Improving tolerance to fluctuating light through adaptive laboratory evolution in the cyanobacterium Synechocystis

Fluctuating light (FL) poses a challenge to cyanobacteria by disrupting photosynthesis and damaging photosystems. Although key FL tolerance components are known, their genetic enhancement remains unexplored. We evolved Synechocystis PCC 6803 under two FL regimes (one lethal to the starter strain, LT) in order to identify adaptive alleles. Our analysis revealed 44 novel mutations, 28 of which impact proteins/RNAs. Mutations in Pam68 (PSII assembly) and Sll0518, present in all strains, enhanced non-lethal FL tolerance in LT. Mutated Pam68 increased PSII abundance and activity. A gain-of-function mutation in RpaB (regulator of phycobilisome association B) significantly increased tolerance to both lethal FL and high-light conditions. This was associated with an increased PSI/PSII ratio and downregulation of light harvesting. In summary, our results suggest that adaptive laboratory evolution can simultaneously identify new FL tolerance factors and their advantageous alleles. The identified point mutations rewire multiple protective responses by as yet unknown molecular mechanisms.