Expression of glycerol-3-phosphate acyltransferase increases neutral lipid accumulation in Nannochloropsis oceanica

Abstract

Microalgae are considered a suitable production platform for high-value lipids and oleochemicals. Several species including Nannochloropsis oceanica produce large amounts of essential \(\omega\)-3 polyunsaturated fatty acids (PUFAs) which are integral components of food and feed and have been associated with health-promoting effects. N. oceanica can further accumulate high contents of neutral lipids with chemical properties that render them a potential replacement for plant oils such as palm oil. However, biomass and lipid productivities obtained with microalgae need to be improved to reach commercial feasibility. Genetic engineering can improve biomass and lipid productivities, for instance by increasing carbon flux to lipids. Here, we report the overexpression of two glycerol-3-phosphate acyltransferases (GPAT) in N. oceanica during favorable growth conditions as a strategy to increase neutral lipid content. Transformants overproducing either an endogenous (NoGPAT) or a heterologous (AoGPAT) GPAT enzyme targeted to the endoplasmic reticulum, had up to 42% and 51% increased neutral lipid contents, respectively, compared to the wild type. Biomass productivities of transformant strains were not substantially impaired, resulting in lipid productivities that were increased by up to 37% and 42% for NoGPAT and AoGPAT transformants, respectively. When exposed to nutrient stress, transformants and wild type had similar lipid contents, suggesting that GPAT enzyme availability is a rate-limiting factor for lipid synthesis in N. oceanica under favorable growth conditions. NoGPAT transformants further accumulated PUFAs in neutral lipids, reaching a total of 6.8% PUFAs per biomass, an increase of 24% relative to the wild type. Overall, our results indicate that GPAT is an interesting target for engineering of lipid metabolism in microalgae, in order to improve neutral lipid and PUFA accumulation in microalgae.