Calculation of Cah3 fraction in both the pyrenoid and the stroma of the chloroplast of both high-CO2 cells and cells acclimating to low CO2 conditions for 3 and 5 h.

Immunogold labelling experiments were carried out using antibodies against Cah3. Evaluation of labelling was made using the computer program UTHSCSA Image Tool version 3.0. The gold particles associated with the pyrenoid and the stroma were counted and the density calculated on the basis of area. To calculate the fraction of Cah3 in the pyrenoid, the particle density of the pyrenoid or stroma was multiplied by the average volume of the compartment (which is 2.4 µm3 and 35.6 µm3, respectively, according to [27], giving the total particles for each compartment. The total number of particles in the pyrenoid was divided by the combined number of particles in the pyrenoid and in the stroma. The data shown are the averages ± SD of 30 samples. Preimmune sera gave immunogold densities of less than 2 particles/µm2.