Fig. 4.

A selective carboxysome would not substantially improve CCM efficiency. The CO₂ concentration in the carboxysome is maximized when it is brought into equilibrium with the cytosolic HCO₃⁻ pool, shown as the black dashed line in A and B. A CCM using the single optimal permeability computed through the pH-aware model (dashed gray line) achieves a carboxysomal CO₂ concentration within 5% of this maximum. Selectivity at the carboxyome shell is thought to increase HCO₃⁻ permeability relative to CO₂ by means of charge interactions in the pores of the carboxysome shell. Selectivity might intuitively result in greater trapping of CO₂ (tracing the purple curve toward lower permeabilities) or faster uptake of HCO₃⁻ (tracing the green curve toward higher permeabilities), but neither of these strategies can increase the carboxysomal CO₂ concentration above equilibrium. As shown in B, selectivity would not substantially reduce the total cost of fixing carbon through the CCM, which is already nearly minimized at the single optimal permeability of 3 × 10⁻⁵ cm/s. Increasing the CO₂ permeability beyond 10⁻⁴ cm/s, however, exponentially increases the cost of fixation due to leakage of CO₂ from the carboxysome to the cytosol.