• Inorganic carbon utilization;
  • Emiliania;
  • photosynthetic oxygen evolution;
  • membrane potential


Inorganic carbon-dependent photosynthetic oxygen evolution was saturated at a photon flux density of 100 μmol m−2s−1for air-grown cells of a low calcifying strain of Emittania huxleyi (Lohmann) Kamptner. Measurement of photosynthetic oxygen evolution at constant inorganic carbon concentration but varying pH showed that exogenous bicarbonate was not a major carbon source for photosynthesis. At pH 8.0 the concentration of dissolved inorganic carbon (DIC) required for the half-maximal rate of photosynthetic O2 evolution (K0.5[DIC]) was 2.86 mm; the rate of non-enzymic dehydration of HCO3 greatly exceeding the rate of CO2 fixation. Carbon dioxide uptake occurs by diffusive entry as shown by the K0.5 [DIC] of 12.5 μM at pH 5.0.

Bicarbonate uptake, measured by the silicone-oil-layer centrigual filtering technique, did not show Michaelis-Menten type kinetics. The electrical membrane potential difference was determined from the distribution of the lipophilic cation tetra[3H]phenylphosphonium (TPP+) between cells and the media. Cells grown at pH 8.0 exhibited a negative membrane potential (inside of cell relative to outside) of about −60 mV.