Bicarbonate transport and extracellular carbonic anhydrase activity in Bering Sea phytoplankton assemblages: Results from isotope disequilibrium experiments

We used a 14 C isotope disequilibrium technique to provide quantitative estimates of both direct HCO 3 ‐ transport and extracellular CA activity in Bering Sea phytoplankton assemblages. The method revealed that direct HCO 3 ‐ 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 HCO 3 ‐ transport was not related to phytoplankton biomass, productivity, or ambient CO 2 concentrations at individual sampling stations. However, HCO 3 ‐ transport and total inorganic C uptake rates decreased in response to elevated CO 2 in direct manipulation experiments. Kinetic analysis of the 14 C time‐course data revealed low levels of extracellular carbonic anhydrase activity at most stations. This activity was related to phytoplankton taxonomic compositions and/or CO 2 concentrations, but was relatively unaffected by direct CO 2 manipulations.