Diffusion barriers and adaptive carbon uptake strategies enhance the modeled performance of the algal CO₂-concentrating mechanism

Abstract

Many photosynthetic organisms enhance the performance of their CO₂-fixing enzyme Rubisco by operating a CO₂-concentrating mechanism (CCM). Most CCMs in eukaryotic algae supply concentrated CO₂ to Rubisco in an organelle called the pyrenoid. Ongoing efforts seek to engineer an algal CCM into crops that lack a CCM to increase yields. To advance our basic understanding of the algal CCM, we develop a chloroplast-scale reaction-diffusion model to analyze the efficacy and the energy efficiency of the CCM in the green alga Chlamydomonas reinhardtii. We show that achieving an effective and energetically efficient CCM requires a physical barrier such as thylakoid stacks or a starch sheath to reduce CO₂ leakage out of the pyrenoid matrix. Our model provides insights into the relative performance of two distinct inorganic carbon uptake strategies: at air-level CO₂, a CCM can operate effectively by taking up passively diffusing external CO₂ and catalyzing its conversion to HCO₃⁻, which is then trapped in the chloroplast; however, at lower external CO₂ levels, effective CO₂ concentration requires active import of HCO₃⁻. We also find that proper localization of carbonic anhydrases can reduce futile carbon cycling between CO₂ and HCO₃⁻, thus enhancing CCM performance. We propose a four-step engineering path that increases predicted CO₂ saturation of Rubisco up to seven-fold at a theoretical cost of only 1.5 ATP per CO₂ fixed. Our system-level analysis establishes biophysical principles underlying the CCM that are broadly applicable to other algae and provides a framework to guide efforts to engineer an algal CCM into land plants.