BICARBONATE UTILIZATION BY MARINE-PHYTOPLANKTON SPECIES

The contribution of bicarbonate to total dissolved inorganic carbon (D IG) utilization was investigated using 18 marine phytoplankton species , including-members of Bacillariophyceae, Dinophyceae, Prymnesiophycea e, and Raphidophyceae, under carbon-replete or -limited conditions Ext racellular carbonic anhydrase (CA) was assayed as an indicator of extr acellular CA-catalyzed HCO3- utilization. For some species, extracellu lar CA was constitutive in others activity was detected under conditio ns of carbon limitation, and in others, even under carbon-limited cond itio-ns, activity was not detected. In species without extracellular C A, direct HCO3- uptake was investigated using a pH drift technique in a closed system, DIC measurements, and the use of the anion exchange i nhibitor 4'4'-diisothiocyanatostilbene-2, 2-disulfonic acid (DIDS). Th ree of these species (Chaetoceros compressus, Thalassiosira pseudonana , and Glenodinium foliaceum) gave a pH drift not inhibited by DIDS, bu t cultures of Chrysochromulina kappa, Gephrocapsa oceanica, and Coccol ithus 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 e ighteen species investigated, only Heterosigma akashiwo did not have t he potential for direct uptake or extracellular CA-catalyzed HCO3- uti lization.