This chapter addresses some of the interactions that occur between primary nitrogen assimilation and photosynthetic and respiratory metabolism. Much of the pioneering work in this area was carried out by Syrett (Syrett, 1953, 1956a,b, 1981) and Bassham, Kanazawa and co-workers (Bassham et al, 1981; Kanazawa et al, 1970, 1972, 1983). It is not coincidental that this work has often employed algal cells. Single-celled algae have C/N ratios between 7 and 12 compared with >20 in higher plants. Therefore, the relative importance of nitrogen in carbon metabolism of single-celled algae is much greater than in higher plants and the interactions between N assimilation and carbon metabolism are much more apparent.

For several years our group has studied the interactions between photosynthesis, respiration and N assimilation in the green alga Selenastrum minutum. Our approach has been to grow this alga in chemostat cultures under N-limited conditions. The addition of a source of inorganic nitrogen (NH4+, NO2- or NO3-) to these cells activates the assimilation of N into amino acids allowing study of the corresponding changes in metabolism. This chapter reviews some of the progress made using this system.

Nitrogen assimilation by N-limited and N-sufficient algal cells

The primary assimilation of inorganic N into amino acids and protein requires ATP, reducing power and carbon skeletons in the form of ketoacids. In photosynthetic tissues, ATP and reducing power are supplied by either photosynthetic or respiratory processes; however, most of the carbon skeletons used in amino acid synthesis are intermediates of respiratory metabolism (Fig. 1).