Contributions of DPOR at low light intensity to chlorophyll biosynthesis and growth in the synechocystis sp. PCC 6803

The chlL gene encoding one component of light-independent (dark) protochlorophyllide oxido reductase (DPOR) was deleted in cyanobacterium Synechocystis sp. PCC 6803 (S.6803). The resulting chlL^- mutant lost DPOR activity. No significant differences of chlorophyll (Chl) content and growth rate were observed between the wild and the mutant strains grown at 50 μE · m^-2 · s^-1 light intensity for photomixtrophic and photoautotrophic growth. However, differences were observed at 1 μE · m^-2 · s^-1 light intensity. For photomixtrophic growth, the mutant Chl content was 50% of the wild content with continuous light and 35.7% of the wild content with a 10 h light/ 14 h dark cycle. For photoautotriphic growth, the mutant Chl level was 76.3% of the wild content with continuous light and 63.2% with a 10 h light/ 14 h dark cycle. The results indicate that DPOR contributes to Chl synthesis and increases the growth rate in cyano-bacteria phototrophically cultured at 1 μE · m^-2 · s^-1 light intensity. In contrast, the photosynthetic capacity on a per-cell basis of the mutant is 5% higher than that of the wild strain with continuous light and 27% higher than that of the wild strain with a 10 h light/14 h dark cycle at 1 μE · m^-2 · s^-1 light intensity for photoautotrophic growth. With the low Chl content, the cyanobacteria have the ability to improve their photosynthetic capacity by decreasing the ratio of PSI to PSII by unknown morphological or physiological means.