Chlorella vulgaris is a fast growing, unicellular microalgae. With its limited nutritional requirements and ability to grow in adverse environments, it is a promising biodiesel feedstock that will not compete with agricultural resources. Published literature has shown C. vulgaris can produce up to 42% lipids per dry mass, the most desired of which are triacylglycerols (TAGs). TAGs consist of three fatty acids esterified to a glycerol backbone and are the most reduced, energy-dense biomolecules. To generate drop-in ready biodiesel, TAGs are transesterified to yield methyl-ester alkyls and a glycerol byproduct. To realistically produce fuel on an industrial scale, microalgal TAG production must be increased beyond current levels. Previous work published by the Kolling Lab increased lipid accumulation in C. vulgaris by using an exogenous carbon source to form a heteromixotrophic culture, in which the cells are energetically supplemented by both exogenous carbon as well as photosynthetic light reactions. This project seeks to further increase TAG production by supplying cells with exogenous glycerol, thus providing a use for the transesterification byproduct while reducing the cellular energy investment for TAG production. While published studies have found 10 g/L glycerol to be optimal for lipid production in C. vulgaris, this was completed heterotrophically. While growing mixotrophically, we have increased lipid accumulation beyond previously published glycerol studies and have increased overall lipid productivity in glycerol concentrations of 31.5 and 63 g/L. We also monitored photosynthetic performance before and after glycerol addition and observed an increased reliance on cyclic electron transport (a mode of photosynthesis that does not result in the production of NADPH), indicating that the ATP need is greater than that of NADPH for TAG formation under photomixotrophic conditions. Finally, using isotopically labeled glycerol, we have used GC-MS to determine the rate of glycerol uptake and that exogenously provided glycerol is directly used for the TAG backbone and not metabolized for the generation of fatty acids.

Enhancement of algal biofeedstocks in a mixotrophic batch culture supplemented with exogenous glycerol

2019 Spring National Meeting

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