Accumulation of medium chain fatty acids in Nannochloropsis oceanica by heterologous expression of Cuphea palustris thioesterase FatB1

Medium chain fatty acids (MCFAs) are compounds of considerable commercial interest that have applications in food, feed, and the production of industrially relevant chemicals. Due to their antipathogenic and health-promoting effects, they are further discussed as potential therapeutic agents for disease treatment and infection prevention and control. Domesticated agricultural crops that can grow in temperate climates lack MCFAs, and increased cultivation of tropical MCFA-rich species such as oil palm is associated with deforestation and a decrease in biodiversity. Alternative sources for more sustainably produced MCFAs are non-domesticated plants such as Cuphea spp., as well as oil crops and oleaginous microalgae that can be genetically engineered to accumulate MCFAs by expression of acyl-acyl carrier protein thioesterases (TEs) from MCFA-producing plants. Here, we report the heterologous expression of a TE from Cuphea palustris in the industrially relevant microalga Nannochloropsis oceanica. Using a recently developed gene expression system, we engineered transformant strains that accumulated up to 2.84 and 4.15% of C8:0 and C10:0 in storage lipids, respectively, and we observed no effect on growth. We further show that MCFA accumulation was negatively correlated with total neutral lipid content, suggesting the presence of regulatory mechanisms that limit MCFA accumulation in Nannochloropsis.