Utilization of inorganic carbon by the coccolithophorid <i>Emiliania huxleyi</i> (Lohmann) Kamptner

Abstract

summary Inorganic carbon‐dependent photosynthetic oxygen evolution was saturated at a photon flux density of 100 μmol m −2 s −1 for 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 O 2 evolution (K 0.5 [DIC]) was 2.86 mm; the rate of non‐enzymic dehydration of HCO 3 greatly exceeding the rate of CO 2 fixation. Carbon dioxide uptake occurs by diffusive entry as shown by the K 0.5 [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[ 3 H]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.