Fig. 5.

A relatively acidic carboxysome would improve CCM performance. The phase space depicts the effect of varying carboxysomal pH and HCO₃⁻ transport on the degree of RuBisCO saturation in the carboxysome. The cytosolic pH was set to 8. The blue region denotes the portion of phase space wherein RuBisCO is saturated with CO₂. When the carboxysomal pH is relatively acidic (roughly pH 7) less HCO₃⁻ uptake is required to saturate RuBisCO because the equilibrium between CO₂ and HCO₃⁻ favors CO₂ more than at pH 8 (SI Appendix). Moreover, RuBisCO has an increased carboxylation k_cat at this pH. The minimum cytosolic HCO₃⁻ concentration (black lines) that allows for saturation of the carboxysomal RuBisCO is 10–20 mM, depending on RuBisCOs CO₂ affinity, which also varies with pH (SI Appendix). The teal region denotes the portion of phase space where the total CO₂ fixation flux is less than 10% of the flux in reference conditions (pH 8 in the cytosol and carboxysome). This region is labeled to emphasize that the fixation flux collapses at basic pH because the carboxylation k_cat is projected to vanish in this regime (SI Appendix, Fig. S4A).