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International audience; Oxygenic photosynthesis is indispensable both for the development and maintenance of life on earth by converting light energy into chemical energy and by producing molecular oxygen and consuming carbon dioxide. This latter process has been responsible for reducing the CO2 from its very high levels in the primitive atmosphere to the present low levels and thus reducing global temperatures to levels conducive to the development of life. Photosystem I and photosystem II are the two multi-protein complexes that contain the pigments necessary to harvest photons and use light energy to catalyse the primary photosynthetic endergonic reactions producing high energy compounds. Both photosystems are highly organised membrane supercomplexes composed of a core complex, containing the reaction centre where electrontransport is initiated, and of a peripheral antenna system, which is important for light harvesting and photosynthetic activity regulation. If on the one hand both the chemical reactions catalysed by the two photosystems and their detailed structure are different, on the other hand they share many similarities. In this review we discuss and compare various aspects of the organisation, functioning and regulation of plant photosystems by comparing them for similarities and differences as obtained by structural, biochemical and spectroscopic investigations.

Photosynthesis in plants, algae and cyanobacteria releases oxygen and is referred to as oxygenic photosynthesis. Among all the photosynthetic reaction centers only PSII provides a redox potential sufficiently strong for water oxidation. While the electron flow in PSII is strictly asymmetric, in PSI different levels of bidirectionality indicate functional flexibility of this complex. Photo-CIDNP is non-Boltzmann nuclear magnetization caused by photochemical reactions and can be observed by NMR spectroscopy as strongly enhanced absorptive or emissive signals. In this thesis photo-CIDNP solid state NMR with selective isotope labeling is applied to get direct access to the heart of large PSII and PSI photosynthetic complexes in intact and isolated systems of Spirodela oligorrhiza and Synechocyctis. For the first time the direct observation of selective atoms within the heart of the PSII and PSI complexes by experiment s on entire plants and whole cells is reported. In this way the conservation of the electronic structure of the PSII electron donor at various levels of biological preparations have been addressed and the electron spin density (ρi) in the active cofactors of PSI has been constructed. In addition the functional heterogeneity of the PSI electron donor among different plant species was probed.

Cyanobacteria are widely used as model organism of oxygenic photosynthesis due to being the simplest photosynthetic organisms containing both photosystem I and II (PSI and PSII). Photochemically induced dynamic nuclear polarization (photo-CIDNP) (13)C magic-angle spinning (MAS) NMR is a powerful tool in understanding the photosynthesis machinery down to atomic level. Combined with selective isotope enrichment this technique has now opened the door to study primary charge separation in whole living cells. Here, we present the first photo-CIDNP observed in whole cells of the cyanobacterium Synechocystis.