High salinity activates CEF and attenuates state transitions in both psychrophilic and mesophilic Chlamydomonas species

In the last decade, studies have revealed the importance of PSI-driven cyclic electron flow (CEF) in stress acclimation in model organisms like C. reinhardtii; however, these studies focused on transient, short-term stress. In addition, PSI-supercomplexes are associated with CEF during state transition response to short-term stress. On the other hand, the role of CEF during long-term stress acclimation is still largely unknown. In this study, we elucidate the involvement of CEF in acclimation response to long-term high salinity in three different Chlamydomonas species displaying varying salinity tolerance. We compared CEF rates, capacity for state transitions, and formation of supercomplexes after salinity acclimation in the model mesophile C. reinhardtii and two psychrophilic green algae C. priscuii (UWO241) and C. sp. ICE-MDV. CEF was activated under high salt in all three species, with the psychrophilic Chlamydomonas spp. exhibiting the highest CEF rates. High salt acclimation was also correlated with reduced state transition capacity and a PSI-supercomplex was associated with high CEF. We propose that under long-term stress, CEF is constitutively activated through assembly of a stable PSI-supercomplex. The proteomic composition of the long-term PSI-supercomplex is distinct from the supercomplex formed during state transitions, and its presence attenuates the state transition response. ### Competing Interest Statement The authors have declared no competing interest.