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Background The development of microalgal strains for enhanced biomass and biofuel production has received increased attention. Moreover, strain development via metabolic engineering for commercial production is being considered as the most efficient strategy. Pyruvate is an essential metabolite in the cells and plays an essential role in amino acid biosynthesis and de novo fatty acid biosynthesis in plastids. Although pyruvate can be a valuable target for metabolic engineering, its transporters have rarely been studied in microalgae. In this study, we aimed to identify the plastidial pyruvate transporter of Phaeodactylum tricornutum and utilize it for strain development. Results We identified putative pyruvate transporter localized in the plastid membrane of Phaeodactylum tricornutum . Transformants overexpressing the pyruvate transporter were generated to increase the influx of pyruvate into plastids. Overexpression of a plastidial pyruvate transporter in P. tricornutum resulted in enhanced biomass (13.6% to 21.9%), lipid contents (11% to 30%), and growth (3.3% to 8.0%) compared to those of wild type during one-stage cultivation. Conclusion To regulate the pyruvate influx and its metabolism in plastids, we generated transformants overexpressing the putative plastidial pyruvate transporter in P. tricornutum . They showed that its overexpression for compartmentalizing pyruvate in plastids could be an attractive strategy for the effective production of biomass and lipids with better growth, via enhanced pyruvate metabolism in plastids.

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Under nitrogen (N) starvation, microalgae increase carbon storage in the form of lipid droplets while also downregulating photosynthesis and eventually terminating growth. To improve lipid yield, we asked whether lipid droplets and N starvation responses can be induced without limiting growth or photosynthesis. In the chlorophyte Chlamydomonas reinhardtii, gametogenesis is induced either by N starvation or by growth with arginine as the sole N source. We showed that arginine cultures supported robust phototrophic growth, constitutively turned on N starvation-induced genes, and increased lipid droplets. The lipids accumulated in arginine cultures exhibited strong enrichment of saturated and monounsaturated fatty acids, a preferred characteristic of biodiesel precursors. The diatom Phaeodactylum tricornutum also accumulated lipid droplets in arginine culture without growth impairment. We document a system wherein N starvation responses are induced without compromising photosynthesis or growth, thereby suited to the producing valuable chemicals and biofuel precursors without requiring stressors in microalgae.

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