As shown in Fig. 2c, the growth of D. salina with nitrate concentrations of 12.0, 18.0, and 24.0 mg/L reached the stationary phase at 10 days and that with 6.0 mg/L nitrate did so at 14 days.
D. salina with a nitrate concentration of 18.0 mg/L produced a maximum PBiomass of 37.5 mg/L/d. As shown in Fig. 2d, the growth of D. tertiolecta with nitrate concentrations of 6.0 and 12.0 mg/L reached the stationary phase at 12 and 14 days and those with 18.0 and 24.0 mg/L nitrate reached it at 10 days. D. tertiolecta with 24.0 mg/L nitrate produced a maximum PBiomass of 44.2 mg/L/d. The low biomass productions of 0.25 g dcw/L and 0.28 g dcw/L were observed in D. salina and D. tertiolecta, respectively, at 10 days after inoculation. Tang et al. [7] reported that the optimal cell growth of different types of microalgae varied individually. Optimal growth of
microalgae was affected by a combination of environmental parameters such as light intensity, photoperiod, CO2 concentration and nutrient composition in the culture system. And the biomass
production of 0.39 g/L for D. tertiolecta was obtained using a light intensity of 350 mmol photon/m2/s, photoperiod of 12 h light:12 h dark, CO2 concentration of 4%. Thus, Dunaliella species requires a higher light intensity and aeration rate than the conditions used in
this study. Overall, D. salina and D. tertiolecta produced lower biomass than I. galbana and N. oculata. I. galbana showed the highest biomass with a nitrate concentration of 24.0 mg/L. The four microalgae cultures showed residual nitrate concentrations of zero at 10e14 days of culture, respectively (data not shown). Thus, I. galbana may have potential for use in oil production.