| In recent years,biodiesel has attracted wide attention because of the lack of fossil fuels and the intensification of the greenhouse effect.Microalgae has many advantages such as high photosynthetic efficiency and fast growth rate,short growth cycle,and no competition for arable land with crops,so microalgae has an advantage in the production of biodiesel,it's crucial for industrial production to increase the lipid yield of microalgae.In this work,the influence of osmotic stress mediated by polyethylene glycol(PEG)on the growth and fatty acid synthesis of microalgae was invested using the screened strain Desmodesmus subspicatus CD1-1.The changes of genes related to fatty acid biosynthesis under stress condition were analyzed by transcriptome,which provided a reference for revealing the mechanisms of lipid production and metabolic regulation in D.subspicatus.In this study,6 algae strains were isolated and screened from different regions,and then morphologically and 18s RNA identification were analyzed.According to the characteristics of algal growth and lipid accumulation,as well as fatty acid compositions,D.subspicatus was chose as the target strain because of the highest fatty acid productivity and the highest biomass.Then,the culture conditions were optimized,the results showed that when the light intensity was fixed at 50?mol/(m~2.s),the p H was7.5,the rotation speed was 150 rpm,and the temperature was 30°C,the biomass of D.subspicatus increased to 0.89 g/L,the lipid content increased to 28.10%,the fatty acid yield increased to 18.27 mg/L/day,and the fatty acid productivity increased by 19.40%compared to pro-optimization.Effect of osmotic stress on the biomass,lipid synthesis and fatty acid quality characteristics of D.subspicatus were investigated by using different concentrations of PEG with different molecular weights.The results showed that the highest lipid content of 63.9%of the dried cell weight was obtained at PEG8000-5%(w/v,63.5 m Osm/kg),which was increased by 113.7%than control.At the same time,the maximum biomass was 0.993 g/L.Moreover,fatty acid quality analysis showed that saturated fatty acid content increased,unsaturated fatty acid content decreased,and fatty acid oxidation stability and combustion performance,and hydrophilicity were enhanced,respectively.In addition,the content of chlorophyll,protein and trehalose were all increased,the starch content was decreased under the stress condition.The molecular mechanism on the intracellular lipid synthesis of D.subspicatus under PEG8000-5%(63.5 m Osm/kg)osmotic stress condition was studied based on transcriptomics.After annotation and analysis,1864 of differential expression genes(DEGs)were obtained,including 648 up-regulation genes and 1246 down-regulation genes.The results of KEGG enrichment analysis showed that the DEGs related to the TAG synthesis pathway was up-regulation.In addition,the DEGs related to photosynthesis and TCA cycle,glycolysis,and pyruvate metabolism were up-regulation.However,these DEGs related to starch metabolism,coenzyme factors and vitamins metabolism were down-regulation.In addition,when microalgae cells were suffered from osmotic stress,the intracellular ROS level decreased,while the GSH and T-AOC content increased.After all,it can be found that the increased intracellular antioxidant capacity and the up-regulation DEGs related to fatty acid synthesis pathway,photosynthesis caused a significant improvement of lipid content in D.subspicatus under the osmotic stress from PEG8000-5%(63.5 m Osm/kg). |
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