Fig. 3: Feasible inorganic carbon uptake strategies for the chloroplast depend on the environmental level of CO₂.

a–i, Results are shown for a model with no barrier to CO₂ diffusion out of the pyrenoid matrix (a–c), a model with thylakoid stacks serving as diffusion barriers (d–f) and a model with an impermeable starch sheath (g–i). a,d,g, Schematics of the modelled chloroplast employing LCIB for passive CO₂ uptake (red), or employing active LCIA^P-mediated HCO₃⁻ pumping across the chloroplast envelope and no LCIB activity (blue). PCCM performance under air-level CO₂ (10 µM cytosolic) (b,e,h) and under very low CO₂ (1 µM cytosolic) (c,f,i) are shown, as measured by normalized CO₂ fixation flux versus ATP spent per CO₂ fixed, for the two inorganic carbon uptake strategies in a, d and g. Solid curves indicate the minimum energy cost necessary to achieve a certain normalized CO₂ fixation flux. Shaded regions represent the range of possible performances found by varying HCO₃⁻ transport rates and LCIB rates. Colour code as in a. In h and i, dashed black curves indicate the optimal PCCM performance of a simplified model that assumes fast intracompartmental diffusion, fast HCO₃⁻ diffusion across the thylakoid membranes, and fast equilibrium between CO₂ and HCO₃⁻ catalysed by CAH3 in the thylakoid tubules inside the pyrenoid (Methods).