Elucidating light-induced changes in excitation energy transfer of photosystem I and II in whole cells of two model cyanobacteria.

Excitation energy transfer between the photochemically active protein complexes is key for photosynthetic processes. Phototrophic organisms like cyanobacteria experience subtle changes in irradiance under natural conditions. Such changes need adjustments to the excitation energy transfer between the photosystems for sustainable growth. Spectroscopic assessments on purified photosystems usually fail to capture these subtle changes. In this study, we examined whole cells from two model cyanobacteria, Synechocystis sp. PCC 6803 and Synechococcus elongatus UTEX 2973, grown under high and low light conditions to decode the high light tolerance of the latter. This allowed us to study photosynthetic machinery in the native state and in this work we particularly focused on the excitation energy transfer within PSII and PSI manifold. Understanding the high-light tolerance mechanism is imperative as it can help design strategies for increasing the light tolerance of cyanobacteria used for carbon neutral bioproduction. Our observations suggest that Synechococcus 2973 employs an uncommon photoprotection strategy, and the absence of hydroxy-echinenone pigment in this strain opens the possibility of an orange carotenoid protein homolog utilizing zeaxanthin as a scavenger of reactive oxygen species to provide photoprotection. Furthermore, the adjustments to the high-light adaptation mechanism involve downregulating the phycobilisome antenna in Synechococcus 2973, but not in Synechocystis 6803. Additionally, the stoichiometric changes to PSII/PSI are more tightly regulated in Synechococcus 2973.