On the question of the light-harvesting role of β-carotene in photosystem II and photosystem I core complexes

beta-Carotene is the only carotenoid present in the core complexes of Photosystems I and II. Its proximity to chlorophyll a molecules enables intermolecular electronic interactions, including beta-carotene to chlorophyll a electronic excitation transfers. However, it has been well documented that, compared to chlorophylls and to phycobilins, the light harvesting efficiency of beta-carotenes for photosynthetic O-2 evolution is poor. This is more evident in cyanobacteria than in plants and algae because they lack accessory light harvesting pigments with absorptions that overlap the beta-carotene absorption. In the present work we investigated the light harvesting role of beta-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective beta-carotene excitation and selective Photosystem detection of photo-induced electron transport to and from the intersystem plastoquinones (the plastoquinone pool). We report that, although selectively excited beta-carotenes transfer electronic excitation to the chlorophyll a of both photosystems, they enable only the oxidation of the plastoquinone pool by Photosystem I but not its reduction by Photosystem II. This may suggest a light harvesting role for the beta-carotenes of the Photosystem I core complex but not for those of the Photosystem II core complex. According to the present investigation, performed with whole cyanobacterial cells, the lower photosynthesis yields measured with beta-Car-absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII. (C) 2014 Elsevier Masson SAS. All rights reserved.