Assimilation of nitrate-nitrogen and carbon dioxide by Scenedesmus acuminatus under different initial nitrogen concentrations

Scenedesmus acuminatus is a unicellular freshwater green microalga that can accumulate high lipid content.Lower nitrogen concentrations can evidently increase lipid accumulation in S.acuminatus,which makes it a promising source for biodiesel production.In this study,the characteristics and phase changes of carbon and nitrogen assimilation of S.acuminatus were studied under different initial nitrogen concentrations (18.0,9.0,6.0,and 3.6 mmol·L^-1) by measuring the activities of nitrate reductase and Rubisco,the intracellular C and N contents,and the NO₃^- concentration in media.Results showed that S.acuminatus achieved the highest cell density,7.9×10⁷ cells·mL^-1 in the 18.0 mmol·L^-1 group.The cellular nitrate reductase and Rubisco activities were also the highest and lasted longest in this group.With the increase of initial nitrogen concentration,the production of NO₂^- increased in the medium,whereas the 18.0 mmol·L^-1 group maintained a relatively high NO₂^- level at the end of the culture period.After 10 d of cultivation,the nitrogen contents in the four experimental groups (from 18.0 to 3.6) were 7.2%,4.1%,2.8%,and 1.9%,respectively,the carbon contents were 46%,52%,54.5%,and 57.4%,respectively,and the molar C/N ratios were 8.0,16.7,24.3,and 35.2,respectively.In conclusion,initial nitrogen concentration had significant impact on the growth of S.acuminatus.Furthermore,the carbon and nitrogen assimilations of the algae interacted with each other,and the fixation of carbon in S.acuminatus was promoted under low nitrogen concentrations.