Bicarbonate Transport and Extracellular Carbonic Anhydrase Activity in Bering Sea Phytoplankton Assemblages: Results from Isotope Disequilibrium Experiments

We used a14C isotope disequilibrium technique to provide quantitative estimates of both direct HCO3 -transport and extracellular CA activity in Bering Sea phytoplankton assemblages. The method revealed that direct HCO3 -transport was the dominant mechanism of inorganic C uptake in both coastal and open ocean waters, accounting for more than half of the total C flux to the phytoplankton. The relative importance of HCO3 -transport was not related to phytoplankton biomass, productivity, or ambient CO2concentrations at individual sampling stations. However, HCO3 -transport and total inorganic C uptake rates decreased in response to elevated CO2in direct manipulation experiments. Kinetic analysis of the14C time-course data revealed low levels of extracellular carbonic anhydrase activity at most stations. This activity was related to phytoplankton taxonomic compositions and/or CO2concentrations, but was relatively unaffected by direct CO2manipulations.