The effects of pH and dissolved inorganic carbon on external carbonic anhydrase activity in Chlorella saccharophila

External carbonic anhydrase (CA) activity in Chlorella saccharophila Is suppressed by growth at high dissolved inorganic carbon and at acid pH. External CA activity was shown to be suppressed by growth at pHs below 7.0, with total repression at pH 5 0. Growth in the presence of the buffer 3-[N-Morpholino]propane-sulphonic add (MOPS) between pH 7 and 8 suppressed CA activity. Cells grown at pH 8.0 aerated at 6 dm(3) h(-1) exhibited external CA activity of 5 units mg(-1) Chl once the dissolved inorganic carbon (DIC) was reduced to 300 mmol m(-3), and this increased to 30 units mg(-1) Chl over a period of 3 d while the DIC dropped to 30 mmol m(-3). Cells aerated at 180 dm(3) h(-1) showed a similar trend in CA activity, although the onset was delayed by 1 d and the DIC did not drop below 300 mmol m(-3). Cells grown at pH 7.8 near an air equilibrium DIC of 300 mmol m(-3) had no detectable external CA activity. It is probable that it is the CO2 supply to the cell, and not total DIC or HCO3-, which controls external CA activity. Cells grown at pH 5.0 had no detectable activity, although they reduced the CO2 concentration to 0.6 mmol m(-3). The loss of CA upon transfer of air-grown cells to 10 mmol mol(-1) CO2 took place over 48 h and was light dependent, while the loss upon transfer from alkaline pH to acid pH took place over 12 h and was independent of light, The effects of pH are independent of the response to CO2.