PARENT SESSION
Posters P4Aa Chlorophyll and bilin based antenna systems. Abstracts (239-271)

Action of phycobilisomes in Photosystem II and Photosystem I of cyanobacteria and the mechanism of phycobilisome protection against photoinhibition. Igor Stadnichuk^*,1, Vladimir Boichenko², Irina Elanskaya³, Marina Rakhimberdieva¹, Navassard Karapetyan¹, ¹ Institute of Biochemistry Russian Academy of Sciences, Moscow, Russia² Institute of Basic Biological Problems Russian Academy of Sciences, Puschino, Moscow Region, Russia³ Department of Biology, Moscow Lomonosov State University, Moscow, Russia

ABSTRACT- Photosynthetic action spectra of PSII-mediated O₂ evolution and of PSI-mediated photoinhibition of respiratory O₂ uptake or H₂ photoevolution in nine species of cyanobacteria, namely Calothrix PCC 7601, Chlorogloea fritschii, Gloeobacter violaceus, Nostoc muscorum, Phormidium uncinatum, Pseudanabaena sp., Spirulina platensis, Synechococcus sp., Synechocystis sp. 6803 as well as in five species of red algae including Cyanidium caldarium, Galdieria partita, Porphyridium cruentum, Rhodella violacea and Porphyra oesoensis were used to determine the participation of phycobilisomes (PBS) in function of PSII and PSI. The connection of phycobilisomes with both photosystems was improved for the mutant strains of the cyanobacterium Synechocystis sp. 6803 lacking polypeptides of PSI or PSII. Summing up of PSI- and PSII-mediated action spectra to the absorption spectrum of each species demonstrated that in average 20-40% of the light energy gathered by PBS is obtained by PSII, while 60-80% is transferred to PSI. Calculation of PBS to chlorophyll ratio for each action spectrum showed that in PSII one PBS is always accounted for 60-80 chlorophylls, whereas in PSI one PBS corresponded as usual to about 350 chlorophylls with the relations of the PBS to the dimer of PSII or to the trimer of PSI. In N-starved cells of cyanobacteria and red algae and in cells of chromatically adapted cyanobacteria grown under green light conditions the PBS stay more tightly bound to PSII while their participation in function of PSI is decreased. The realization of direct energy transfer from PBS to PSI or the transfer mediated by PSII depends on the relative contents of PBS, PSII and PSI in the algal species. It was found that excess of energy absorbed by PBS in vivo is quenched by preliminary illumination with unsaturated ultraviolet/blue light. A novel regulatory mechanism of energy dissipation which protects PBS and the photosynthetic apparatus from photoinactivation is proposed.

KEY WORDS: cyanobacteria, phycobilisome (PBS), action spectra, photosystems I and II