Quantification of the rate of CO2 formation in the periplasmic space of microalgae during photosynthesis. A comparison of whole-cell rate constants for CO2 and HCO3– uptake among three species of the green alga Chlorella
As previously described, the absolute rate of photosynthesis due to a limited concentration of dissolved inorganic carbon at alkaline pH, where the rate of CO2 formation is strictly limited, plotted as a function of chlorophyll (Chl) concentration, will take the form of a rectangular hyperbola combined with a linear rate directly proportional to [Chl], which are, respectively, due to the contribution of CO2 and HCO3– to photosynthesis. This model represents that the mathematical asymptote of absolute rate of photosynthesis versus cell density is described by the whole-cell rate constant for HCO3– uptake and the maximum rate of CO2 formation in the extracellular space. This means that any trace modification of the CO2 formation rate outside the cell will alter the photosynthetic rate and should be detectable experimentally. In air-grown Chlorella ellipsoidea and C. kessleri and in high CO2-grown C. saccharophila, the graph of the absolute rate of photosynthesis against [Chl] clearly followed the mathematical model described above and the actual CO2 formation rates outside the cells were not significantly different from the calculated rates. It also indicated that the whole-cell rate constants for CO2 and HCO3– uptake in air-grown C. ellipsoidea and C. saccharophila were similar at ≈ 300 and 2·0 mm3μg–1 Chl min–1, respectively, whereas those in air-grown C. kessleri were ≈ 550 and 15 mm3μg–1 Chl min–1. These results indicate that no acidification of the periplasmic space occurs, and there is no trace activity of external carbonic anhydrase in these microalgae.