The presence and characteristics of the CO 2 -concentrating mechanism in the freshwater eustigmatophyte Eustigmatos vischeri was investigated. The capacity of cells to remove CO 2 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 CO 2 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 CO 2 was corroborated when cells were pulsed with pure CO 2 in the light and in the dark and the rates of CO 2 disappearance were faster than those measured in a buffer solution. CO 2 transport in the light and in the dark was inhibited by low concentrations (100? µ M) of the mitochondrial respiration inhibitor cyanide, and cells treated with azide, which effectively blocks mitochondrial complex IV, also lost their capacity to remove CO 2 . 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 CO 2 supply; at pH 8.0, the rates of O 2 evolution exceeded the CO 2 supply rate 4-fold, indicating that this alga was able to take up . Cyanide and azide also inhibited the uptake of . These results demonstrate that E. vischeri uses both CO 2 and as external carbon sources for photosynthesis and indicate a respiratory control of the Ci acquisition process.
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