Gas Exchange and C Allocation in Dunaliella salina Cells in Response to the N Source and CO2 Concentration Used for Growth

The halotolerant alga Dunaliella salina was cultured on 10 mM NH4+ or NO3- with air CO2 or 5% (v/v) CO2. Cells grown on NH4+ rather than NO3- were up to 17% larger in volume but had similar division rates. The photosynthetic K0.5 of dissolved inorganic C per cell was reduced, but the light- and CO2-saturated photosynthesis, dark respiration, and light-independent fixation rates were increased. The cells exhibited 2- to 5-fold greater activities of ribulose-1,5-bisphosphate carboxylase/oxygenase, phosphoenolpyruvate carboxylase and carboxykinase, and carbonic anhydrase and more soluble and ribulose-1,5-bisphosphate carboxylase/oxygenase protein. Chlorophyll and [beta]-carotene also increased by 30 to 70%. However, starch and glycerol decreased, indicating that C was reallocated from carbohydrates into protein and pigments by growth on NH4+. Algae cultured on air-CO2 rather than a high CO2 concentration were 44% smaller with 55 to 67% lower cell division rates and thus appeared C-limited, despite the operation of a CO2-concentrating mechanism. Cells cultured on air-CO2 had less protein and starch and 28% more glycerol, but the pigment content was unchanged. In only one growth regime was the cell glycerol concentration sufficient to maintain osmotic equilibrium with the external medium, indicating that an additional osmoticum was required. It appears that the N source, as well as the growth [CO2], substantially modifies photosynthetic and growth characteristics, light-independent C metabolism, and C-allocation patterns of D. salina cells.