Regulation of the induction of bicarbonate uptake by dissolved CO2 in the marine diatom, Phaeodactylum tricornutum

Physiological properties of photosynthesis were determined in the marine di atom, Phaeodactylum tricornutum UTEX640, during acclimation from 5% CO2 to air and related to H2CO3 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 CO2-grown cells was 1009 mmol m(-3) but was reduced three-fold by the addition of bovine carbonic an hydrase (CA), whereas in air-grown cells K-0.5[DIC] was 71 mmol m(-3), irre spective of the presence of CA. The maximum rate of photosynthesis (P-max) values varied between 300 and 500 mu mol O-2 mg Chl(-1) h(-1) regardless of growth pCO(2). Bicarbonate dehydration kinetics in artificial seawater wer e re-examined to evaluate the direct HCO3- uptake as a substrate for photos ynthesis. The uncatalysed CO2 formation rate in artificial seawater of 31.6 5 parts per thousand of salinity at pH 8.2 and 25 degreesC 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 s lower than the rates of photosynthesis exhibited in air- and high CO2-grown cells, respectively, These data indicate that even high CO2-grown cells of P. tricornutum can take up both CO2 and HCO3- as substrates for photosynth esis and HCO3- use improves dramatically when the cells are grown in air. D etailed time courses were obtained of changes in affinity for DIC during th e acclimation of high CO2-grown cells to air, The development of high-affin ity photosynthesis started after a 25 h lag period, followed by a steady in crease over the next 15 h, This acclimation time course is the slowest to b e described so far. High CO2-grown cells were transferred to controlled DIC conditions, at which the concentrations of each DIC species could be defin ed, 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 [CO2(aq)] in the m edium but not to HCO3-, CO32-, total [DIC] or the pH of the medium and indi cate that the critical signal regulating the affinity of cells for DIC in t he marine diatom, P. tricornutum, is [CO2(aq)] in the medium.