THE EFFECTS OF PH AND DISSOLVED INORGANIC CARBON ON EXTERNAL CARBONIC-ANHYDRASE ACTIVITY IN CHLORELLA-SACCHAROPHILA

External carbonic anhydrase (CA) activity in Chlorella saccharophila I s suppressed by growth at high dissolved inorganic carbon and at acid pH. External CA activity was shown to be suppressed by growth at pHs b elow 7.0, with total repression at pH 5 0. Growth in the presence of t he buffer 3-[N-Morpholino]propane-sulphonic add (MOPS) between pH 7 an d 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 dis solved 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 dr opped to 30 mmol m(-3). Cells aerated at 180 dm(3) h(-1) showed a simi lar trend in CA activity, although the onset was delayed by 1 d and th e 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 a ctivity. 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 co ncentration to 0.6 mmol m(-3). The loss of CA upon transfer of air-gro wn cells to 10 mmol mol(-1) CO2 took place over 48 h and was light dep endent, 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 in dependent of the response to CO2.