Fig. 5: The proposed CO₂-enrichment and transport mechanisms of the MOF/C. pyrenoidosa assembly.

Scenario 1: natural carbon fixation of microalgae growth is limited by the CO₂ concentration in the cell ([CO₂]₁) and enzymatic kinetic rate k_1. Scenario 2: in this work, MOFs as a CO₂ concentrating entrepot are applied to accelerate the transport of CO₂. The captured CO₂ in MOF is hydrated to HCO₃⁻ by the excreted eCA adsorbed on MOF. The higher concentration of HCO₃⁻ outside is transported into the cell by the transporter, and then in pyrenoid, HCO₃⁻ is converted to CO₂ by iCA to feed the carboxylation process of Rubisco. Compared to the intrinsic carbon concentrating mechanisms that hydrate dissolved CO₂ to HCO₃⁻ for inorganic carbon supplementation, the artificial CO₂-enrichment and transport pathways not only increase the CO₂ concentration ([CO₂]₂) in the cell but also accelerate the rate of Rubisco (k₂) for CO₂ fixation, which increases the apparent photo conversion efficiency to 1.9 folds, up to 9.8% in ambient air.