The red alga Porphyridium purpureum has been known to produce polyunsaturated fatty acids, especially arachidonic acid (ARA), under stressful conditions. However, there is no consistent conclusion about the response of ARA in this alga to nitrogen (N) stress. Also, no research has been done to clearly elucidate the underlying molecular mechanisms of N stress. In this work, P. purpureum CoE1 was cultivated under nitrogen limitation conditions and the putative Δ5-desaturase related gene FADSD5 was isolated. The results showed that the fatty acids in P. purpureum CoE1 were significantly higher in the N limited cultures (54.3 mg g−1) than in the N-replete cultures (45.3 mg g−1) at the 18th day (t-test, p < 0.001), which was attributed to the upregulated abundance of the putative Δ5-desaturase related protein, Δ5-Des. The study also indicated that the expression of the putative Δ5-desaturase related gene, FADSD5, increased with cell growth, demonstrating considerable potentials for ARA biosynthesis in P. purpureum CoE1. These results might guide the direction in illuminating the biosynthetic pathway of fatty acids with molecular evidence and enable genetic modifications of P. purpureum CoE1 for enhancing the ARA accumulation.
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