Vonlanthen, SE; (2013) Analysis and Manipulation of Storage Lipids in Microalgae. Doctoral thesis , UCL (University College London).

Abstract

The exploitation of microalgal lipids for production of biodiesel will require strains that meet a number of basic criteria including high biomass productivity, high lipid content, and tolerance to various biotic and abiotic stresses. The freshwater green alga, Chlorella sorokiniana is a thermo-tolerant species with exceptionally high growth rates. Work in this thesis has explored the optimum conditions for growth of this species, with reduced doubling times at elevated temperatures and by supplement with an organic carbon source such as acetate or glucose. Similar to many other green algal species, C. sorokiniana accumulates neutral lipids in the form of triglycerides (TAGs) during nutrient deprivation. Nitrogen starvation experiments showed an increase in TAG content whilst a decrease in polar membrane lipids and change in the fatty acid profile towards more saturated and monounsaturated fatty acids. However, the nitrogen deprivation also results in reduced biomass production and severe degradation of pigments. Alternative strategies to increase the lipid content of algal cells are necessary to increase the overall yield in large-scale productions. Several papers have shown that by blocking starch biosynthesis in the model species Chlamydomonas reinhardtii, cells hyper-accumulate lipids (Work et al., 2010; Wang et al., 2009). To test this theory further, C. sorokiniana starch mutants were isolated after UV mutagenesis. Several mutants showed reduced amount of storage starch, of which one mutant, ST68, defected in an isoamylase, necessary for correct formation of starch, showed the most promise. An alternative approach to increase lipid yield is to use genetic engineering to overexpress enzymes in the lipid biosynthesis pathway. Several strategies were taken to overexpress two important enzymes, the diacylglycerol acyltransferase (DGAT) and glycerol-3-phosphate dehydrogenase (GPD) in C. reinhardtii. Although transgenic lines were successfully generated, expression could not be detected and the issue with gene silencing in the nucleus of C. reinhardtii is discussed.

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