PLANT PHYSIOLOGY , Vol 115, Issue 3 1049-1056, Copyright © 1997 by American Society of Plant Biologists
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WHOLE PLANT, ENVIRONMENTAL, AND STRESS PHYSIOLOGY |
Gas Exchange and C Allocation in Dunaliella salina Cells in Response to the N Source and CO2 Concentration Used for Growth
M. Giordano and G. Bowes
Facolta di Scienze, Universita di Ancona, Via Brecce Bianche, 60131 Ancona, Italy (M.G.)
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.
