title: On the question of the light-harvesting role of β-carotene in photosystem II and photosystem I core complexes.
abstract: β-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 β-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 β-carotenes for photosynthetic O2 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 β-carotene absorption. In the present work we investigated the light harvesting role of β-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective β-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 β-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 β-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 β-Car-absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII.
authors: Institute of Biosciences and Applications, National Center for Scentific Research Demokritos, Aghia Paraskevi, Attikis 15310, Greece. Electronic address: kstam@bio.demokritos.gr.
authors: Kostas
authors: K
authors: Stamatakis
authors: Athanasiou Phylactou str. 3, Nicosia CY-1000, Cyprus.
authors: Merope
authors: M
authors: Tsimilli-Michael
authors: Institute of Biosciences and Applications, National Center for Scentific Research Demokritos, Aghia Paraskevi, Attikis 15310, Greece.
authors: George C
authors: GC
authors: Papageorgiou
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D004579:Electron Transport; D005453:Fluorescence; D008027:Light; D010100:Oxygen / Q000378:metabolism*; D010788:Photosynthesis; D045331:Photosystem I Protein Complex / Q000378:metabolism*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D013050:Spectrometry, Fluorescence; D046940:Synechococcus / Q000378:metabolism* / Q000528:radiation effects; D019207:beta Carotene / Q000378:metabolism*
chemical_list: D045331:Photosystem I Protein Complex; D045332:Photosystem II Protein Complex; D019207:beta Carotene; D002734:Chlorophyll; D010100:Oxygen
keywords: 77 K fluorescence spectra; Chlorophyll fluorescence; Cyanobacteria; State transitions; β-Carotene