Inorganic carbon uptake and intracellular assimilation by subarctic Pacific phytoplankton assemblages

We report the results of inorganic carbon (C) uptake experiments and activity measurements for carbonic anhydrase, ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), and phosphoenolpyruvate carboxylase (PEPC) in offshore and coastal regions of the eastern subarctic Pacific Ocean. HCO3- was the dominant source of inorganic C taken up by phytoplankton at all sampling locations, accounting for similar to 60-90% of total C uptake. The uptake of HCO3- occurred primarily through a direct transport system, while indirect HCO3- use, mediated by extracellular carbonic anhydrase (eCA), played a minor role in the C uptake system. Direct HCO3- transport and eCA activity were not related to ambient nutrient or CO2 concentrations or to phytoplankton biomass (chlorophyll a [Chl a]) or primary productivity. There was significant variability in the biomass-normalized activities of Rubisco, PEPC, and total (intracellular and extracellular) carbonic anhydrase. The activities of all of the enzymes measured exhibited significant correlations with both CO2 concentrations and Chl a. PEPC activity averaged 20% of Rubisco activity (range 0.5-110%), and the PEPC: Rubisco ratio was positively correlated with CO2 concentrations and negatively correlated with Chl a. Carbonic anhydrase activity was strongly anticorrelated with CO2 and positively correlated with Chl a. The results provide evidence for the importance Of CO2-regulated carbon concentrating mechanisms in marine waters.