A Palmitic Acid Elongase Affects Eicosapentaenoic Acid and Plastidial Monogalactosyldiacylglycerol Levels in Nannochloropsis

Nannochloropsis species are oleaginous eukaryotes containing a plastid limited by four membranes, deriving from a secondary endosymbiosis. In Nannochloropsis, thylakoid lipids, including monogalactosyldiacylglycerol (MGDG), are enriched in eicosapentaenoic acid (EPA). The need for EPA in MGDG is not understood. Fatty acids are de novo synthesized in the stroma, then converted into very-long-chain polyunsaturated fatty acids (FAs) at the endoplasmic reticulum (ER). The production of MGDG relies therefore on an EPA supply from the ER to the plastid, following an unknown process. We identified seven elongases and five desaturases possibly involved in EPA production in Nannochloropsis gaditana. Among the six heterokont-specific saturated FA elongases possibly acting upstream in this pathway, we characterized the highly expressed isoform D0-ELO1. Heterologous expression in yeast (Saccharomyces cerevisiae) showed that NgD0-ELO1 could elongate palmitic acid. Nannochloropsis D0-elo1 mutants exhibited a reduced EPA level and a specific decrease in MGDG. In NgD0-elo1 lines, the impairment of photosynthesis is consistent with a role of EPA-rich MGDG in nonphotochemical quenching control, possibly providing an appropriate MGDG platform for the xanthophyll cycle. Concomitantly with MGDG decrease, the level of triacylglycerol (TAG) containing medium chain FAs increased. In Nannochloropsis, part of EPA used for MGDG production is therefore biosynthesized by a channeled process initiated at the elongation step of palmitic acid by D0-ELO1, thus acting as a committing enzyme for galactolipid production. Based on the MGDG/ TAG balance controlled by D0-ELO1, this study also provides novel prospects for the engineering of oleaginous microalgae for biotechnological applications.