Enhanced biomass production by Phaeodactylum tricornutum overexpressing phosphoenolpyruvate carboxylase

Phaeodactylum tricornutum is an extensively studied model diatom and a promising candidate for biochemical and biotechnological engineering aimed at increasing biomass production. Phosphoenolpyruvate carboxylase (PEPCase), which converts phosphoenolpyruvate (PEP) to oxaloacetate (OAA) using bicarbonate, is a key metabolic enzyme of the tricarboxylic acid (TCA) cycle and the C4 photosynthetic pathway. Two transgenic P. tricornutum lines overexpressing PtPEPCase1 (PtPEPC1, JGI protein ID: 27976) were constructed and their photosynthetic productivity, cell growth, and biomass were characterized. The levels of PtPEPC1 mRNAs in the two transgenic lines were increased by 2.3 and 11.2-fold and the amounts of the PEPCase protein by 1.3 and 2.3-fold, respectively. PtPEPC1 was targeted to mitochondria. Addition of bicarbonate to the P. tricornutum culture increased biomass of the transformants by about 12% compared to wild type, and their maximum specific growth rate in exponential phase was about 10% greater than that of wild type. The transformants also exhibited higher photosynthetic productivity. We conclude that overexpression of mitochondrial PtPEPC1 enhanced both photosynthetic productivity and TCA cycle activity in P. tricornutum, thereby enhancing biomass production in the presence of dissolved inorganic carbon. Our findings suggest that P. tricornutum overexpressing PtPEPC1 can be used to mitigate rising atmospheric CO2 levels or for sustainable microalgal biomass production.