Regulation of the induction of bicarbonate uptake by dissolved CO₂ in the marine diatom, <i>Phaeodactylum tricornutum</i>

ABSTRACT Physiological properties of photosynthesis were determined in the marine diatom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO 2 to air and related to H 2 CO 3 dissociation kinetics and equilibria in artificial seawater. The concentration of dissolved inorganic carbon at half maximum rate of photosynthesis ( K 0·5 [DIC]) value in high CO 2 ‐grown cells was 1009 mmol m − 3 but was reduced three‐fold by the addition of bovine carbonic anhydrase (CA), whereas in air‐grown cells K 0·5 [DIC] was 71 mmol m − 3 , irrespective of the presence of CA. The maximum rate of photosynthesis ( P max ) values varied between 300 and 500 μ mol O 2 mg Chl − 1 h − 1 regardless of growth p CO 2 . Bicarbonate dehydration kinetics in artificial seawater were re‐examined to evaluate the direct HCO 3 − uptake as a substrate for photosynthesis. The uncatalysed CO 2 formation rate in artificial seawater of 31·65°/ oo of salinity at pH 8·2 and 25 °C was found to be 0·6 mmol m − 3 min − 1 at 100 mmol m − 3 DIC, which is 53·5 and 7·3 times slower than the rates of photosynthesis exhibited in air‐ and high CO 2 ‐grown cells, respectively. These data indicate that even high CO 2 ‐grown cells of P. tricornutum can take up both CO 2 and HCO 3 − as substrates for photosynthesis and HCO 3 − use improves dramatically when the cells are grown in air. Detailed time courses were obtained of changes in affinity for DIC during the acclimation of high CO 2 ‐grown cells to air. The development of high‐affinity photosynthesis started after a 2–5 h lag period, followed by a steady increase over the next 15 h. This acclimation time course is the slowest to be described so far. High CO 2 ‐grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defined, and were allowed to acclimate for more than 36 h. The K 0·5 [DIC] values in acclimated cells appeared to be correlated only with [CO 2(aq) ] in the medium but not to HCO 3 − , CO 3 2 − , total [DIC] or the pH of the medium and indicate that the critical signal regulating the affinity of cells for DIC in the marine diatom, P. tricornutum , is [CO 2(aq) ] in the medium.

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