Diverse Chromatic Acclimation Processes Regulating Phycoerythrocyanin and Rod-Shaped Phycobilisome in Cyanobacteria.

Cyanobacteria have evolved various photoacclimation processes to perform oxygenic photosynthesis under different light environments. Chromatic acclimation (CA) is a widely recognized and ecologically important type of photoacclimation, whereby cyanobacteria alter the absorbing light colors of a supermolecular antenna complex called the phycobilisome. To date, several CA variants that regulate the green-absorbing phycoerythrin (PE) and/or the red-absorbing phycocyanin (PC) within the hemi-discoidal form of phycobilisome have been characterized. In this study, we identified a unique CA regulatory gene cluster encoding yellow-green-absorbing phycoerythrocyanin (PEC) and a rod-membrane linker protein (CpcL) for the rod-shaped form of phycobilisome. Using the cyanobacterium Leptolyngbya sp. PCC 6406, we revealed novel CA variants regulating PEC (CA7) and the rod-shaped phycobilisome (CA0), which maximize yellow-green light-harvesting capacity and balance the excitation of photosystems, respectively. Analysis of the distribution of CA gene clusters in 445 cyanobacteria genomes revealed eight CA variants responding to green and red light, which are classified based on the presence of PEC, PE, cpcL, and CA photosensor genes. Phylogenetic analysis further suggested that the emergence of CA7 was a single event and preceded that of heterocystous strains, whereas the acquisition of CA0 occurred multiple times. Taken together, these results offer novel insights into the diversity and evolution of the complex cyanobacterial photoacclimation mechanisms.