(A–B) P700 redox kinetics in dark acclimated ambient-air (black) and high-CO2 E. timida (blue) (A) and ambient-air (red) and high-CO2 Acetabularia (purple) (B) upon exposure to a 780 ms high-light pulse. The ambient-air Acetabularia data are the same as in Figure 4B and are shown here for reference. (C–D) P700 redox kinetics during a second light pulse, fired 10 s after the first pulse, shown in panels A-B, in ambient-air and high-CO2 E. timida (C) and ambient-air and high-CO2 Acetabularia (D). The ambient-air Acetabularia data are the same as in Figure 4B inset and are shown here for reference. (E–F) RLC measurements from dark acclimated ambient-air (black solid line) and high-CO2 E. timida (blue dashed line) (E) and ambient-air (red solid line) and high-CO2 Acetabularia (purple dashed line) (F). Illumination at each light intensity (PPFD) was continued for 90 s prior to firing a saturating pulse to determine relative electron transfer rate of PSII (rETR). E. timida individuals used in RLC measurements were fixed in 1% alginate for the measurements (see ‘Materials and methods’ and Figure 5—figure supplement 1 for details). (G–H) Fluorescence traces and NPQ during the RLC measurements from ambient-air and high-CO2 E. timida (G) and ambient-air and high-CO2 Acetabularia (H). P700+ transients were double normalized to their respective dark levels and to the P700+ peak measured immediately after the onset of the pulse. Curves in (A) are averages from 7 (ambient-air E. timida) and 8 (high-CO2 E. timida) biological replicates, 7 and 8 in (C), and 3 and 3 in (E,G), respectively. High-CO2 Acetabularia curves in (B,D) are averages from three biological replicates. Ambient-air and high-CO2 Acetabularia curves in (F,H) are averages from three biological replicates. Shaded areas around the curves and error bars show SE. rETR and NPQ were calculated as described in ‘Materials and methods’. All E. timida individuals used in panels (A,C,E,G) were allowed to incorporate the chloroplasts for an overnight dark period in the absence of Acetabularia prior to the measurements. See Figure 5—source data 1 for original data.