Abstract
The halotolerant green alga Dunaliella tertiolecta accumulates starch and triacylglycerol (TAG) amounting to 70% and 10-15% of total cellular carbon, respectively, when exposed to nitrogen (N) deprivation. The purpose of this study was to clarify the inter-relationships between the biosynthesis of TAG, starch, and polar lipids (PLs) in this alga. Pulse labeling with [14C]bicarbonate was utilized to label starch and [14C]palmitic acid (PlA) to label lipids. Transfer of 14C into TAG was measured and used to calculate rates of synthesis. About two-thirds of the carbon in TAG originates from starch, and one-third is made de novo by direct CO2 assimilation. The level made from degradation of pre-formed PLs is estimated to be very small. Most of the de novo synthesis involves fatty acid transfer through PLs made during the first day of N deprivation. The results suggest that starch made by photosynthetic carbon assimilation at the early stages of N deprivation is utilized for synthesis of TAG. Trans-acylation from PLs is the second major contributor to TAG biosynthesis. The utilization of starch for TAG biosynthesis may have biotechnological applications to optimize TAG biosynthesis in algae.
Highlights
Under stress conditions such as nitrogen (N) deprivation, green microalgae synthesize large amounts of starch and/ or of triacylglycerol (TAG) (Razeghifard, 2013; Slocombe et al, 2015)
In order to obtain an accurate measure of the levels of TAG and of starch accumulated under N deprivation, we performed a complete labeling of cells with [14C]bicarbonate and followed the changes in the level of 14C incorporated into starch and into TAG during N deprivation (Fig. 2A; Supplementary Table S1)
These values are in good agreement with starch levels of 20 ± 6 μg starch 106 cells−1 and 70 ± 16 μg starch 106 cells−1 that were determined in parallel experiments in complete and N-deprived growth media, respectively, by the iodine method (Supplementary Table S2; Avidan et al, 2015)
Summary
Under stress conditions such as nitrogen (N) deprivation, green microalgae synthesize large amounts of starch and/ or of triacylglycerol (TAG) (Razeghifard, 2013; Slocombe et al, 2015). Recent studies indicate that in eukaryotic algae the process may be more complicated and involve more diverse metabolic pathways. In some algae such as in Chlamydomonas reinhardtii, it was shown that TAG biosynthesis takes place inside the chloroplast (Fan et al, 2011; Goodson et al, 2011). At least three different metabolic pathways may allocate carbon to FA biosynthesis in the chloroplast of green algae (Goncalves et al, 2016): (i) de novo synthesis directly from CO2 assimilation by the Calvin cycle, formation of acetyl-CoA by pyruvate dehydrogenase, and its assimilation via the fatty acid synthesis (FAS) cycle (pathway 1; Fig. 1); (ii) FA acyl transfer from pre-formed polar lipids (PLs), of chloroplast galactoglycerolipids
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