Fig. 2. Middle Proterozoic ε_P estimates as compared to empirical cyanobacterial ε_P values.

(A) Histogram of estimated ε_P values between 1.0 and 1.8 Ga. Boxed vertical ranges represents ε_P values from a cyanobacterial mat system [black (26)] and ε_P values reported here in cultures of WT (green) and Δccm mutant (blue) Synechococcus sp. PCC 7002 strains. (B) Measured values of ε_P increase at higher dissolved CO₂ levels in cultures of Synechococcus sp. PCC 7002. In the WT strain (green squares), ε_P values covary with [CO_2(aq)]⁻¹ (green line, fig. S5; R², 0.96). Blue diamonds are experimental results for the Δccm mutant, which requires ≥36 × PAL CO₂ to grow under our experimental conditions. The shaded blue band represents calculations from a 1D reaction-diffusion model trained on physiological observations of the Δccm mutant. Horizontal axes refer to CO₂ in the culture headspace relative to PAL (1 PAL = 280 ppm CO₂; upper axis) and the corresponding dissolved CO₂ in the culture medium (micromole per kilogram; lower axis). Data points represent biological replicates (n = 6 for each condition).