Metabolic Engineering for Redirecting Carbon to Enhance the Fatty Acid Content of Synechocystis sp. PCC6803

Biofuels produced by photosynthetic microorganisms are considered a renewable, sustainable, and eco-friendly alternative to fossil fuels that cause a negative environmental impact. Microalgae can accumulate the end-products and precursors required for biodiesel production with higher productivity and better sustainability than conventional energy crops. High lipid content, a metabolic rate faster than that of higher plants, and an ability to grow under poor conditions are the desirable characteristics of microalgae that make them an efficient feedstock for the economical production of biodiesel. In this study, we attempted to improve the lipid content of microalgae by metabolically redirecting the carbon flux from carbohydrate or cyanophycin synthesis to lipid synthesis. Synechocystis sp. PCC6803, a model microalga for genetic modification studies, was used to study the effect of deletion of specific metabolic genes and the introduction of an exogenous gene. In Synechocystis, glycogen and cyanophycin primarily store carbon, and are accumulated in the absence of nitrogen sources. The genes encoding the enzymes ADP-glucose pyrophosphorylase (glgC: slr1176) and cyanophycin synthase (cphA: slr2002), which are involved in glycogen and cyanophycin synthesis, respectively, were knocked out to block cyanophycin or glycogen synthesis and increase the carbon pool for lipid synthesis. Blocking glycogen synthesis decreased the carbohydrate content by up to 71% and increased the fatty acid content by up to 24% compared with the wild-type strain cultivated under nitrate-deficient conditions. Blocking cyanophycin synthesis did not affect the fatty acid content. Our results could be used to construct genetically engineered Synechocystis for the large-scale production of fatty acids.