Inorganic carbon acquisition in the acid‐tolerant alga Chlorella kessleri

The ability of the freshwater alga, Chlorella kessleri, to maintain a carbon concentrating mechanism when grown at acid pH was investigated. The alga grows over the pH range 4.0–9.0 and was found to take up bicarbonate and CO₂actively when grown at pH 6.0. However, when grown at acid pH (below 5.5), it does not have active CO₂uptake. The acidotolerant species maintained an internal pH of 6.1–7.5 over the external 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 CO₂at acid external pH. Mass spectrometric monitoring of O₂and CO₂fluxes by suspensions of C. kessleri, grown at acid pH, and maintained at pH 7.5 showed that the rates of O₂evolution did not exceed those of CO₂uptake. The final CO₂compensation concentrations of 14.0–17.7 µM reached by photosynthetic cells were above the CO₂equilibrium concentration in the external medium, indicating a lack of active CO₂uptake at acid pH. Chlorella kessleri accumulated CO₂with internal concentrations that were 9.9, 18.7 and 22.7‐fold that of the external medium for cells grown, respectively, at pH 4.5, 5.0 and 5.5. The ability of C. kessleri cells to accumulate high intracellular concentrations of inorganic carbon at acid pH would provide a sufficiently high concentration of CO₂at the active site of Rubisco thus allowing the alga to maintain growth rates similar to those at alkaline pH.