CO 2 and HCO 3 ߚ uptake in marine diatoms acclimated to different CO 2 concentrations

Rates of cellular uptake of CO 2 and HCO 3 − during steadyߚstate photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum , acclimated to CO 2 partial 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 CO 2 availability. Both species responded to diminishing CO 2 supply with an increase in eCA and iCA activity. In P. tricornutum , eCA activity was close to the detection limit at higher CO 2 concentrations. Simultaneous uptake of CO 2 and HCO 3 − was observed in both diatoms. At airߚequilibrated CO 2 levels (360 ppmv), T. weissflogii took up CO 2 and HCO 3 − at approximately the same rate, whereas CO 2 uptake exceeded HCO 3 − uptake by a factor of two in P. tricornutum . In both diatoms, CO 2 :HCO 3 − uptake ratios progressively decreased with decreasing CO 2 concentration, whereas substrate affinities of CO 2 and HCO 3 − uptake increased. Halfߚsaturation concentrations were always <=5 mM CO 2 for CO 2 uptake and <700 mM HCO 3 − for HCO 3 − uptake. Our results indicate the presence of highly efficient uptake systems for CO 2 and HCO 3 − in both diatoms at concentrations typically encountered in ocean surface waters and the ability to adjust uptake rates to a wide range of inorganic carbon supply.