Measurement of internal pH in the coccolithophoreEmiliania huxleyi using 2',7'-bis-(2-carboxyethyl)-5(and-6)carboxyfluorescein acetoxymethylester and digital imaging microscopy.

Internal pH (pHi) was determined inEmiliania huxleyi (Lohmann) using the probe 2',7'-bis-(2-carboxyethyl)-5(and-6)carboxyfluoresceinacetoxymethylester (BCEF-AM) and digital imaging microscopy. The probe BCECF-AM was taken up and hydrolysed to the free acid by the cells. A linear relationship was established between pHi and the 490/450 fluorescence ratio of BCECF-AM over the pH range 6.0 to 8.0 using the ionophore nigericin. Two distinct pH domains were identified within the cell, the cytoplasmic domain (approx. pH 7.0) and the chloroplast domain (approx. pH 8.0). The average pHi was 7.29 (±0.11) for cells in the presence of 2 mM HCO 3 (-) . In the absence of HCO 3 (-) the pHi was decreased by 0.8 pH unit. The importance of these changes in pHi is considered in relation to inorganic-carbon uptake.