Lipid Metabolism Research And Transcriptome Analysis Of Chlorella Sp.under Nitrogen Starvation

With the lack of traditional fossil energy and the disadvantages such as unfriendliness to environment,it is urgent for people to seek alternative energy for sustainable development.However,the production of biodiesel by microalgae has great advantages so it has been attracted more and more attention at home and abroad However,the traditional screening methods of oleaginous microalgae have high cost and slow effect,but we can obtain the target algae faster because of the feature that microalgae is easily to be genetically engineered.However,the regulation mechanism of the metabolic pathways and its related pathways of synthesis of lipid in microalgae cells is not fully understood,so in order to further understand the metabolic mechanisms we choose Chlorella sp.as research object that the algae can grow rapidly with glucose as the organic carbon source and significantly increase the lipid content in the condition of nitrogen starvation.We put the algae in different culture conditions with different nitrogen concentration and choose the 10%nitrogen concentration condition which is very different with 100%nitrogen concentration in cell growth situation and lipid accumulation situation,finding that the lipid content in the algae can reach(31.41±1.02)%of cell dry weight in nitrogen starvation condition and(14.71± 1.44)%of cell dry weight in nitrogen sufficiency condition.Meanwhile,changes in the fatty acid composition in total lipid also occur which the proportion of saturated and monounsaturated fatty acids has risen.In addition,the starch content in the algae is(510.01±14.04)mg/L in nitrogen starvation condition but only(162.32±10.12)mg/L in nitrogen sufficiency condition.But because of poor nutrition in nitrogen starvation condition,the biomass and chlorophyll content of the algae cells in the growth process decrease,then the photosynthesis also decreases.We then interchanged the nitrogen starvation condition and nitrogen sufficiency condition to cultivate the algae again to obtain chlorella in four kinds of growth situations with different responses to nitrogen starvation,and transcriptome sequencing and analysis were performed.We reconstructed the synthesis pathway of TAG and starch in the chlorella and the related pathway of carbon source and we found that under the condition of nitrogen starvation,the key enzymes involved in TAG synthesis had up-regulated expression,and expression of the key enzymes involved in starch synthesis began to decline,but the related enzymes' expression of starch hydrolysis pathway began to increase and meanwhile the most key enzymes involved in fatty acid synthesis pathway had down-regulated expression.It is indicated that the synthesis of TAG in the chlorella which grows in nitrogen starvation and amphitrophy conditions may depend more on starch as carbon source rather than de novo synthesis pathway.Taken together,through the analysis of transcriptome on Chlorella sp.in this study we reconstructed the synthesis pathway of lipid and analyzed the key enzymes in it and it is essential for further study about the analysis of genes expression involved in the synthesis of lipid metabolism and further application of genetic engineering in the production of biodiesel from microalgae.