Carbon acquisition in relation to CO 2 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 O 2 evolution, CO 2 and HCO uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO 2 Ϫ 3 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 CO 2 concentration in S. costatum and P. globosa, consistently low values were obtained for E. huxleyi. No clear trends with pCO 2 were observed in iCA activity for any of the species tested. Half saturation concentrations (K 1/2 ) for photosynthetic O 2 evolution, which were highest for E. huxleyi and lowest for S. costatum, generally decreased with decreasing CO 2 concentration. In contrast, K 1/2 values for P. globosa remained unaffected by pCO 2 of the incubation. CO 2 and HCO were taken up simultaneously by all species. The relative contribution of HCO to total carbon uptakegenerally increased with decreasing CO 2 , yet strongly differed between species. Whereas K 1/2 for CO 2 and HCO Ϫ 3 uptake was lowest at the lowest pCO 2 for S. costatum and E. huxleyi, it did not change as a function of pCO 2 in P. globosa. The observed taxon-specific differences in CO 2 sensitivity, if representative for the natural environment, suggest that changes in CO 2 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.Marine phytoplankton account for approximately 50% of global primary production (Falkowski et al. 1998). Changes in the oceanic primary production over geological timescales have influenced biogeochemical cycles and thus atmospheric pCO 2 levels. Of the approximately 20,000 phytoplankton species (Falkowski and Raven 1997), however, only a relatively small number of key species control the cycling of carbon and other bioelements. Among these, bloom-forming phytoplankton play a major role in determining vertical fluxes of particulate material. With respect to their specific effects on biogeochemical cycling, phytoplankton can be separated into so-called functional groups (Falkowski et al. 1998), such as silicifying and calcifying phytoplankton, flagellates, and N 2 -fixating cyanobacteria. The relative contribution of each of these groups to marine primary production largely determines biogeochemical cycling in the ocean and the interplay between the various bioelements. What determines the distribution and succession of phytoplankton in space and time, especially with respect to the different func-1...

Rates of cellular uptake of CO 2 and HCO 3 Ϫ during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO 2 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 CO 2 availability. Both species responded to diminishing CO 2 supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO 2 concentrations. Simultaneous uptake of CO 2 and HCO 3 Ϫ was observed in both diatoms. At air-equilibrated CO 2 levels (360 ppmv), T. weissflogii took up CO 2 and HCO 3Ϫ at approximately the same rate, whereas CO 2 uptake exceeded HCO 3 Ϫ uptake by a factor of two in P. tricornutum. In both diatoms, CO 2 : HCO 3 Ϫ uptake ratios progressively decreased with decreasing CO 2 concentration, whereas substrate affinities of CO 2 and HCO 3 Ϫ uptake increased. Half-saturation concentrations were always Յ5 M CO 2 for CO 2 uptake and Ͻ700 M HCO 3 Ϫ for HCO 3 Ϫ uptake. Our results indicate the presence of highly efficient uptake systems for CO 2 and HCO 3 Ϫ 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.Primary production by marine phytoplankton takes place in an environment that is characterized by high and relatively constant HCO 3 Ϫ concentrations (ϳ2 mM) but low and variable concentrations of molecular dissolved CO 2 [CO 2,aq ] (ϳ5-25 M). Variation in [CO 2,aq ] of ocean surface waters is mainly caused by intense photosynthesis during phytoplankton blooms, differences in water temperature, or mixing with deep water of different CO 2 content. On longer timescales, rising CO 2 concentrations in the upper layers of the ocean are expected in response to the present increase in atmospheric CO 2 partial pressure (pCO 2 ; Houghton et al. 1996). Because these changes in [CO 2,aq ] are always accompanied by changes in pH, concentrations of HCO 3 Ϫ vary much less because of concomitant shifts in the relative proportions of the inorganic carbon (C i ) species.The response of phytoplankton growth to changes in CO 2 supply is largely determined by the mechanism of C i uptake. Several studies indicate that both CO 2 and HCO 3 Ϫ in the bulk seawater are utilized by marine eukaryotic microalgae (e.g., Colman and Rotatore 1995;Rotatore et al. 1995;Korb et al.

Abstract-Stable carbon isotope fractionation ( p ) was measured in four marine diatom and one dinoflagellate species of different cell sizes. Monospecific cultures were incubated under high-light and nutrient-replete conditions at 16 h : 8 h and 24 h : 0 h light/dark cycles in dilute batch cultures at six CO 2 concentrations, [CO 2,aq ], ranging from ca. 1 to 38 mol kg Ϫ1. In all species, p increased with increasing [CO 2,aq ]. Among the diatoms, the degree of CO 2 -related variability in p was inversely correlated with cell size. Isotopic fractionation in the dinoflagellate differed in several aspects from that of the diatoms, which may reflect both morphological and physiological differences between taxa. Daylength-related changes in instantaneous growth rate, defined as the rate of C assimilation during the photoperiod, affected p to a similar or greater extent than differences in experimental [CO 2,aq ] in three of the species tested. In contrast, the irradiance cycle had no effect on p in 2 other species. With the exception of Phaeodactylum tricornutum, growth rate of all species declined below a critical [CO 2,aq ]. At these concentrations, we observed a reversal in the CO 2 -related p trend, which we attribute to a decline in carbon assimilation efficiency. Although uncatalyzed passive diffusion of CO 2 into the cell was sufficient to account for gross carbon uptake in most treatments, our results indicate that other processes contribute to inorganic carbon acquisition in all species even at [CO 2,aq ] Ͼ 10 mol kg Ϫ1 . These processes may include active C transport and/or catalyzed conversion of HCO 3 Ϫ to CO 2 by carbonic anhydrase. A comparison of our results with data from the literature indicates significant deviations from previously reported correlations between p and /[CO 2,aq ], even when differences in cellular carbon content and cell geometry are accounted for.

The effect of variable concentrations of dissolved molecular carbon dioxide, [CO 2,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 [CO 2,aq ]. Among these species, the magnitude of change in C : N : P was similar over the experimental CO 2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO 2 concentration below which a further decrease in [CO 2,aq ] led to large changes in C : N : P ratios. Significant CO 2 -related changes in elemental ratios were observed at [CO 2,aq ] Ͻ 10 mol kg Ϫ1 and correlated with a CO 2 -dependent decrease in growth rate. At [CO 2,aq ] typical for ocean surface waters, variation in C : N : P was relatively small under our experimental conditions. No general pattern for CO 2 -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 [CO 2,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 CO 2 -related variation. In this species, the CO 2 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 CO 2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO 2 -related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle.

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