Mechanism Of Monoethanolamine On Metabolism Of Carbon Fixation And Lipid Biosynthesis In Chlorella Vulgaris

In recent years,with the task of carbon neutrality and carbon peaking put forward in China,CO₂ emission reduction has attracted more and more attention.In terms of CO₂emission reduction,microalgae have unique advantages,which not only grow fast,high carbon sequestration efficiency,but also can be used to produce biodiesel,thereby reducing China’s dependence on fossil energy.However,the microalgae biodiesel process at present is still immature,and its biomass growth rate cannot meet the process needs;at the same time,the lipid content of algae cells still has room to improve.In this paper,the physiological and biochemical characteristics of the photosynthetic growth process of microalgae under different CO₂ concentrations were first discussed by taking a green alga（Chlorella vulgaris）as the research object;secondly,the effect of MEA on Chlorella vulgaris fixation carbon and the accumulation of lipid was studied by monoethanolamine（MEA）as a CO₂ chemical absorbent;finally,transcriptome analysis of Chlorella vulgaris cultured with different concentrations of MEA was used to grasp the metabolic regulation of MEA on carbon fixation and lipid production of Chlorella vulgaris.The main conclusions are as follows:（1）5%,10%,15%,20%and 40%vol CO₂ were introduced into the Chlorella vulgaris culture respectively,and the biomass obtained was 15%vol CO₂>10%vol CO₂>5%vol CO₂>20%vol CO₂>40%vol CO₂ in high to low order,and the biomass of 15%vol CO₂ after 14 days of culture was 2004 mg/L,which was 3.1 times higher than that of the air control group.The CO₂ fixation rate obtained by 5%～20%vol CO₂ in the experimental group was significantly higher than that in the control group,and the CO₂ fixation rate reached the maximum on the 7th day,which was 74.85～55.26 mg/L/d.The total lipid content under 40%vol CO₂ culture conditions can reach 15.31%on 6th,but its total lipid yield is the lowest.Total lipid yield obtained was 15%vol CO₂>10%vol CO₂>5%vol CO₂>20%vol CO₂>Air,so 15%vol CO₂ is used as a subsequent incubation condition.（2）Adding 25 mg/L,50 mg/L,100 mg/L,200 mg/L and 300 mg/L MEA to the common Chlorella vulgaris culture solution supplied with 15%vol CO₂,the experimental group with 300 mg/L obtained the maximum biomass of 2755.89 mg/L after 14 days,which was 19.14%higher than the control group without MEA.However,the concentration of chlorophyll a in the experimental group with 100 mg/L was the highest,which was 26.238 mg/L.The addition of MEA was beneficial to the accumulation of DIC in microalgae culture medium,and the concentration of dissolved inorganic carbon（DIC）increased from 5.53%to 62.69%compared with the control group when the addition of MEA was 25～300 mg/L.With the addition of 100 mg/L MEA,Chlorella vulgaris cells obtained the maximum lipid content and lipid yield,which were 35.19%and 887.27 mg/L respectively.With the increase of MEA concentration,the relative content of C18 fatty acids in Chlorella vulgaris cells increased from 54.09%to 62.7%.（3）After transcriptome sequencing of Chlorella vulgaris cultured by MEA,123,751differentially expressed genes were obtained.According to the analysis of KEGG metabolic pathway database,the addition of MEA promoted the up-regulation of malate dehydrogenase in C4 pathway,he up-regulation of ribulose kinase,glyceraldehyde-3-phosphate dehydrogenase and Rubisco enzyme in C3 pathway,and the up-regulation of pyruvate phosphokinase in CAM pathway,which further promoted its CO₂ fixation efficiency.Up-regulation of expression levels of coding genes DN28633 and DN27246of acetyl-Co A carboxylase,DN12793 of malonyl-Co A:ACP transacylase,DN27018 of3-oxoacyl-ACP synthase reducer and DN22119 of beta-ketoacyl synthase in fatty acid synthesis pathway shows that the addition of MEA can promote fatty acid synthesis.In addition,the up-regulation of the expression levels of the coding genes DN71060 of glycerol-3-phosphate acyltransferase,DN118032 of lipid phosphate phosphatase and DN32461 and DN2788 of diacylglycerol in the coding genes indicates that the addition of MEA can also promote the synthesis of triglyceride.