Lipid and DHA-production in Aurantiochytrium sp. \u2013 Responses to nitrogen starvation and oxygen limitation revealed by analyses of production kinetics and global transcriptomes

Tonje M B Heggeset,Inga Marie Aasen,Olav Vadstein,Helga Ertesvåg,Bin Liu,Trond E Ellingsen

doi:10.1038/s41598-019-55902-4

Tonje M B Heggeset, Inga Marie Aasen + Show 4 more

Open Access

https://doi.org/10.1038/s41598-019-55902-4

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Abstract

Thraustochytrids of the genera Schizochytrium and Aurantiochytrium accumulate oils rich in the essential, marine n3 fatty acid docosahexaenoic acid (DHA). DHA production in Aurantiochytrium sp T66 was studied with the aim to provide more knowledge about factors that affect the DHA-productivities and the contributions of the two enzyme systems used for fatty acid synthesis in thraustochytrids, fatty acid synthetase (FAS) and PUFA-synthase. Fermentations with nitrogen starvation, which is well-known to initiate lipid accumulation in oleaginous organisms, were compared to fermentations with nitrogen in excess, obtained by oxygen limitation. The specific productivities of fatty acids originating from FAS were considerably higher under nitrogen starvation than with nitrogen in excess, while the specific productivities of DHA were the same at both conditions. Global transcriptome analysis showed significant up-regulation of FAS under N-deficient conditions, while the PUFA-synthase genes were only marginally upregulated. Neither of them was upregulated under O2-limitation where nitrogen was in excess, suggesting that N-starvation mainly affects the FAS and may be less important for the PUFA-synthase. The transcriptome analysis also revealed responses likely to be related to the generation of reducing power (NADPH) for fatty acid synthesis.

Highlights

Thraustochytrids of the genera Schizochytrium and Aurantiochytrium accumulate oils rich in the essential, marine n3 fatty acid docosahexaenoic acid (DHA)
In experiments with nitrogen starvation, Aurantiochytrium sp. strain T66 was cultivated on a defined medium containing nitrogen as glutamate, sufficient to produce approximately 30 g/l catalytic biomass (‘fat-free’ cell mass), and with glycerol as carbon source in excess
The lipids synthesised under N-starvation contained a higher fraction of the fatty acids originating from the fatty acid synthetase, and a lower fraction of DHA, than the lipids synthesised when nitrogen was in excess

Summary

Introduction

Thraustochytrids of the genera Schizochytrium and Aurantiochytrium accumulate oils rich in the essential, marine n3 fatty acid docosahexaenoic acid (DHA). DHA production in Aurantiochytrium sp T66 was studied with the aim to provide more knowledge about factors that affect the DHA-productivities and the contributions of the two enzyme systems used for fatty acid synthesis in thraustochytrids, fatty acid synthetase (FAS) and PUFA-synthase. Thraustochytrids use a standard fatty acid synthase (FAS) enzyme complex for synthesis of saturated fatty acids, mainly C14:0 and C16:0, while DHA and docosapentaenoic acid (DPA, C22:5 n6) is synthesized by a PKS-enzyme complex, the PUFA-synthase[4]. Despite many years of research, the mechanisms governing the relative activities of the two enzyme systems and the distribution of the carbon flow between them, are not known This is essential knowledge needed to increase the DHA-productivity. In many of the studies of DHA-production by thraustochytrids dissolved oxygen has not been measured, which makes data interpretation difficult

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