| Chlorella has a superior position in the production of biofuel because of their advantages of rapid growth rates and high lipid productivity.In addition,Chlorella can synthesize various high-value compounds including proteins,pigments,and vitamins.Currently,the main obstacles of microalgae biofuel industrialization are lower lipid yield and higher production cost.Increasing the lipid yield of microalgae is crucial for its industrial production.Compared with other cultivation conditions,the components of the medium are the most important factors that affect cell growth and lipid production of microalgae.In this work,effects of nutrient elements such as nitrogen,phosphorus,and sulfur on the growth,intracellular components,lipid synthesis,ROS,and antioxidant defense systems were investigated,and the responses of cell growth and fatty acid biosynthesis of Chlorella pyrenoidosa suffering from oxidative stress induced by nutrient regimes were analyzed.The regulation methods improving the productivity of fatty acids in Chlorella pyrenoidosa were finally achieved.In this study,the growth performance,lipid accumulation,ROS,fatty acid compositions,intracellular oxidative stress of Chlorella pyrenoidosa were dtermined using two-stage cultivation method,and the concentrations of nitrogen,phosphorus,and sulfur were changed in the second stage.The final biomass concentration of C.pyrenoidosa was 1.32 g·L-1 in full-medium.Significant growth decreases were found for N deficiency?P<0.001?,P deficiency?P<0.01?,S deficiency?P<0.001?,and 3P?P<0.001?,and the corresponding biomass concentrations at 120 h were 1.05,1.18,0.72and 0.37 g·L-1,respectively.Meanwhile,the chlorophyll content decreased due to the inhibition to cell growth.In addition,the growth kinetics models on nitrogen effect was established.The predicted values of the model are basically consistent with the experimental data,which can better describe the growth regularity of algae cells under the influence of different nitrogen sources concentration.The results show that the lipid content in cells increased significantly under the condition of nitrogen deficiency.The final lipid content of Chlorella pyrenoidosa was25.10%in full-medium.The maximal lipid content?48.90%?and lipid productivity(72.04 mg·L-1·d-1)were all obtained as nitrogen deficiency,which increased 94.82%?P<0.001?and 58.23%?P<0.001?,respectively.The main fatty acid components were palmitic acid?C16:0?,palmitic acid?C16:1?,stearic acid?C18:0?,oleic acid?C18:1?,linoleic acid?C18:2?,and alpha-linolenic acid?C18:3?in the testing,which accounted for more than 95%of the total fatty acids.In addition,SV,IV,CN,DU,LCSF and CFPP were respectively calculated according to the compositions of fatty acids.The result indicates that lipid content,combustion quality and ignition delay time,and oxidation stability of the produced lipids were all improved under nitrogen,phosphorus,and sulfur-deficient conditions.In summary,the lipid productivity and quality can be improved significantly under nitrogen deficiency condition.Furthermore,the role of cellular oxidative stress and the relationship between intracellular reactive oxygen species and fatty acid content under various nutritional conditions were analyzed.The Pearson correlation coefficient between reactive oxygen species and lipid content was 0.777,which means high positive correlation between reactive oxygen species and fatty acid content.In addition,the correlation between individual reactive oxygen species?hydrogen peroxide,superoxide anion,and hydroxyl radical?and lipid content was also investigated,and the Pearson correlation coefficient were 0.770,0.476,and 0.860,respectively.A higher positive correlation existed between hydroxyl radical and lipid content.Meanwhile,changes in antioxidant defense systems under different nutrient conditions were investigated.It was found that SOD,CAT,APX,and biomarker MDA were all increased.It means that stress response occurred in the intracellular antioxidant system. |
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