CYANOBACTERIAL ACCLIMATION TO RAPIDLY FLUCTUATING LIGHT IS CONSTRAINED BY INORGANIC CARBON STATUS 1

Acclimation to rapidly fluctuating light, simulating shallow aquatic habitats, is altered depending on inorganic carbon (C i ) availability. Under steady light of 50 μmol photons·m −2 ·s −1 , the growth rate of Synechococcus elongatus PCC7942 was similar in cells grown in high C i (4 mM) and low C i (0.02 mM), with induced carbon concentrating mechanisms compensating for low C i . Growth under fluctuating light of a 1‐s period averaging 50 μmol photons·m −2 ·s −1 caused a drop in growth rate of 28%±6% in high C i cells and 38%±8% in low C i cells. In high C i cells under fluctuating light, the PSI/PSII ratio increased, the PSII absorption cross‐section decreased, and the PSII turnover rate increased in a pattern similar to high‐light acclimation. In low C i cells under fluctuating light, the PSI/PSII ratio decreased, the PSII absorption cross‐section decreased, and the PSII turnover remained slow. Electron transport rate was similar in high and low C i cells but in both was lower under fluctuating than under steady light. After acclimation to a 1‐s period fluctuating light, electron transport rate decreased under steady or long‐period fluctuating light. We hypothesize that high C i cells acclimated to exploit the bright phases of the fluctuating light, whereas low C i cells enlarged their PSII pool to integrate the fluctuating light and dampen the variation of the electron flux into a rate‐restricted C i pool. Light response curves measured under steady light, widely used to predict photosynthetic rates, do not properly predict photosynthetic rates achieved under fluctuating light, and exploitation of fluctuating light is altered by C i status.