Delta-5 elongase knockout reduces DHA and TAG synthesis coupled with an increase of heat sensitivity in a marine diatom

Recent global marine lipidomic analysis reveals a strong relationship in the ocean between temperature and phytoplanktonic abundance of omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs), especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), which are essential for human nutrition and primarily from phytoplankton in marine food webs. In phytoplanktonic organisms, EPA may play a major role in regulating the phase transition temperature of membranes, while the function of DHA remains to be explored. In the oleaginous diatom Phaeodactylum tricornutum , DHA is distributed mainly on extraplastidial phospholipids, which is very different from the EPA enriched in thylakoid lipids. Here, CRISPR/Cas9-mediated knockout of ptELO5a , which encodes a delta-5 elongase catalyzing the elongation of EPA to synthesize DHA, led to a substantial interruption of DHA synthesis in P. tricornutum . The ptELO5a mutants show significant alterations in transcriptome and glycerolipidomes including membrane lipids and triacylglycerols under normal temperature (22°C), and are more sensitive to elevated temperature (28°C) than wild type. We conclude that the PtELO5a-mediated synthesis of small amounts of DHA has indispensable functions in regulating the membrane lipid, and indirectly contributing storage lipid accumulation and maintaining thermomorphogenesis in P. tricornutum . This study also highlights the significance of DHA synthesis and lipid composition for environmental adaptation of P. tricornutum .