The contribution of bicarbonate to total dissolved inorganic carbon (DIC) utilization was investigated using 18 marine phytoplankton species, including members of Bacillariophyceae, Dinophyceae, Pqmnesiophyceae, and Raphidophyceae, was assayed as an indicator of extracellular GI-catalyzed HC@ utilization. For some species, extracellular CA was constitutive, in others activity was detected under conditions of carbon limitation, and in others, even under carbon-limited conditions, activity was not detected. I n species without extracellular CA, direct HC@ uptake was investigated using a pH dnji technique in a closed system, DIC measurements, and the use of the anion exchange inhibitor 4'4'-diisothiocyanatostilbene-2,2-disulfonic acid (DLDS) . Three of these species (Chaetoceros compressus, Thalassiosira pseudonana, and Glenodinium foliaceum) gave a p H drift not inhibited by DIDS, but cultures of Chrysochromulina kappa, Gephrocapsa oceanica, and Coccolithus pelagicus, in which DLDS inhibited DIC uptake, did not gzve a pH dn$. This result shows that direct H C G transport may occur ! y a n anion exchange-type mechanism in some species but not others. Of the eighteen species investigated, only Heterosigma akashiwo did not have the potential fm direct uptake or extracellular U-catalyzed H C G utilization.Marine phytoplankton species acquire their inorganic carbon (Ci) for photosynthesis from the dissolved inorganic carbon (DIC) of the seawater.Within the usual seawater pH range of 8.0-8.3, the bulk of total DIC is HCOi, and CO, is less than 1 % of the total DIC (Skirrow 1975) when the system is in equilibrium with atmospheric CO,. Marine phytoplankton able to use HCO; may have a competitive advantage over phytoplankton using exclusively CO, when other essential nutrients or light are not rate-limiting for growth. The mechanism of DIC utilization is species dependent and some species can rapidly acclimate to changes in the concentration of dissolved CO, or total DIC (Coleman 1991, . In the direct uptake of HCO;

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This study investigated inorganic carbon accumulation in relation to photosynthesis in the marine dinoflagellate Prorocentrum micans. Measurement of the internal inorganic carbon pool showed a 10-fold accumulation in relation to external dissolved inorganic carbon (DIC). Dextran-bound sulfonamide (DBS), which inhibited extracellular carbonic anhydrase, caused more than 95% inhibition of DIC accumulation and photosynthesis. We used real-time imaging of living cells with confocal laser scanning microscopy and a fluorescent pH indicator dye to measure transient pH changes in relation to inorganic carbon availability. When steady-state photosynthesizing cells were DIC limited, the chloroplast pH decreased from 8.3 to 6.9 and cytosolic pH decreased from 7.7 to 7.1. Readdition of HCO 3 ؊ led to a rapid re-establishment of the steadystate pH values abolished by DBS. The addition of DBS to photosynthesizing cells under steady-state conditions resulted in a transient increase in intracellular pH, with photosynthesis maintained for 6 s, the amount of time needed for depletion of the intracellular inorganic carbon pool. These results demonstrate the key role of extracellular carbonic anhydrase in facilitating the availability of CO 2 at the exofacial surface of the plasma membrane necessary to maintain the photosynthetic rate. The need for a CO 2 -concentrating mechanism at ambient CO 2 concentrations may reflect the difference in the specificity factor of ribulose-1,5 bisphosphate carboxylase/oxygenase in dinoflagellates compared with other algal phyla.Marine phytoplankton species are the major dominant fixers of inorganic carbon in the oceans (Raven, 1994; 1997a), removing about 35 Pg of inorganic carbon per year from the ecosphere (Raven, 1997a). The key enzyme of photosynthetic CO 2 fixation, Rubisco (EC 4.1.1.39) (Raven, 1995), catalyzes the initial assimilation reaction of CO 2 and also the oxygenation of ribulose bisphosphate, initiating the photorespiratory pathway (Lorimer et al., 1973). The ratio of these reactions determines the specificity factor () of the enzyme, which can indicate enzyme type. The diverse universal type I enzyme found in oxygenic phototrophs has a substantially higher specificity for CO 2 than for oxygen (Jordan and Ogren, 1981) compared with type II, the more oxygen-sensitive, homomeric form of the enzyme found in heterotrophic anaerobic proteobacteria and cyanobacteria (Delgado et al., 1995; Tabita, 1995; Raven, 1997a). Recently, among the dinoflagellates, a major component of marine phytoplankton, the species tested were found to possess the oxygen-sensitive homomeric type-II form of the enzyme (Morse et al., 1995; Whitney and Yellowlees, 1995; Rowan et al., 1996; Whitney and Andrews, 1998).Dinoflagellates are morphologically and physiologically diverse, abundant in the marine ecosystem, and ecologically important; they make a major contribution to the global biological carbon pump (Raven and Johnston, 1991). Relatively little, however, is known about their mechanism of inorganic carb...