Sources of inorganic carbon for phytoplankton in the eastern Subtropical and Equatorial Pacific Ocean

We present the results of a field study examining inorganic carbon utilization by phytoplankton assemblages in the eastern Subtropical and Equatorial Pacific Ocean. Data front isotope disequilibrium experiments demonstrate that HCO3- was the principal form of inorganic carbon taken tip by all of the in Situ phytoplankton populations we sampled. In a cyanobacteria-dominated assemblage, HCC3- uptake occurred chiefly through a direct transmembrane transport mechanism. Diatom-dominated assemblages expressed extracellular carbonic anhydrase and transported CO2 derived front the catalyzed dehydration of HCO3-. Direct HCO3- transport by the diatoms may have also occurred. In a 3-d incubation experiment. we observed the CO2-dependent regulation of inorganic C uptake ill diatom-dominated phytoplankton assemblages. Phytoplankton assemblages grown at 150 ppm CO2 possessed external carbonic anhydrase activity and took up HCO3- following its dehydration to CO2. In contrast. the assemblages Cultured with 750 ppm CO2 appeared to lack external carbonic anhydrase activity and rely solely on CO2 as an exogenous source of carbon for photosynthesis. The CO2 effect on inorganic C utilization occurred in the absence of a detectable difference in phytoplankton growth rates between the 50 and 750 ppm CO2 treatments. Our field data provide compelling evidence that HCO3- utilization is prevalent in natural marine phytoplankton communities and is regulated by ambient CO2 concentrations. We discuss the ecological and biogeochemical implications of these results.