Diversity and engineering of diatom metabolism for new and improved sterol products

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Diatoms are a large group of eukaryotic microalgae that arose through secondary endosymbiosis and are renowned for their wide ecological distribution. Diatoms have genetically diversified their physiology, metabolism and natural products, while adapting to dynamic environments. Among these metabolic products are an expanded repertoire of phytosterols, a class of essential terpenoids that are involved in the regulation of membrane dynamics and signalling functions. Phytosterols are considered a marker of eukaryotic life and have been used to identify and date evolutionary events. They are also useful natural products due to their wide range of biological applications. The global phytosterol market is growing at about 7-9% per annum. In order to meet this demand, diatom microalgae are proposed as an alternative source of natural products. The function, distribution and biosynthesis of sterols is well characterised and conserved in model animal, plant and fungal organisms. However, the biological role and metabolism of the high diversity of sterols produced by diatoms is not well understood. To establish diatoms as a suitable platform for phytosterols production, in this PhD project we provide insight into key aspects of sterol compounds from diatoms: i) The response of sterol levels to changes in environmental conditions, ii) The reconstruction of the sterol biosynthesis pathways of multiple diatom species, and iii) Genetic investigations and engineering of diatoms to alter sterol product profiles. In Chapter 1, I provide an updated review of the phytosterol repertoire in diatoms, including the biology and regulation of sterol biosynthesis. In the first data chapter, Chapter 2, I investigated the occurrence of different sterol types in twelve different diatom species, as well as the effect of temperature reduction and changes in salinity on the sterol contents of three model diatoms. In Chapter 3, I experimentally examined the sterol biosynthesis pathways of three divergent diatom species, using empirical biochemical profiling and comparative โ€˜omics. This Chapter experimentally explored hypotheses with regard to what extent the sterol biosynthesis pathways of three diatom species are conserved, and where each of these has diverged to produce different phytosterols. This study introduces in-depth multi-species analyses in order to compare and contrast the biosynthesis pathways of distantly related species. The results expand our understanding of sterol biosynthesis in diatoms. Finally, in Chapter 4, I implemented and performed genetic engineering technologies to test the extent to which natural sterol levels can be manipulated in the diatoms ๐˜›๐˜ฉ๐˜ข๐˜ญ๐˜ข๐˜ด๐˜ด๐˜ช๐˜ฐ๐˜ด๐˜ช๐˜ณ๐˜ข ๐˜ฑ๐˜ด๐˜ฆ๐˜ถ๐˜ฅ๐˜ฐ๐˜ฏ๐˜ข๐˜ฏ๐˜ข and ๐˜—๐˜ฉ๐˜ข๐˜ฆ๐˜ฐ๐˜ฅ๐˜ข๐˜ค๐˜ต๐˜บ๐˜ญ๐˜ถ๐˜ฎ ๐˜ต๐˜ณ๐˜ช๐˜ค๐˜ฐ๐˜ณ๐˜ฏ๐˜ถ๐˜ต๐˜ถ๐˜ฎ.
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