Effects of the Pattern of Light and a Dark Recovery Period on Photosynthesis and Growth in Synechocystis sp. PCC 6803.

Optimizing the application of light is a central challenge in the cultivation of cyanobacteria because light is needed for growth, but excessive light can cause cellular damage. Here, we demonstrate that the temporal pattern of the light intensity, rather than the time-integrated daily total, determines photosynthetic stability and growth in Synechocystis sp. PCC 6803. Cells were cultivated in a turbidostatic flat-panel photobioreactor using three different light patterns, however, with identical time-integrated daily total (3.19 × 107 µmol photons m-2 day-1): a simulated solar light/dark (L/D) pattern, with increasing and then decreasing light intensity during the light period; a 'rectangular' L/D pattern, with a constant light intensity during the light period; and a constant light pattern, with no dark period. The rectangular L/D pattern led to the most stable and productive photosynthesis: 7161.3 ± 321.2 µmol O2 L-1 day-1 and the volumetric biomass productivity (0.072 ± 0.002 g DCW L-1 day-1); these values were approximately 1.2-fold higher than those under the simulated solar L/D pattern. In contrast, constant light intensity at 370 µmol photons m-2 s-1 initially increased O2 evolution, but photosynthetic activity decreased over time. A constant light intensity of 150 µmol photons m-2 s-1 prevented this collapse but also decreased O2 evolution. During the light period, Western blotting of the D1 protein of PSII indicated damage to this protein and measurements of chlorophyll content indicated pigment-related stress. Cells recovered from this damage during the dark period. These findings show that recovery during a dark period was required for the sustained photosynthesis of Synechocystis sp. PCC 6803 under conditions of continuous cultivation and that the rectangular L/D pattern was the most effective of the three temporal patterns.