In green algae and plants, state transitions act as a short-term acclimation process that balances excitation between PSI and PSII. During this process, LHC trimers are phosphorylated, dissociated from PSII, and reassociated with PSI. We determined the Cryo-EM structure of the PSI supercomplex from the green alga Chlamydomonas reinhardtii in state 2, revealing the structural details of the binding site of the phospho-LHCII trimers to the PSI–LHCI supercomplex (1). A subsequent study elucidated the Cryo-EM structure of the PSI supercomplex from the low-light-grown prasinophytic alga Ostreococcus tauri, which is ubiquitously present in the ocean. This structure exhibits a unique composition, involving a phospho-Lhcp trimer bound to the PSI core along with two additional Lhcp trimers, suggesting the possibility of state transition capability in this early-branching green alga (2). To investigate this hypothesis, we conducted a series of biochemical and physiological experiments. Initially, the absorption spectra showed a distinct difference between PSI and PSII, particularly at blue-green wavelengths. Subsequently, we observed that specific excitation of Lhcp with green light induced its phosphorylation and led to the formation of the PSI-LHCI-Lhcp supercomplex. Furthermore, the functional antenna size of PSI could reversibly expand in response to green light/darkness, demonstrating that O. tauri undergoes state transitions. These findings not only highlight a unique photoacclimation to the marine environment performed by O. tauri but also suggest an ancestral role of state transitions in green plants, considering the phylogenetic position of prasinophytes.