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.