south of the frontal zone (42–47°S). Picoeukaryotes (<2 μm) and cyanobacteria populations, resolved by flow cytometry, were present all along the transect. However, their abundance decreased southward up to the quasi-disappearance of cyanobacteria at the southernmost station (52°S) that is characteristic of antarctic waters. The presence of prochlorophytes that is exclusive to the subtropical surface waters, and the low carbon fixation activity associated with these waters, may be linked to the specific hydrological features encountered. In contrast, the marked reduction in the cyanobacteria and the abundance of picoeukaryotes along the north-south transect is more likely to be a result of the reduction in temperature through the frontal zone.

Influence of nitrogen enrichment on size-fractionated in vitro carboxylase activities of phytoplankton from Thau Lagoon (Coastal Mediterranean Lagoon, France)

The influence of dissolved inorganic and organic nitrogen (DIN and DON) enrichments on pools of enzymes responsible for CO2 fixation by the Calvin-Benson (Rubisco) and beta-carboxylation pathways (beta-carboxylases) were studied in a natural plankton assemblage. The plankton community from a coastal Mediterranean lagoon were incubated in situ for 24 h with initially ammonium, nitrate and DON (taurine) enrichments and compared to a control without any enrichment. An increase of small picophytoplankton and diatom biomass was observed in the enriched inorganic nitrogen treatments 7 - 10 h after the initial enrichment. Phytoplankton biomass decreased in the control and under taurine enrichment suggesting an inorganic nitrogen limitation of phytoplankton growth.

Undamaged sedimented coccolithophorids in a deep environment (continental slope of the Gulf of Lions)

Abstract At different periods of the year, undisturbed sediment samples from the continental slope of the Gulf of Lions were collected with a multiple gravity corer. The overlying water (0–20 cm above the sediment) and the surface water masses were also collected. The following stations were sampled: R 4 , an interfluve (a platform between two canyons), 760 m deep; R 5 , at the junction of two canyon floors, 1540 m and R 6 , at the top of the wall, 1280 m. At all stations, the top sediment layer and the overlying water were enriched with coccoliths. During a late spring cruise (March 22–April 5, 1991) a thin microphyte-enriched layer at the water-sediment interface of station R 5 contained encysted diatoms, dinophytes and coccolithophorids. Several of the coccolithophorid species were in good preservation, among which Emiliania huxleyi was dominant and showed a considerable range of morphological variations in coccolith length and degree of calcification. The presence of tintinnids that use coccoliths to build their loricae reflects the species composition in the water masses from which this biodeposit originated. Homogeneous hydrological conditions favoured the accumulation of this deep biodeposit.