Inorganic Carbon Uptake at Acid pH by the Alga Chlorella kessler

The mechanism of inorganic carbon (Ci) uptake was examined in the freshwater green microalga Chlorella kessleri grown at acid pH. The CCM components investigated were active HCO3 - and CO2 uptake in cells grown at the pH range of 4.0 to 6.0. The photosynthetic characteristics of Ci uptake, indicate that C. kessleri has a high apparent affinity for CO 2 with a low K1/2(CO2) between 10.76 to 14.30 muM at acid pH. Cells grown at pH 6.0 had a low affinity for CO2 with a high K1/2(CO2) of 23.88 muM. Acid-grown cells were shown to have no capacity for bicarbonate uptake because the rates of oxygen evolution in the light were lower than the calculated rate of spontaneous CO2 formation from HCO3-. The internal pH of the cells was determined by equilibration with 14C-labelled benzoic acid at pH 4.5 and 5.0, and with 14C-labelled 5,5- dimethyloxazolidine-2,4-dione over the pH range 5.5-7.5. The acidotolerant species maintained an internal pH of 6.1-7.5 over the pH range 4.5-7.5, thus the pH difference between the cell interior and the external medium was large enough to allow for the diffusive uptake of CO2 at acid pH. Mass spectrometric monitoring of O2 and CO2 fluxes by suspensions of C. kessleri at pH 7.5 showed that the rates of O 2 evolution did not exceed those of CO2 uptake. The final CO2 compensation concentrations of 14.1 to 17.7 muM reached by photosynthetic cells were above CO2 equilibrium concentration in the external medium, indicating a lack of active CO2 uptake at acid pH. C. kessleri accumulated CO2 with internal concentrations that were 9.9, 18.7 and 22.7-fold higher than that in the external medium for cells grown at pH 4.5, 5.0 and 5.5 respectively.;Cells grown at pH 6.0 were shown to exhibit both active CO2 and bicarbonate uptake. The O2 evolution rate was equal to the spontaneous CO2 formation rate indicating active bicarbonate uptake. The final CO2 compensation concentrations of 15.3 muM reached by photosynthetic cells at this pH were below the CO2 equilibrium concentration in the external medium, indicating active CO2 uptake. At pH 6.0, C. kessleri cells were found to have an internal Ci concentration of 91.96 +/- 3.36 mM with an accumulation that is 25.1-fold higher than the external medium.;Results indicate that the alga does not have a CCM based on the active uptake of CO2 or HCO3- but maintains an alkaline internal pH which allows an internal accumulation of inorganic carbon solely by the diffusive uptake of CO2. The ability of C. kessleri cells to accumulate high intracellular concentrations of CO2 internally at acid pH would provide a high concentration of CO2 at the active site of Rubisco suppressing photorespiration.