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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.

The light harvesting-reaction center complex (LH-RC) of Roseiflexus castenholzii binds bacteriochlorophylls a (BChls a ), B800 and B880, absorbing around 800 and 880 nm, respectively. We comparatively investigated the interband excitation energy transfer (EET) dynamics of the wild-type LH-RC (wt - LH-RC) of Rfl . castenholzii and its carotenoid (Car)-less mutant (m-LH-RC) and found that Car can boost the B800 → B880 EET rate from (2.43 ps) -1 to (1.75 ps) -1 , accounting for 38% acceleration of the EET process. Interestingly, photoexcitation of wt - LH-RC at 800 nm induced pronounced excitation dynamics of Car despite the insufficient photon energy for direct Car excitation, a phenomenon which is attributed to the BChl-Car exciplex 1 [B800(↑↑)···Car(↓↓)]*. Such an exciplex is suggested to play an essential role in promoting the B800 → B880 EET process, as corroborated by the recently reported cryo-EM structures of wt - LH-RC and m - LH-RC. The mechanism of Car-mediated EET will be helpful to deepen the understanding of the role of Car in bacterial photosynthesis.