Light-Quality-Adapted Carotenoid Photoprotection in the Photosystem of Roseiflexus castenholzii .

The photosystem of filamentous anoxygenic phototroph Roseiflexus ( Rfl .) castenholzii comprises a light-harvesting (LH) complex encircling a reaction center (RC), which intensely absorbs blue-green light by carotenoid (Car) and near-infrared light by bacteriochlorophyll (BChl). To explore the influence of light quality (color) on the photosynthetic activity, we compared the pigment compositions and triplet excitation dynamics of the LH-RCs from Rfl . castenholzii was adapted to blue-green light ( bg -LH-RC) and to near-infrared light ( nir -LH-RC). Both LH-RCs bind γ-carotene derivatives; however, compared to that of nir -LH-RC (12%), bg -LH-RC contains substantially higher keto-γ-carotene content (43%) and shows considerably faster BChl-to-Car triplet excitation transfer (10.9 ns vs 15.0 ns). For bg -LH-RC, but not nir -LH-RC, selective photoexcitation of Car and the 800 nm-absorbing BChl led to Car-to-Car triplet transfer and BChl-Car singlet fission reactions, respectively. The unique excitation dynamics of bg -LH-RC enhances its photoprotection, which is crucial for the survival of aquatic anoxygenic phototrophs from photooxidative stress.