Development and Analysis of High-Light Stress Tolerant Strain of Synechocystis Sp. PCC 6803

Cyanobacteria adapt to the light intensity in the environment by changing the photosynthetic apparatus. However, when the energy supply exceeds the capacity of the photosynthesis under the high-light condition, the excess energy becomes stress and decreases the cell growth. Adaptive evolution is a powerful strategy to develop the stress tolerant strain by long-term cultivation under the target stress conditions. In this study, to understand the high-light tolerance mechanism in cyanobacteria, we developed the high-light tolerant strain by adaptive evolution experiment, and analyzed the tolerance mechanism by multi-omics analysis.

Synechocystis sp. PCC 6803 GT strain was used as the parental strain, and cells were cultivated with aeration of air and continuous light illumination using point source white LED. To develop the high-light tolerant strain, the serial transfer experiment was performed under 7,000 μmol/m²/s light intensity in which the growth rate of the parental strain was decreased. After 56 days of the serial transfer experiment, the high-light tolerant strain was obtained. The tolerant strain could grow under 9,000 μmol/m²/s in which the growth of the parental strain was severely inhibited. The high-light tolerance of the tolerant strain was not changed after 15 days of the serial transfer experiment using the tolerant strain under low-light condition (40 μmol/m²/s). This suggested that the high-light tolerance was not acclimation but probably caused by mutations in the genome of the tolerant strain. Actually non-synonymous mutations in the two genes were identified in the tolerant strain by whole genome sequencing. Moreover, transcriptomic analysis identified that the expression levels of the genes related the photosystem and phycobilisome in the tolerant strain were lower than the parent strain. This suggests that the decrease of the amount of photon received in the cell by repression of the light harvesting system is one possible mechanism of the high-light tolerance.