Bicarbonate utilization by marine phytoplankton species

The contribution of bicarbonate to total dissolved inorganic carbon (DIG) utilization was investigated using 18 marine phytoplankton species, including-members of Bacillariophyceae, Dinophyceae, Prymnesiophyceae, and Raphidophyceae, under carbon-replete or -limited conditions Extracellular carbonic anhydrase (CA) was assayed as an indicator of extracellular CA-catalyzed HCO3- 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 conditio-ns, activity was not detected. In species without extracellular CA, direct HCO3- uptake was investigated using a pH drift technique in a closed system, DIC measurements, and the use of the anion exchange inhibitor 4'4'-diisothiocyanatostilbene-2, 2-disulfonic acid (DIDS). Three of these species (Chaetoceros compressus, Thalassiosira pseudonana, and Glenodinium foliaceum) gave a pH drift not inhibited by DIDS, but cultures of Chrysochromulina kappa, Gephrocapsa oceanica, and Coccolithus pelagicus, in which DIDS inhibited DIC uptake, did not give a pH drift. This result shows that direct HCO3- transport may occur by an anion exchange-type mechanism in some species but not others. Of the eighteen species investigated, only Heterosigma akashiwo did not have the potential for direct uptake or extracellular CA-catalyzed HCO3- utilization.