BICARBONATE UTILIZATION BY MARINE PHYTOPLANKTON SPECIES

Abstract

ABSTRACTThe contribution of bicarbonate to total dissolved inorganic carbon (DIC) 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 HCO−3 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. In species without extracellular CA, direct HCO−3 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 (DLDS). 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 DLDS 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 HCO−3 utilization.