CO2 and HCO3- uptake in marine diatoms acclimated to different CO2 concentrations

Rates of cellular uptake of CO2 and HCO3- during steady-state photosynthesi s were measured in the marine diatoms Thalassiosira weissflogii and Phaeoda ctylum tricornutum, acclimated to CO2 partial pressures of 36, 180, 360, an d 1,800 ppmv. In addition, in vivo activity of extracellular (eCA) and intr acellular (iCA) carbonic anhydrase was determined in relation to CO2 availa bility. Both species responded to diminishing CO2 supply with an increase i n eCA and iCA activity. In P. tricornutum, eCA activity was close to the de tection limit at higher CO2 concentrations. Simultaneous uptake of CO2 and HCO3- was observed in both diatoms. At air-equilibrated CO2 levels (360 ppm v), T. weissflogii took up CO2 and HCO3- at approximately the same rate, wh ereas CO2 uptake exceeded HCO3- uptake by a factor of two in P. tricornutum . In both diatoms, CO2 and HCO3- uptake ratios progressively decreased with decreasing CO, concentration, whereas substrate affinities of CO, and HCO. - uptake increased. Half-saturation concentrations were always less than or equal to5 muM CO2 for CO2 uptake and < 700 muM HCO3- for HCO3- uptake, Our results indicate the presence of highly efficient uptake systems for CO2 a nd HCO3- in both diatoms at concentrations typically encountered in ocean s urface waters and the ability to adjust uptake rates to a wide range of ino rganic carbon supply.