Choreography of Transcriptomes and Lipidomes ofNannochloropsisReveals the Mechanisms of Oil Synthesis in Microalgae  

Abstract

To reveal the molecular mechanisms of oleaginousness in microalgae, transcriptomic and lipidomic dynamics of the oleaginous microalga Nannochloropsis oceanica IMET1 under nitrogen-replete (N+) and N-depleted (N-) conditions were simultaneously tracked. At the transcript level, enhanced triacylglycerol (TAG) synthesis under N-conditions primarily involved upregulation of seven putative diacylglycerol acyltransferase (DGAT) genes and downregulation of six other DGAT genes, with a simultaneous elevation of the other Kennedy pathway genes. Under N-conditions, despite downregulation of most de novo fatty acid synthesis genes, the pathways that shunt carbon precursors from protein and carbohydrate metabolic pathways into glycerolipid synthesis were stimulated at the transcript level. In particular, the genes involved in supplying carbon precursors and energy for de novo fatty acid synthesis, including those encoding components of the pyruvate dehydrogenase complex (PDHC), glycolysis, and PDHC bypass, and suites of specific transporters, were substantially upregulated under N-conditions, resulting in increased overall TAG production. Moreover, genes involved in the citric acid cycle and b-oxidation in mitochondria were greatly enhanced to utilize the carbon skeletons derived from membrane lipids and proteins to produce additional TAG or its precursors. This temporal and spatial regulation model of oil accumulation in microalgae provides a basis for improving our understanding of TAG synthesis in microalgae and will also enable more rational genetic engineering of TAG production.

Nitrate Reductase Knockout Uncouples Nitrate Transport from Nitrate Assimilation and Drives Repartitioning of Carbon Flux in a Model Pennate Diatom

“…FAME composition in both wild-type and NR-KO14 cells was dominated by C16:0, C16:1, and C20:5 FAs (Supplemental Figures 7A to 7E), consistent with previous reports for diatom FAME composition (Abida et al, 2015;Shen et al, 2016;Yu et al, 2009). As previous studies have established (Bender et al, 2014;Hockin et al 2012;Li et al, 2014;Rismani-Yazdi et al, 2012;Shifrin and Chisholm, 1981;Yang et al 2013;Yoon et al, 2012), N-limited conditions in photosynthetic microalgae induce accumulation of lipids, predominantly TAG, which is stored in lipid droplets. Therefore, it was not surprising to observe very rapid increases in TAG accumulation in NR-KO14 cells ( Figure 8C) as N-stress is sensed amid NO 3 2 -replete conditions.…”

“…Four genes annotated for lipid recycling, transport, and signaling ( Figures 11F to 11I) were all more highly expressed in NR-KO14 than in wild-type cells (18 h). One of the lipases, EG00718, is predicted to be patatin-like galactolipases, which in microalgae and plants release FAs from membrane lipids, such as monogalactosyldiacylglycerol, under N-stress conditions Li et al, 2014). Other genes encoded putative proteins with roles in phospholipid biosynthesis and phosphatidylinositol signaling (PITPs) ( Figures 11G to 11I), choline transport, fatty acid biosynthesis, elongation, and scavenging (Supplemental Figure 8C and Supplemental Data Set 1).…”

“…In P. tricornutum and other microalgae, the decrease in C20:5 FA during N-stress is believed to reflect the efforts by alga to pursue a second source for TAG by recycling C20:5 and C16 FA moieties from chloroplast thylakoid membrane glycerolipids (Abida et al, 2015;Li et al, 2012;Martin et al, 2014;Simionato et al, 2013). Specific galactoglycerolipid lipases, known in plants as patatin lipases (reviewed in Scherer et al, 2010), have been functionally characterized in C. reinhardtii and Nannochloropsis oceanica Li et al, 2014) as thylakoid glycerolipid recyclers. Ten transcriptionally active, patatin-like lipases candidate genes are expressed in both NR-KO14 and wild-type transcriptomes, and one, EG00718, is more highly expressed in NR-KO14 cells through 42 h ( Figure 11F).…”

“…Two desaturases involved in TAG biosynthesis and C20:5 biosynthesis (J9316 and J28797) were ;10 times more highly upregulated at 18 h in NR-KO14 cells as compared with the wild type ( Figures 10C and 10D). J9316, a palmitoyl-acyl-carrier protein desaturase (PAD), observed in other microalgae as highly responsive to N-stress Li et al, 2014;Dolch and Maréchal, 2015), is localized to the chloroplast stroma and converts C16 to C16:1. Its elevated expression was correlated to the timing of increased C16:1 FA accumulation in NR-KO14 cells (Supplemental Figures 7C and 7E).…”

Light Remodels Lipid Biosynthesis in Nannochloropsis gaditana by Modulating Carbon Partitioning between Organelles

“…Lipid accumulation in several algal species is triggered by nitrogen (N) depletion, and these conditions have been heavily investigated to understand lipid metabolism regulation (Miller et al, 2010;Simionato et al, 2013;Li et al, 2014;Lu et al, 2014;Martin et al, 2014;Abida et al, 2015;Jia et al, 2015). Nitrogen deprivation also leads to accumulation of storage sugars like starch or chrysolaminarin, depending on the organism (Jia et al, 2015), and the impairment of the storage sugar metabolism was observed to further increase TAGs (Blaby et al, 2013).…”