J. Craig Venter Institute, San Diego, California 92121, USA.
Nature. 2011 May 12;473(7346):203-7. doi: 10.1038/nature10074.
Diatoms dominate the biomass of phytoplankton in nutrient-rich conditions and form the basis of some of the world's most productive marine food webs. The diatom nuclear genome contains genes with bacterial and plastid origins as well as genes of the secondary endosymbiotic host (the exosymbiont), yet little is known about the relative contribution of each gene group to diatom metabolism. Here we show that the exosymbiont-derived ornithine-urea cycle, which is similar to that of metazoans but is absent in green algae and plants, facilitates rapid recovery from prolonged nitrogen limitation. RNA-interference-mediated knockdown of a mitochondrial carbamoyl phosphate synthase impairs the response of nitrogen-limited diatoms to nitrogen addition. Metabolomic analyses indicate that intermediates in the ornithine-urea cycle are particularly depleted and that both the tricarboxylic acid cycle and the glutamine synthetase/glutamate synthase cycles are linked directly with the ornithine-urea cycle. Several other depleted metabolites are generated from ornithine-urea cycle intermediates by the products of genes laterally acquired from bacteria. This metabolic coupling of bacterial- and exosymbiont-derived proteins seems to be fundamental to diatom physiology because the compounds affected include the major diatom osmolyte proline and the precursors for long-chain polyamines required for silica precipitation during cell wall formation. So far, the ornithine-urea cycle is only known for its essential role in the removal of fixed nitrogen in metazoans. In diatoms, this cycle serves as a distribution and repackaging hub for inorganic carbon and nitrogen and contributes significantly to the metabolic response of diatoms to episodic nitrogen availability. The diatom ornithine-urea cycle therefore represents a key pathway for anaplerotic carbon fixation into nitrogenous compounds that are essential for diatom growth and for the contribution of diatoms to marine productivity.
硅藻在富营养条件下主导着浮游植物的生物量，构成了世界上一些生产力最高的海洋食物网的基础。硅藻核基因组包含具有细菌和质体起源的基因，以及次生内共生宿主（外共生体）的基因，但对于每个基因群对硅藻代谢的相对贡献知之甚少。在这里，我们表明，外共生体衍生的鸟氨酸-尿素循环与后生动物的类似，但在绿藻和植物中不存在，促进了从长期氮限制中快速恢复。RNA 干扰介导的线粒体氨甲酰磷酸合酶的敲低会损害氮限制硅藻对氮添加的反应。代谢组学分析表明，鸟氨酸-尿素循环的中间体特别耗尽，三羧酸循环和谷氨酰胺合成酶/谷氨酸合酶循环都与鸟氨酸-尿素循环直接相关。其他几个耗尽的代谢物是由从细菌侧向获得的基因的产物从鸟氨酸-尿素循环中间体生成的。这种细菌和外共生体衍生蛋白的代谢偶联似乎是硅藻生理学的基础，因为受影响的化合物包括主要的硅藻渗透剂脯氨酸和形成细胞壁时沉淀硅所需的长链多胺的前体。到目前为止，鸟氨酸-尿素循环仅因其在后生动物中去除固定氮的重要作用而被人们所知。在硅藻中，该循环作为无机碳和氮的分配和重新包装中心，对硅藻对间歇性氮供应的代谢反应有重要贡献。因此，硅藻的鸟氨酸-尿素循环代表了一种关键途径，可将固定碳固定到氮化合物中，这对硅藻的生长和硅藻对海洋生产力的贡献至关重要。
