Fatty Acid Production and Direct Acyl Transfer through Polar Lipids Control TAG Biosynthesis during Nitrogen Deprivation in the Halotolerant Alga Dunaliella tertiolecta.

Omri Avidan,Sergey Malitsky,Uri Pick

doi:10.3390/md19070368

Omri Avidan, Sergey Malitsky + Show 1 more

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https://doi.org/10.3390/md19070368

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Abstract

The aims of this work were to evaluate the contribution of the free fatty acid (FA) pool to triacylglyceride (TAG) biosynthesis and to try to characterize the mechanism by which FA are assimilated into TAG in the green alga Dunaliella tertiolecta. A time-resolved lipidomic analysis showed that nitrogen (N) deprivation induces a redistribution of total lipidome, particularly of free FA and major polar lipid (PL), in parallel to enhanced accumulation of polyunsaturated TAG. The steady-state concentration of the FA pool, measured by prolonged 14C-bicarbonate pre-labeling, showed that N deprivation induced a 50% decrease in total FA level within the first 24 h and up to 85% after 96 h. The abundance of oleic acid increased from 50 to 70% of total free FA while polyunsaturated FA (PUFA) disappeared under N deprivation. The FA flux, measured by the rate of incorporation of 14C-palmitic acid (PlA), suggests partial suppression of phosphatidylcholine (PC) acyl editing and an enhanced turnover of the FA pool and of total digalactosyl-diacylglycerol (DGDG) during N deprivation. Taken together, these results imply that FA biosynthesis is a major rate-controlling stage in TAG biosynthesis in D. tertiolecta and that acyl transfer through PL such as PC and DGDG is the major FA assimilation pathway into TAG in that alga and possibly in other green microalgae. Increasing the availability of FA could lead to enhanced TAG biosynthesis and to improved production of high-value products from microalgae.

Highlights

Many green microalgae accumulate large amounts of triacylglycerides (TAG), amounting to 15–60% of their dry weight, under adverse environmental conditions such as nutrient deprivation and high light [1,2,3]
The increase in abundance of oleic acid is in line with previous studies, showing that this fatty acid (FA) is a major constituent of TAG in green algae, and with the reported increased activity of an MGDGspecific lipase like Plastid Galactoglycerolipid Degradation1 (PGD1) (C. reinhardtii), which preferentially hydrolyzes oleic acid from special newly-formed molecular species of MGDG, to be eventually incorporated into the sn1/3 positions in TAG [43,53]
These results suggest that PG 34:1, 34:2, 34:3, and MGDG 34:2 serve in acyl transfer into TAG at the early stages of TAG biosynthesis; whereas MGDG 34:3 and DGDG

Summary

Introduction

Many green microalgae accumulate large amounts of triacylglycerides (TAG), amounting to 15–60% of their dry weight, under adverse environmental conditions such as nutrient deprivation and high light [1,2,3]. A widely utilized approach to induce TAG accumulation in microalgae is to subject cultures to nitrogen deprivation. Such growth conditions partly uncouple the carbon-to-nitrogen (C/N) ratio, leading to attenuated chloroplast-cytosolmitochondria communication, inducing photo-oxidative stress due to the accumulation of reducing equivalents and superoxide, all of which eventually result in the cessation of growth [10]. It has long been argued that N deprivation induces TAG accumulation as a protective mechanism against over-energization because its biosynthesis consumes large amounts of reducing equivalents. The synthesis of each C-18 FA requires 24 NADPH molecules, de novo TAG biosynthesis could serve as an ultimate electron sink under photo-oxidative stress conditions [5]

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