Changes in growth, carbon and nitrogen enzyme activity and mRNA accumulation in the halophilic microalga Dunaliella viridis in response to NaCl stress

Abstract

Many species of microalga Dunaliella exhibit a remarkable tolerance to salinity and are therefore ideal for probing the effects of salinity. In this work, we assessed the effects of NaCl stress on the growth, activity and mRNA level of carbon and nitrogen metabolism enzymes of D. viridis. The alga could grow over a salinity range of 0.44 mol L−1 to 3.00 mol L−1 NaCl, but the most rapid growth was observed at 1.00 mol L−1 NaCl, followed by 2.00 mol L−1 NaCl. Paralleling these growth patterns, the highest initial and total Rubisco activities were detected in the presence of 1.00 mol L−1 NaCl, decreasing to 37.33% and 26.39% of those values, respectively, in the presence of 3.00 mol L−1 NaCl, respectively. However, the highest extracellular carbonic anhydrase (CA) activity was measured in the presence of 2.00 mol L−1 NaCl, followed by 1.00 mol L−1 NaCl. Different from the two carbon enzymes, nitrate reductase (NR) activity showed a slight change under different NaCl concentrations. At the transcriptional level, the mRNAs of Rubisco large subunit (rbcL), and small subunit (rbcS), attained their highest abundances in the presence of 1.00 and 2.00 mol L−1 NaCl, respectively. The CA mRNA accumulation was induced from 0.44 mol L−1 to 3.00 mol L−1 NaCl, but the NR mRNA showed the decreasing tendency with the increasing salinity. In conclusion, the growth and carbon fixation enzyme of Rubisco displayed similar tendency in response to NaCl stress, CA was proved be salt-inducible within a certain salinity range and NR showed the least effect by NaCl in D. viridis.