| Microalgae energy has become the hot topic in the global bioenergy research field. Nitrogen (N), phosphorus (P) and iron (Fe) deficiency stress can effectively promote lipid synthesis and accumulation in microalgae cells, and are the main approaches to regulate the lipid accumulation in oleaginous microalgae culture process. Current research on physiological and biochemical changes and lipid metabolism under stress conditions are limited to model alga like Chlamydomonas, and similar research on industrial algae strains that are able to grow in large-scale was rarely reported, which limit the industrialization progress of microalgae energy. Our group initiated a novel cultivation strategy called "Sequential Heterotrophy-Dilution-Photoinduction"(SHDP) which can increase the lipid content of Chlorella in a short period of time. However, the physiological and biochemical changes as well as the transcription profile of key gene have yet been in-depth studied.In this study, the changes of biochemical compositions and photosynthetic physiology in the process of SHDP and N/P/Fe deficiency cultivation were studied. Besides, based on the Chlorella pyrenoidosa genomic information obtained in previous study of our group, the expression levels of key genes in lipid biosynthesis pathways were also investigated. The changes in physiological and biochemical characteristics as well as gene transcription profile of Chlorella under different lipid-producing modes were systematically studied. The main contents and results are as follows:Firstly, lipid and other biochemical compositions of Chlorella were measured, the content of many biochemical compositions changed dramatically after photoinduction, the total lipid and neutral lipid content of heterotrophic algal cell increased from24.5%and18.0%up to33.7%and25.9%within24hours’photoinduction, respectively. The lipid content of Chlorella under all nutritional deficiencies enhanced significantly, especially for N deficiency, the total lipid and neutral lipid content reached50.32%and34.29%of the dry cell weight within7days, respectively.Secondly, according to chlorophyll fluorescence parameters (Fv/Fm and NPQ), the active oxygen content and antioxidant enzyme activities, the physiological parameters of algal cells in the initial stage of photoinduction changed significantly, indicating that high light can trigger the rehibilation of photosynthetic system in the heterotrophic algal cell, but the lasting radiation would inhibit the photosynthetic activity of algal cell and impose physiological stress to algal cell. The continuous increase of MDA content suggests that algal cell was severely harmed in phosphorus deficiency. New balance of producing and scavenging ROS was achieved in nitrogen deficiency, while iron deficiency didn’t affect the variation of ROS significantly.Finally, the transcription profile of the key genes in the central carbon metabolism pathway and lipid synthesis pathway was investigated. In the photo-induction phase, the expression level of me g4297and g6562genes increased2.5times and3.5times respectively; the expression level of dgat g3280and g7566gene increased by10times and47times, respectively. Under nutrient deficiency conditions, the expression levels of rbc and pepc genes exhibited down-regulation, the transcription abundance of me, acc and dgat genes increased significantly. The correlation analysis of lipid content with cultivation modes and gene expression level has revealed that nitrogen deficiency is the most effective approach to trigger the expression of me, acc and dgat genes, and that accD, dgat g3280and me g6562genes contributed most to the lipid synthesis in algal cell. |
