CO2and HCO3 -Uptake in Marine Diatoms Acclimated to Different CO2Concentrations

Rates of cellular uptake of CO2and HCO3 -during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO2partial pressures of 36, 180, 360, and 1,800 ppmv. In addition, in vivo activity of extracellular (eCA) and intracellular (iCA) carbonic anhydrase was determined in relation to CO2availability. Both species responded to diminishing CO2supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO2concentrations. Simultaneous uptake of CO2and HCO3 -was observed in both diatoms. At air-equilibrated CO2levels (360 ppmv), T. weissflogii took up CO2and HCO3 -at approximately the same rate, whereas CO2uptake exceeded HCO3 -uptake by a factor of two in P. tricornutum. In both diatoms, CO2: HCO3 -uptake ratios progressively decreased with decreasing CO2concentration, whereas substrate affinities of CO2and HCO3 -uptake increased. Half-saturation concentrations were always ≤5 μM CO2for CO2uptake and$