Description: Carbon acquisition in relation to CO2 supply was investigated in three marine bloom-forming microalgae, the diatom Skeletonema costatum, the flagellate Phaeocystis globosa, and the coccolithophorid Emiliania huxleyi. In vivo activities of extracellular (eCA) and intracellular (iCA) carbonic anhydrase activity, photosynthetic O2 evolution, CO2 and HCO uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO2 levels of 36, 180, 360, and 1,800 ppmv. Large differences were obtained between species both with regard to the efficiency and regulation of carbon acquisition. While eCA activity increased with decreasing CO2 concentration in S. costatum and P. globosa, consistently low values were obtained for E. huxleyi. No clear trends with pCO2 were observed in iCA activity for any of the species tested. Half saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest for E. huxleyi and lowest for S. costatum, generally decreased with decreasing CO2 concentration. In contrast, K1/2 values for P. globosa remained unaffected by pCO2 of the incubation. CO2 and HCO3- were taken up simultaneously by all species. The relative contribution of HCO3- to total carbon uptake generally increased with decreasing CO2, yet strongly differed between species. Whereas K1/2 for CO2 and HCO3- uptake was lowest at the lowest pCO2 for S. costatum and E. huxleyi, it did not change as a function of pCO2 in P. globosa. The observed taxon-specific differences in CO2 sensitivity, if representative for the natural environment, suggest that changes in CO2 availability may influence phytoplankton species succession and distribution. By modifying the relative contribution of different functional groups, e.g., diatomaceous versus calcareous phytoplankton, to the overall primary production this could potentially affect marine biogeochemical cycling and air-sea gas exchange.

Description: Diatoms have evolved a multitude of morphologies, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protection or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. Ar scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller scale of diatom cells, however, turbulence creates a roughly linear shear and nutrients can locally become strongly depleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent environments, Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The model results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects of sinking.

Description: EisenEx�the second in situ iron enrichment experiment in the Southern Ocean�was performed in the Atlantic sector over 3 weeks in November 2000 with the overarching goal to test the hypothesis that primary productivity in the Southern Ocean is limited by iron availability in the austral spring. Underwater irradiance, chlorophyll a (Chl a), photochemical efficiency, and primary productivity were measured inside and outside of an iron-enriched patch in order to quantify the response of phytoplankton to iron fertilization. Chl a concentration and photosynthetic rate (14C uptake in simulated in situ incubations) were measured in pico-, nano-, and microphytoplankton. Photochemical efficiency was studied with fast repetition rate fluorometry and xenon-pulse amplitude modulated fluorometry. The high-nutrient low-chlorophyll waters outside the Fe-enriched patch were characterized by deep euphotic zones (63-72 m), low Chl a (48-56 mg m-2), low photosynthetic efficiency (Fv/Fm ~ 0.3), and low daily primary productivity (130-220 mg C m-2 d-1). Between 70 and 90% of Chl a was found in pico- and nanophytoplankton. During the induced bloom, Fv/Fm increased up to ;0.55, primary productivity and Chl a reached the maximum values of 790 mg C m-2 d-1 and 231 mg Chl a m-2, respectively. As a consequence, the euphotic depth decreased to ~41 m. Picophytoplankton biomass hardly changed. Nano- and microphytoplankton biomass increased. In the first 2 weeks of the experiment, when the depth of the upper mixed layer was mostly 〈40 m, primary productivity was highly correlated with Chl a. In the third week, productivity was much lower than predicted from Chl a, probably because of a reduction in photosynthetic capacity as a consequence of increased physical variability in the upper water column. These results provide unequivocal evidence that iron supply is the central factor controlling phytoplankton primary productivity in the Southern Ocean, even if the mixing depth is 〉80 m.

Description: The carbon isotopic composition of marine phytoplankton varies significantly with growth conditions. Aqueous CO2 concentration [CO2] and algal growth rate (µ) have been suggested to be important factors determining isotope fractionation (ep). Here we examine ep of the coccolithophorid Emiliania huxleyi in relation to CO2 concentration and light conditions in dilute batch cultures. Cells were incubated at different irradiance cycles, photon flux densities (PFDs), and [CO2]. Isotope fractionation varied between 6.7 and 12.3‰ under 16 : 8 h light : dark cycle (L :D) and between 14.7 and 17.8‰ at continuous light. ep was largely independent of ambient [CO2], varying generally by less than 2‰ over a range of [CO2] from 5 to 34 mmol L-1. Instantaneous carbon-specific growth rates (µC) and PFDs, ranging from 15 to 150 mmol m-2 s-1, positively correlated with ep. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between ep and µ. In the present study the effect of PFDs on ep was stronger than that of mand thus resulted in a positive relationship between µ and ep. In addition, the L:D cycle of 16 : 8 h resulted in significantly lower ep values compared to continuous light. Since the observed offset of about 8‰ could not be related to daylength dependent changes in µC, this implies a direct influence of the irradiance cycle on ep. These findings are best explained by invoking active carbon uptake in E. huxleyi. If representative for the natural environment, these results complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications.

Description: Rates of cellular uptake of CO2 and HCO3- during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO2 partial pressures of 36, 180, 360, and 1,800 ppmv. In addition, in vivo activity of extracellular (eCA) and intracellular (iCA) carbonic anhydrase was determined in relation to CO2 availability. Both species responded to diminishing CO2 supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO2 concentrations. Simultaneous uptake of CO2 and HCO3- was observed in both diatoms. At air-equilibrated CO2 levels (360 ppmv), T. weissflogii took up CO2 and HCO3- at approximately the same rate, whereas CO2 uptake exceeded HCO3- uptake by a factor of two in P. tricornutum. In both diatoms, CO2 :HCO3- uptake ratios progressively decreased with decreasing CO2 concentration, whereas substrate affinities of CO2 and HCO3- uptake increased. Half-saturation concentrations were always 〈=5 mM CO2 for CO2 uptake and 〈700 mM HCO3- for HCO3- uptake. Our results indicate the presence of highly efficient uptake systems for CO2 and HCO3- in both diatoms at concentrations typically encountered in ocean surface waters and the ability to adjust uptake rates to a wide range of inorganic carbon supply.

Description: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,aq], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO2,aq]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO2,aq] led to large changes in C:N:P ratios. Significant CO2-related changes in elemental ratios were observed at [CO2,aq] 〈 10 mu mol kg-l and correlated with a CO2-dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern far CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO2-related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle

Description: Diatoms have evolved a multitude of morphologics, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protecticn or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. At scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller stall: of diatom cells, however, turbulence drcates a roughly linear shear and nutrients can locally become strongly dl=pleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent cnvironmerits. Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The mod=1 results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects ol’ sinking

Description: Blooms of the marine diatom Skeletonema costatum were initiated in closed-system batch cultures with P-deficient medium under two different initial concentrations of dissolved molecular CO2([CO2,aq]: 20.6 and 4.5 µmol L-1). Algal C: N: P ratios strongly increased with decreasing P concentration. In the exponential growth phase, C: N ratios were 1.3 mol mol-1 higher in the low relative to the high [CO2,aq] treatment. There was no [CO2,aq] effect on C: N: P ratios during P-limited growth. Carbon isotope fractionation («p ) was 2-3‰ higher in the high [CO2,aq] treatment. With growth rate decreasing due to P limitation, ep increased in both [CO2,aq] treatments by 2-3‰ despite decreasing [CO2,aq]. Under these conditions the effect of decreasing growth rate on isotope fractionation strongly dominated over that of declining CO2 availability. When extrapolated to the natural environment, these results imply that systematic changes in algal growth, as occurring during the course of phytoplankton blooms, may affect algal isotope fractionation. These results severely complicate the interpretation of carbon isotope measurements in suspended and sedimentary organic matter