Munro, R. J. L.; Andreeva, E. A.; Hartmann, E.; Goor, Q.; El Zein, H.; Nizinski, S.; Wilson, A.; De Zitter, E.; Effantin, G.; Coquelle, N.; Zala, N.; Appleby, M. V.; Bar-Zvi, S.; Bacellar, C.; Beale, E.; Bignon, E.; Brutscher, B.; Byrdin, M.; Cirelli, C.; Dworkowski, F.; Foucar, L.; Gotthard, G.; Gorel, A.; Grunbein, M. L.; Hilpert, M.; Johnson, P. J. M.; Kloos, M.; Knoop, G.; Nass, K.; Nass Kovacs, G.; Ozerov, D.; Milne, C. J.; Burdinski, G.; Chipot, C.; Karami, Y.; Dehez, F.; Weik, M.; Doak, R. B.; Shoeman, R. L.; Schiro, G.; Sliwa, M.; Kirilovsky, D.; Schlichting, I.; Colletier, J.-P.

Cyanobacteria have produced Earth oxygen for 2.4 billion years by adapting to fluctuating irradiance. This adaptation relies on orange carotenoid protein (OCP), which mediates light-intensity -- dependent photoprotective energy dissipation using a unique two- photon absorption mechanism. Photon absorption by ground-state OCP (OCPO) generates a metastable intermediate (OCP1hv) that either relaxes thermally or, upon absorption of a second photon within ~1 s, converts to the active photoprotective state (OCPR). By integrating static and time-resolved crystallography, cryo-EM, computation, spectroscopy and biochemistry, we assign the structure of OCP1hv;, establish its functional relevance and capture structural snapshots along the OCPO --> OCP1hv and OCP1hv --> OCPR photochemical pathways. We elucidate the molecular mechanism of OCP, which serves as a unique biological circuit breaker protecting the photosynthetic machinery from high light flux.