Role of carbonic anhydrase in photosynthesis in Chlorella derived from kinetic analysis of 14CO2 fixation1

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Abstract
Time courses of photosynthetic 14CO2 fixation and its simulation are presented for Chlorella cells grown under low CO2 concentration (low-CO2 cells) and subsequently exposed to 0.2 mM NaH14CO3 or 130 ppm 14CO2 in the presence or absence of carbonic anhydrase (CA) in the suspending medium. It was shown that Chlorella cells utilized only free CO2 when NaHCO3 was given in the presence or absence of CA, or when CO2 was bubbled in the absence of CA. However, the present simulation indicated that both CO3 and HCO3− were utilized when CO2 was given in the presence of CA. Based on these results, we concluded that 1) Chlorella cells absorb only free CO2 and 2) this gas is provided to algal cells in two ways, i.e., by direct and indirect CO2 supply. Usually, the dissolved CO2 is directly utilized by the algal cells (direct supply of CO2). However, when the concentration of dissolved CO2 is extremely low and when there is CA, CO2 reconverted from HCO3− is also utilized by Chlorella cells (indirect supply of CO2). The utilization of HCO3− indicated by the above simulation was explained by the indirect supply of CO2. We further assumed that the indirect supply of CO2 to ribulose 1,5-bisphosphate carboxylase occurs mainly in the chloroplasts of low-CO2 cells containing high CA. Thus, under low CO2 concentrations, low-CO2 cells can carry out more efficient CO2 fixation than high-CO2 cells, resulting in the lower apparent Km(CO2).