Inducible expression of Haematococcus oil globule protein in the diatom Phaeodactylum tricornutum: Association with lipid droplets and enhancement of TAG accumulation under nitrogen starvation

Microalgae accumulate the storage lipids triacylglycerols (TAG) in lipid droplets (LDs) in response to environmental cues. Mechanistic insights into LD biogenesis in microalgal cells hold great promise for the manipulation of TAG production. In this report, we demonstrate the heterologous expression of a major LD protein of the green microalga Haematococcus pluvialis in the diatom Phaeodactylum tricornutum. The N-terminus of Haematococcus oil globule protein (HOGP) was fused with EGFP and expressed under the control of the strong light-inducible fcpA gene promoter and the novel nitrogen-starvation-inducible DGAT1 gene promoter. Fatty acid feeding enabled fast monitoring of LDs formation under nitrogen-replete conditions and detection of a fluorescent EGFP-HOGP signal as ring-like structures adjacent to the periphery of LDs. During the prolonged nitrogen starvation, EGFP-HOGP was expressed only under the control of the DGAT1 promoter and was similarly associated with LDs. Furthermore, expression of HOGP under nitrogen starvation enhanced TAG accumulation. The tools and approaches described in this report open the door for characterizing endogenous LD proteins in P. tricornutum. The results of this study indicate the significant biotechnological potential of such LD-associated protein genes in the metabolic engineering of microalgae for enhanced oil productivity.