J. Plant Physiol. 158. 577–581 (2001)  Urban & Fischer Verlag http://www.urbanfischer.de/journals/jpp Interactions between C and N metabolism in Dunaliella salina cells cultured at elevated CO 2 and high N concentrations Mario Giordano* Istituto di Scienze del Mare, Facoltà Di Scienze, Università di Ancona, Italy Received August 17, 2000 · Accepted November 29, 2000 Summary The green alga Dunaliella salina UTEX 200 was cultured at high (5 %) [CO 2 ] in a medium containing 10 mmol/L of either NO 3 – or NH 4 + as the sole N source. Specific growth rate was 50 % higher for NH 4 + -grown cells than for their counterparts cultured in the presence of NO 3 – . Cell size, protein con- tent, Rubisco protein, phosphoenolpyruvate carboxylase (PEPC) activity, and light independent car- bon fixation were enhanced by growth in the presence of NH 4 + . On the other hand, maximal photo- synthetic rate and cell glycerol concentration were lower when N was supplied as NH 4 + . The activity of glutamine synthetase was affected very little by the N-source. D. salina UTEX 200 showed some peculiarities in its mechanism of adaptation to high [N] in comparison to other strains previously used for similar studies. This allowed dissection of the underlying mechanism of the growth re- sponse to high [N], highlighting the potential role of PEPC, the main anaplerotic enzyme, as a pivotal player in the adaptation of cells to these conditions. Key words: ammonium – anaplerosis – nitrate – PEPC – photosynthesis Abbreviations: DIC dissolved inorganic carbon. – GS glutamine synthetase. – NR nitrate reductase. – PEP phosphoenolpyruvate. – PEPC PEP carboxylase. – P max light- and CO 2 -saturated photosyn- thetic rate. – PPFD photosynthetic photon flux density. – Rubisco ribulosebisphosphate carboxyl- ase/oxygenase. – SD standard deviation Introduction The halotolerant, wall-less flagellate Dunaliella salina (Volvo- cales, Chlorophyta) is abundant in salt lakes, with either NO 3 – or NH 4 + as the main and sometimes the only N-source (Stephens and Gillespie 1976, Post 1977). For N-sufficient cells, NH 4 + is the preferred source of N because of the lower demand of energy for its assimilation. However, NH 4 + can act * E-mail corresponding author: giordano@popcsi.unian.it as an uncoupler of photophosphorylation (Krogman et al. 1959, Padan and Schuldiner 1978, Post 1993). Also, NH 4 + / NH 3 influx into the cell can only be partially controlled, espe- cially at high external concentrations and at pH above 7, when free NH 3 becomes available (Kleiner 1981, Ullrich 1987, 1992); under these conditions, NH 4 + /NH 3 influx is strongly influenced by the concentration gradient between the cell and the external environment. Consequently, in the presence of high [NH 4 + ], substantial expenditures for initial compart- mentalization (Pick et al. 1991, Post 1993) and subsequent 0176-1617/01/158/05-577 $ 15.00/0