title: Effects of exogenous β-carotene, a chemical scavenger of singlet oxygen, on the millisecond rise of chlorophyll a fluorescence of cyanobacterium Synechococcus sp. PCC 7942.
abstract: Singlet-excited oxygen (1O2*) has been recognized as the most destructive member of the reactive oxygen species (ROS) which are formed during oxygenic photosynthesis by plants, algae, and cyanobacteria. ROS and1O2*are known to damage protein and phospholipid structures and to impair photosynthetic electron transport and de novo protein synthesis. Partial protection is afforded to photosynthetic organism by the β-carotene (β-Car) molecules which accompany chlorophyll (Chl) a in the pigment-protein complexes of Photosystem II (PS II). In this paper, we studied the effects of exogenously added β-Car on the initial kinetic rise of Chl a fluorescence (10-1000 μs, the OJ segment) from the unicellular cyanobacterium Synechococcus sp. PCC7942. We show that the added β-Car enhances Chl a fluorescence when it is excited at an intensity of 3000 μmol photons m-2s-1but not when excited at 1000 μmol photons m-2s-1. Since β-Car is an efficient scavenger of1O2*, as well as a quencher of3Chl a*(precursor of1O2*), both of which are more abundant at higher excitations, we assume that the higher Chl a fluorescence in its presence signifies a protective effect against photo-oxidative damages of Chl proteins. The protective effect of added β-Car is not observed in O2-depleted cell suspensions. Lastly, in contrast to β-Car, a water-insoluble molecule, a water-soluble scavenger of1O2*, histidine, provides no protection to Chl proteins during the same time period (10-1000 μs).
authors.affiliation: Institute of Biosciences and Applications, National Center of Scientific Research "Demokritos", 15310, Athens, Greece.
authors.forename: Kostas
authors.initials: K
authors.lastname: Stamatakis
authors.affiliation: Institute of Biosciences and Applications, National Center of Scientific Research "Demokritos", 15310, Athens, Greece. gcpap@ath.forthnet.gr.
authors.forename: George C
authors.initials: GC
authors.lastname: Papageorgiou
authors.affiliation: Department of Plant Biology, Department of Biochemistry, and Center of Biophysics & Comparative Biology, University of Illinois at Urbana- Champaign, Champaign, IL, 61801, USA.
authors.lastname: Govindjee
mesh_terms: D002734:Chlorophyll / Q000819:agonists*; D000077194:Chlorophyll A; D005453:Fluorescence; D006639:Histidine / Q000494:pharmacology; D007700:Kinetics; D008027:Light; D010788:Photosynthesis / Q000187:drug effects*; D026082:Singlet Oxygen / Q000378:metabolism*; D046940:Synechococcus / Q000187:drug effects* / Q000502:physiology; D013997:Time Factors; D019207:beta Carotene / Q000494:pharmacology*
chemical_list: D019207:beta Carotene; D002734:Chlorophyll; D026082:Singlet Oxygen; D006639:Histidine; D000077194:Chlorophyll A
keywords: Chlorophyll fluorescence; Cyanobacteria; Singlet oxygen; Synechococcus sp. PCC7942; β-Carotene