DOI: 10.1007/s11099-015-0129-y PHOTOSYNTHETICA 53 (3): 369-377, 2015 369 Structural and functional changes in the photosynthetic apparatus of Chlamydomonas reinhardtii during nitrogen deprivation and replenishment É. PREININGER *+ , A. KÓSA * , ZS. LŐRINCZ * , P. NYITRAI ** , J. SIMON * , B. BÖDDI * , Á. KERESZTES * , and I. GYURJÁN *† Department of Plant Anatomy, Eötvös Loránd University, Pázmány P. sétány 1/C, H-1117 Budapest, Hungary * Department of Plant Physiology and Molecular Plant Biology, Eötvös Loránd University, Pázmány P. sétány 1/C, H-1117 Budapest, Hungary ** Abstract Nitrogen is an essential factor for normal plant and algal development. As a component of nucleic acids, proteins, and chlorophyll (Chl) molecules, it has a crucial role in the organization of a functioning photosynthetic apparatus. Our aim was to study the effects of nitrogen starvation in cultures of the unicellular green alga, Chlamydomonas reinhardtii, maintained on nitrogen-free, and then on nitrogen-containing medium. During the three-week-long degreening process, considerable changes were observed in the Chl content, the ratio of Chl-protein complexes, and photosynthetic activity of the cultures as well as in the ultrastructure of single chloroplasts. The regreening process was much faster then the degradation; total greening of the cells occurred within four days. The rate of regeneration depended on the nitrogen content. At least 50% of the normal nitrogen content of Tris-Acetate-Phosphate (TAP) medium was required in the medium for the complete regreening of the cells and regeneration of chloroplasts. Additional key words: electron microscopy; nitrogen starvation; O2 evolution; 77K fluorescence. Introduction Nitrogen deficiency is an extensively studied subject in plant physiology and phycology since it often occurs in unbalanced environmental conditions. In higher plants, nitrogen starvation is known to cause reduced leaf ex- pansion and yellowing (Bouma 1970, Zhao et al. 2005). This is accompanied by the degradation of the chloroplast thylakoids (Kutík et al. 1993, Doncheva et al. 2001), and a decline of photosynthesis (Baszyński et al. 1975, Lawlor et al. 1989). All of these changes, together with a loss of plastid DNA, lead to leaf senescence (Scott and Possingham 1983). The green alga, Chlamydomonas reinhardtii, is a popular object in many fields of biology, e.g. for study of structure, physiology, genetics, etc. (Goodenough and Levine 1969, Gyurján et al. 1980, 1982; Davies and Grossman 1998). Fast response to many environmental changes is characteristic for this alga and it is a great advantage for experiments. A good example for the great plasticity of C. reinhardtii is adaptation to lack of nitrogen (Fernandez and Galvan 2007, 2008), which includes a complex differentiation process. The alga acquires nitrogen by autophagy, develops mating capacity (e.g. synthesizes glycoproteins for fusion of gametes developing from the vegetative cells), and prepares itself for long-term survival (accumulates starch and lipids) (Martin and Goodenough 1975). These symptoms occur under the activation of 21 genes (Abe et al. 2005). The early responding genes are induced within 2 h, the late ones between 5 and 8 h after nitrogen removal (Abe et al. 2004). Similarly to yellowing of higher plant, a decrease of the Chl content and transformation of the photosynthetic apparatus are characteristic for nitrogen-starved cells. Nitrogen starvation unequally changes the amounts of the Chl-protein complexes that can be observed via significant changes in the fluorescence emission spectra, i.e. the dominance of the short-wavelength emission bands in the ——— Received 29 August 2014, accepted 22 January 2015. + Corresponding author; e-mail: preininger@ludens.elte.hu Abbreviations: Fv/Fm - maximal quantum yield of PSII photochemistry; Chl - chlorophyll; CP43 and CP47 - core complex proteins of PSII; DG  degreening process; P680 - primary electron donor of PSII; RG  regreening process; TAP – Tris-Acetate-Phosphate. Acknowledgements: A. Kósa is grateful for the Ferenc Deák Scholarship of the Hungarian Ministry of Education and Culture (DFÖ 0021/2009). This work was supported by Richter Centennial Foundation.