Time‑resolved transcriptome analysis during transitions of sulfur nutritional status provides insight into triacylglycerol (TAG) and astaxanthin accumulation in the green alga Chromochloris zofingiensis

Abstract

Background: Chromochloris zofingiensis, an oleaginous microalga, is a promising feedstock for the co-production of triacylglycerol (TAG)-based biodiesel and the high-value product astaxanthin. To reveal the molecular mechanism of TAG and astaxanthin biosynthesis during transitions of sulfur nutritional status, namely sulfur-starvation (SS) and sulfur-replenishment (SR), the physiological responses and the transcriptomic dynamics of C. zofingiensis were examined. Results: The results revealed a reversible TAG and astaxanthin accumulation under SS, which is correlated with the reduction of cell growth and protein content, indicating the reallocation of carbon. By correlating the data on the physiological and transcriptional responses to different sulfur nutritional status, a model for the underlying mechanism of TAG and astaxanthin accumulation in C. zofingiensis was postulated, which involved up-regulation of key genes including diacylglycerol acyltransferase (DGTT5) and beta-carotene ketolase (BKT1), increased energy and NADPH supply by elevating the tricarboxylic acid (TCA) cycle and the oxidative pentose phosphate (OPP) pathway, and the increased carbon precursors (pyruvate and acetyl-CoA) through central carbon metabolism. In addition, the net enhancement of the de novo biosynthesis of fatty acids and the redirection of the terpenoid precursors toward the branch catalyzed by lycopene beta cyclase (LCYb) and BKT1 escalated the substrate availability for the biosynthesis of TAG and astaxanthin, respectively. Conclusions: In this study, the time-resolved transcriptional analysis of C. zofingiensis under SS and SR conditions was reported for the first time to elucidate the regulatory roles of key enzymes, including DGTT5, BKT1 and LCYb, in the underlying mechanisms of TAG and astaxanthin accumulation.

The oleaginous astaxanthin-producing alga Chromochloris zofingiensis: potential from production to an emerging model for studying lipid metabolism and carotenogenesis

“…The chloroplast-localized ACCase in C. zofingiensis is a tetrasubunit enzyme consisting of α-carboxyltransferase, β-carboxyltransferase, biotin carboxyl carrier protein, and biotin carboxylase. These subunits are well correlated at the transcriptional level [ 18 , 33 , 37 , 39 ]. Malonyl-CoA has to be converted to malonyl-acyl carrier protein (ACP), through the action of MCT, before entering the subsequent condensation reactions for acyl chain extension.…”

“…Compared to C. reinhardtii that has only one gene copy for each MGD , DGD , SQD1 and SQD2 [ 164 ], C. zofingiensis harbors one copy for MDG , SQD1 and SQD2 each yet three copies for DGD [ 37 ]. Upon exposure of C. zofingiensis to stress conditions, MGDG, DGDG and SQDG all decreased, yet their biosynthetic pathways showed no transcriptional down-regulation [ 13 , 17 , 18 , 37 , 39 ].…”

“…C. zofingiensis is predicted to contain two CDS genes, one PGPS gene and one PGP gene [ 37 ]. Similarly, the transcriptional expression pattern of these genes is inconsistent with PG decrease observed under stress conditions [ 18 , 37 , 39 ]. PI also uses CDP-DAG as the precursor for synthesis, catalyzed by phosphatidylinositol synthase (PIS).…”

“…Considering that the decreases of membrane glycerolipids upon stress conditions are accompanied with no transcriptional down-regulation of their biosynthetic pathways [ 18 , 37 , 39 ], we hypothesize that their biosyntheses are maintained yet catabolic pathways mediated by lipases are likely stimulated leading to net decreases of these lipids. Microalgae harbor a number of genes encoding putative lipases, yet Plastid Galactoglycerolipid Degradation1 (PGD1) from C. reinhardtii is the only one that has been demonstrated to be involved in membrane lipid turnover [ 169 ].…”

Chromochloris zofingiensis (Chlorophyceae) Divides by Consecutive Multiple Fission Cell-Cycle under Batch and Continuous Cultivation

“…In addition, C. zofingiensis can tolerate media based on low-quality water and produce valuable biocommodities, such as lipids, starch, and the carotenoid pigments lutein, canthaxanthin, and astaxanthin [ 14 , 15 , 16 , 17 , 18 ]. Recently, the chromosome-level assembly of the genome [ 19 ] and time-resolved carotenoid profiling and transcriptomic analysis for C. zofingiensis were published [ 20 , 21 ]. These analyses advance the understanding of carotenoid production in this alga and enhance prospects for improving commercial production of C. zofingiensis .…”