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

Rates of cellular uptake of CO2 and HCO3 during steady-state photosynthesis were measured in the marine diatoms Thalassiosira weissflogii and Phaeodactylum tricornutum, acclimated to CO2 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 CO2 availability. Both species responded to diminishing CO2 supply with an increase in eCA and iCA activity. In P. tricornutum, eCA activity was close to the detection limit at higher CO2 concentrations. Simultaneous uptake of CO2 and HCO3 was observed in both diatoms. At air-equilibrated CO2 levels (360 ppmv), T. weissflogii took up CO2 and HCO3 at approximately the same rate, whereas CO2 uptake exceeded HCO3 uptake by a factor of two in P. tricornutum. In both diatoms, CO2 : HCO3 uptake ratios progressively decreased with decreasing CO2 concentration, whereas substrate affinities of CO2 and HCO3 uptake increased. Half-saturation concentrations were always #5 mM CO2 for CO2 uptake and ,700 mM HCO3 for HCO3 uptake. Our results indicate the presence of highly efficient uptake systems for CO2 and HCO3 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. Primary production by marine phytoplankton takes place in an environment that is characterized by high and relatively constant HCO3 concentrations (;2 mM) but low and variable concentrations of molecular dissolved CO2 [CO2,aq] (;5–25 mM). Variation in [CO2,aq] of ocean surface waters is mainly caused by intense photosynthesis during phytoplankton blooms, differences in water temperature, or mixing with deep water of different CO2 content. On longer timescales, rising CO2 concentrations in the upper layers of the ocean are expected in response to the present increase in atmospheric CO2 partial pressure (pCO2; Houghton et al. 1996). Because these changes in [CO2,aq] are always accompanied by changes in pH, concentrations of HCO3 vary much less because of concomitant shifts in the relative proportions of the inorganic carbon (Ci) species. The response of phytoplankton growth to changes in CO2 supply is largely determined by the mechanism of Ci uptake. Several studies indicate that both CO2 and HCO3 in the bulk seawater are utilized by marine eukaryotic microalgae (e.g., Colman and Rotatore 1995; Rotatore et al. 1995; Korb et al. 1 Corresponding author ([email protected]).