Effect of dissolved inorganic carbon on oxygen evolution and uptake by Chlamydomonas reinhardtii suspensions adapted to ambient and CO2-enriched air.

Mass spectrometric measurements of (16)O2 and (18)O2 isotopes were used to compare the rates of gross O2 evolution (E0), O2 uptake (U0) and net O2 evolution (NET) in relation to different concentrations of dissolved inorganic carbon (DIC) by Chlamydomonas reinhardtii cells grown in air (air-grown), in air enriched with 5% CO2 (CO2-grown) and by cells grown in 5% CO2 and then adapted to air for 6h (air-adapted).At a photon fluence rate (PFR) saturating for photosynthesis (700 μmol photons m(-2) s(-1)), pH=7.0 and 28°C, U0 equalled E0 at the DIC compensation point which was 10μM DIC for CO2-grown and zero for air-grown cells. Both E0 and U0 were strongly dependent on DIC and reached DIC saturation at 480 μM and 70 μM for CO2-grown and air-grown algae respectively. U0 increased from DIC compensation to DIC saturation. The U0 values were about 40 (CO2-grown), 165 (air-adapted) and 60 μmol O2 mg Chl(-1) h(-1) (air-grown). Above DIC compensation the U0/E0 ratios of air-adapted and air-grown algae were always higher than those of CO2-grown cells. These differences in O2 exchange between CO2- and air-grown algae seem to be inducable since air-adapted algae respond similarly to air-grown cells.For all algae, the rates of dark respiratory O2 uptake measured 5 min after darkening were considerably lower than the rates of O2 uptake just before darkening. The contribution of dark respiration, photorespiration and the Mehler reaction to U0 is discussed and the energy requirement of the inducable CO2/HCO3 (-) concentrating mechanism present in air-adapted and air-grown C. reinhardtii cells is considered.