Research article On the question of the light-harvesting role of b-carotene in photosystem II and photosystem I core complexes Kostas Stamatakis a, * , Merope Tsimilli-Michael b , George C. Papageorgiou a a Institute of Biosciences and Applications, National Center for Scentific Research Demokritos, Aghia Paraskevi, Attikis 15310, Greece b Athanasiou Phylactou str. 3, Nicosia CY-1000, Cyprus article info Article history: Received 30 October 2013 Accepted 22 January 2014 Available online xxx Keywords: Chlorophyll fluorescence Cyanobacteria b-Carotene 77 K fluorescence spectra State transitions abstract b-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 b-carotene to chloro- phyll a electronic excitation transfers. However, it has been well documented that, compared to chlo- rophylls and to phycobilins, the light harvesting efficiency of b-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 b-carotene absorption. In the present work we investigated the light harvesting role of b-carotenes in the cyanobacterium Synechococcus sp. PCC 7942 using selective b-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 b-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 b-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 b-Car- absorbed light can be attributed to the different excitation trapping efficiencies in the reaction centers of PSI and PSII. Ó 2014 Elsevier Masson SAS. All rights reserved. 1. Introduction In 1942, Emerson and Lewis (1942), reported that the quantum yield of photosynthesis in the cyanobacterium Chroococcus was about 70% lower for light absorbed by carotenes (450e520 nm) than for light absorbed by C-phycocyanin (CPC) and chlorophyll (Chl) a. Similar, but shallower, blue troughs in quantum yield spectra of photosynthesis were reported for the green alga Chlorella pyr- enoidosa (Emerson and Lewis, 1943) and the red alga Porphyridium cruentum (Brody and Emerson, 1959), which contain accessory pigments (Chl b and phycoerythrin, respectively) that absorb in the same spectral region as carotenes. Early experiments of Goedheer (1961) (performed, however, after the discovery of the two light reactions (Emerson and Rabinowitch, 1960) and the formulation of the Z-scheme (Hill and Bendall, 1961) for oxygenic photosynthesis) with light petroleum-extracted cyanobacteria indicated that only in Photosystem I (PSI) do the b-carotenes (b-Cars) sensitize Chl a fluorescence and, hence, do harvest light for photosynthesis (for historical details, see Govindjee, 1999). In contrast, modern ultra- fast spectroscopy did prove that singlet excitation energy (EE) transfer from b-Car to Chl a occurs in all photosystem I (PSI) (Kennis et al., 2001; De Weerd et al., 2003a; Holt et al., 2004; Hilbert et al., 2004; Wehling and Walla, 2005) and PSII holochromic complexes (De Weerd et al., 2003b; Holt et al., 2004), although with lower efficiency in PSII. EE transfer from carotenoids to Chl a was also reported recently by means of 77 K emission spectra (Mimuro et al., 2011). In cyanobacteria, the PSI core complex is trimeric, containing per monomer 96 Chls a and 22 b-Cars (Jordan et al., 2001). Twenty one b-Cars are in van der Waals contact with one or more Chl a headgroups (Wang et al., 2004) and several b-Cars show extensive pep electron stacking with them, thereby facilitating efficient b- Car to Chl a EE transfer, as well as quenching of Chl a excited triplets. The PSII core complex exists as a dimer and contains 35 Chls a and 11 b-Cars per monomer, distributed in its holochromic Abbreviations: APC, allophycocyanin; Chl, chlorophyll; CPC, C-phycocyanin; DPC, 1,5-diphenylcarbazide; DCMU, 3-(3,4-dichlorophenyl)-1,1 0 -dimethylurea; EE, excitation energy; PQ, plastoquinone; PS, photosystem; PSI RC , PSI reaction center complex; PSII RC , PSII reaction center complex. * Corresponding author. Tel.: þ30 210 650 3518; fax: þ30 210 651 1767. E-mail address: kstam@bio.demokritos.gr (K. Stamatakis). Contents lists available at ScienceDirect Plant Physiology and Biochemistry journal homepage: www.elsevier.com/locate/plaphy 0981-9428/$ e see front matter Ó 2014 Elsevier Masson SAS. All rights reserved. http://dx.doi.org/10.1016/j.plaphy.2014.01.014 Plant Physiology and Biochemistry xxx (2014) 1e7 Please cite this article in press as: Stamatakis, K., et al., On the question of the light-harvesting role of b-carotene in photosystem II and photosystem I core complexes, Plant Physiology and Biochemistry (2014), http://dx.doi.org/10.1016/j.plaphy.2014.01.014