Carbon dioxide availability, intracellular pH and growth rate of the coccolithophore Emiliania huxleyi

Growth of a high-calcifying strain of Emiliania huxleyi (Lohmann) Hay & Mohler was investigated in cultures aerated with varying concentrations of CO2 in air and compared with growth in 0.03 % (v/v) CO2. Cultures aerated with 0.1 % (v/v) C02/air under identical conditions resulted in approximately 40% reduction in cell number and final cell yield. A concentration of 0.5% (v/v) CO2 completely inhibited growth. In the virtual absence of CO,, cells could grow to the same levels as in those cultures aerated with air-equilibrated levels of CO2, i.e. 0.03% (v/v) CO2. Measurement of internal pH (pH,) gave comparable results using either the 5,5-dimethyl-2['4C]oxazalidine-2,4-dione (DMO) method or a fluorescent probe technique (BCECF-AM). At external pH 8.3, intracellular pH of cells aerated with air was 6.9 whilst pH, of cells aerated with 0.1 % (v/v) CO2 was 6.4. Lowering external pH decreased growth rate for cultures aerated with 0.03 "., (v/v) CO2. A 30 "/o reduction in cell number and final cell yield occurred at pH 7.8 increasing to almost 600 at pH 7.0; pH, decreased at more acidic external pH down to 6.38 at pH 7.0. Carbon dioxide concentration and external pH appear to be equally important in the growth of high-calcifying cells. The significance of these results is considered in relation to the development of mesoscale blooms of E. huxleyi.