The long-standing paradigm establishing that global production of Omega-3 (n–3) long-chain polyunsaturated fatty acids (LC-PUFA) derived almost exclusively from marine single-cell organisms, was recently challenged by the discovery that multiple invertebrates possess methyl-end (or ωx) desaturases, critical enzymes enabling the biosynthesis of n–3 LC-PUFA. However, the question of whether animals with ωx desaturases have complete n–3 LC-PUFA biosynthetic pathways and hence can contribute to the production of these compounds in marine ecosystems remained unanswered. In the present study, we investigated the complete enzymatic complement involved in the n–3 LC-PUFA biosynthesis in Tigriopus californicus, an intertidal harpacticoid copepod. A total of two ωx desaturases, five front-end desaturases and six fatty acyl elongases were successfully isolated and functionally characterized. The T. californicus ωx desaturases enable the de novo biosynthesis of C₁₈ PUFA such as linoleic and α-linolenic acids, as well as several n–3 LC-PUFA from n–6 substrates. Functions demonstrated in front-end desaturases and fatty acyl elongases unveiled various routes through which T. californicus can biosynthesize the physiologically important arachidonic and eicosapentaenoic acids. Moreover, T. californicus possess a Δ4 desaturase, enabling the biosynthesis of docosahexaenoic acid via the ‘Δ4 pathway’. In conclusion, harpacticoid copepods such as T. californicus have complete n–3 LC-PUFA biosynthetic pathways and such capacity illustrates major roles of these invertebrates in the provision of essential fatty acids to upper trophic levels.

长期存在的范式确立了 Omega-3 (n-3) 长链多不饱和脂肪酸 (LC-PUFA) 的全球生产几乎完全来自海洋单细胞生物，但最近发现多种无脊椎动物具有甲基-末端（或ω x）去饱和酶，是能够生物合成 n-3 LC-PUFA 的关键酶。然而，具有ω x 去饱和酶的动物是否具有完整的 n-3 LC-PUFA 生物合成途径并因此有助于在海洋生态系统中产生这些化合物的问题仍未得到解答。在本研究中，我们研究了在Tigriopus californicus（一种潮间带 harpacticoid 桡足类）中参与 n-3 LC-PUFA 生物合成的完整酶补体。一共两个ω x 去饱和酶、五种前端去饱和酶和六种脂肪酰基延长酶被成功分离和功能表征。T. californicus ω x 去饱和酶使C ₁₈ PUFA（例如亚油酸和 α-亚麻酸）以及来自 n-6 底物的几种 n-3 LC-PUFA的从头生物合成成为可能。在前端去饱和酶和脂肪酰基延伸酶中展示的功能揭示了加利福尼亚T. californicus可以生物合成生理上重要的花生四烯酸和二十碳五烯酸的各种途径。此外，T. californicus具有 Δ4 去饱和酶，能够通过“Δ4 途径”生物合成二十二碳六烯酸。总之，harpacicoid桡足类如T. californicus具有完整的 n-3 LC-PUFA 生物合成途径，这种能力说明了这些无脊椎动物在为上层营养水平提供必需脂肪酸方面的主要作用。