Intracellular pH of symbiotic dinoflagellates

Intracellular pH (pH(i)) is likely to play a key role in maintaining the functional success of cnidarian-dinoflagellate symbiosis, yet until now the pH(i) of the symbiotic dinoflagellates (genus Symbiodinium) has never been quantified. Flow cytometry was used in conjunction with the ratiometric fluorescent dye BCECF to monitor changes in pH(i) over a daily light/dark cycle. The pH(i) of Symbiodinium type B1 freshly isolated from the model sea anemone Aiptasia pulchella was 7.25 +/- A 0.01 (mean +/- A SE) in the light and 7.10 +/- A 0.02 in the dark. A comparable effect of irradiance was seen across a variety of cultured Symbiodinium genotypes (types A1, B1, E1, E2, F1, and F5) which varied between pH(i) 7.21-7.39 in the light and 7.06-7.14 in the dark. Of note, there was a significant genotypic difference in pH(i), irrespective of irradiance.