Adaptation of Dunaliella salina (Volvocales, Chlorophyta) to growth on NH4+ as the sole N source

The green, wall-less microalga Dunaliella salina.(Dunal) Teodoresco (Volvocales, Chlorophyceae) UTEX 200 was cultured at air-equilibrium CO2 concentration, at 1.5 M NaCl, in the presence of either 10 mM NO3− or 10 mM NH4+ as the sole N source. The N source strongly influenced photosynthesis and C metabolism, and it altered the cell size. Growth on NH4+ caused a 20% increase of the cell volume. Chlorophyll, β-carotene, and protein content were higher by 49%, 75%, and 104%, respectively, in NH4+-grown cells compared to their NO3−-grown counterparts. By contrast, glycerol decreased by about 20% when N was supplied as NH4+ rather than as NO3−. Cells adapted to growth on NH4+ were more efficient at using light and inorganic C for photosynthesis and showed a greater affinity for photons and CO2 than cells cultured in medium containing NO3−. Ribulose bisphosphate carboxylase/oxygenase (Rubisco) protein and activity were present at about the same level in both NO3−- and NH4+-grown cells, but in vivo Rubisco activity was about three times higher in algae cultured on NH4+. Phosphoenolpyruvate (PEP) carboxylase, PEP carboxykinase, and carbonic anhydrase activities were stimulated by growth on NH4+. The mechanisms of adaptation to growth at high NH4+ concentrations are discussed.