Fig. 3. Modeled relationships between ε_P and CO₂ concentration for β-cyanobacteria without a CCM incorporating estimated middle Proterozoic O₂ levels [0.1 to 10% PAL (8)].

The gray band represents the estimated middle Proterozoic distribution of ε_P values (95th percentile; 8 to 24‰). The blue field represents calculations extending the observed fractionation by the Δccm mutant across possible Proterozoic CO₂ and O₂ levels. The red fields represent calculations incorporating the measured kinetics of ancestral form 1B rubisco (table S3) (42) and the full range of known intrinsic isotope effects for rubisco (ε_fix = 10, 15, and 30‰; the SM).