Fig. 7.

Colimitation of autotrophic growth by CO₂− and HCO₃⁻-dependent carboxylation reactions can explain the growth improvements associated with expressing CAs and Ci transporters. (A) In autotrophs using the CBB cycle, nearly all biomass carbon derives from rubisco-catalyzed CO₂ fixation. However, autotrophs also require HCO₃⁻ for carboxylation reactions in lipid, nucleic acid, and arginine biosynthesis (49–51). We expressed this diagram as a mathematical model, which we applied to understand why CA and Ci uptake improved rubisco-dependent growth. (B) The model exhibited two regimes: one wherein growth was limited by rubisco flux and another where it was limited by HCO₃⁻-dependent carboxylation (“bicarboxylation”) flux. At low rubisco levels (lighter-colored lines), growth was rubisco limited: increased rubisco activity produced faster growth, but the growth rate was insensitive to CA activity because slow spontaneous CO₂ hydration provided sufficient HCO₃⁻ to keep pace with rubisco. At higher rubisco levels (maroon lines), growth was bicarboxylation limited and increased CA activity was required for increasing rubisco activity to translate into faster growth. Increasing Ci uptake led to similar effects (SI Appendix, Fig. S12). In panel (C), color indicates the ratio of total Ci leakage (J_L,tot = J_L,C + J_L,H) to biomass production flux (J_B) at fixed rubisco activity across wide ranges of CA and Ci uptake activities. J_L,tot was calculated as the sum of CO₂ and HCO₃⁻ leakage rates (J_L,C + J_L,H) with J_L,C ≫ J_L,H in most conditions due to the much greater membrane permeability of CO₂. The so-called futile cycling, where leakage greatly exceeds biomass production (J_L,tot / J_B ≫ 1), occurs when CA and Ci uptake are coexpressed at extreme levels (redder colors). See SI Appendix for detailed description of the colimitation model.