SOURCE OF INORGANIC CARBON FOR PHOTOSYNTHESIS IN TWO MARINE DINOFLAGELLATES1

Abstract

Inorganic carbon uptake was investigated in two marine dinoflagellates, Amphidinium carterae Hulburt and Heterocapsa oceanica Stein. Mass spectrometric and potentiometric assays indicated that both species lacked external carbonic anhydrase (CA). The presence of internal CA was demonstrated by potentiometric assay and by the inhibition of photosynthesis upon the addition of 500 μM ethoxyzolamide a membrane‐permeable inhibitor of CA. The capacity for bicarbonate transport was investigated by comparing the calculated rate of spontaneous CO2 formation at pH 8.2 and 25°C with the rate of photosynthesis after the addition of 100 μM NaHCO3. Both species appeared to have a very limited capacity for direct bicarbonate uptake. Monitoring of CO2 and O2 fluxes in both species by mass spectrometry demonstrated a rapid uptake of CO2 on illumination, to concentrations below the CO2 equilibrium concentration, indicating an effective selective uptake of CO2. This dependence of photosynthesis on free CO2 alone suggests that these species are CO2 limited in their natural environment because the CO2 concentration of seawater is very low.