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 limit ed concentration of dissolved inorganic carbon at alkaline pH, where the ra te of CO2 formation is strictly limited, plotted as a function of chlorophy ll (Chl) concentration, will take the form of a rectangular hyperbola combi ned with a linear rate directly proportional to [Chl], which are, respectiv ely, due to the contribution of CO2 and HCO3- to photosynthesis. This model represents that the mathematical asymptote of absolute rate of photosynthe sis versus cell density is described by the whole-cell rate constant for HC O3- 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 experi mentally, In air-grown Chlorella ellipsoidea and C, kessleri and in high CO 2-grown C, saccharophila, the graph of the absolute rate of photosynthesis against [Chl] clearly followed the mathematical model described above and t he actual CO2 formation rates outside the cells were not significantly diff erent 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, sacc harophila were similar at approximate to 300 and 2.0 mm(3) mu g(-1) Chi min (-1), respectively, whereas those in air-grown C, kessleri were approximate to 550 and 15 mm(3) mu g(-1) Chl min(-1). These results indicate that no a cidification of the periplasmic space occurs, and there is no trace activit y of external carbonic anhydrase in these microalgae.