Characterization of the CO2-concentrating mechanism in the unicellular alga Eustigmatos vischeri

The presence and characteristics of the CO2-concentrating mechanism in the freshwater eustigmatophyte Eustigmatos vischeri was investigated. The capacity of cells to remove CO2 actively from the medium was established by mass spectrometry; illumination of a cell suspension incubated with inorganic carbon ( Ci) caused a rapid drop in the extracellular CO2 concentration to levels below its equilibrium value with bicarbonate. No activity of external carbonic anhydrase was detected in these experiments. The occurrence of the active uptake of CO2 was corroborated when cells were pulsed with pure CO2 in the light and in the dark and the rates of CO2 disappearance were faster than those measured in a buffer solution. CO2 transport in the light and in the dark was inhibited by low concentrations ( 100 mu M) of the mitochondrial respiration inhibitor cyanide, and cells treated with azide, which effectively blocks mitochondrial complex IV, also lost their capacity to remove CO2. The ability of cells to take up bicarbonate was also examined by comparing the rate of photosynthetic O-2 evolution with the calculated rate of spontaneous CO2 supply; at pH 8.0, the rates of O-2 evolution exceeded the CO2 supply rate 4-fold, indicating that this alga was able to take up HCO3-. Cyanide and azide also inhibited the uptake of HCO3-. These results demonstrate that E. vischeri uses both CO2 and HCO3- as external carbon sources for photosynthesis and indicate a respiratory control of the Ci acquisition process.