Biologia Vegetal

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Evolution and metabolic significance of the urea cycle in photosynthetic diatoms
(Nature, 2011-05-12) Allen, Andrew E.; Dupont, Christopher L.; Oborník, Miroslav; Horák, Aleš; McCrow, John P.; Zheng, Hong; Johnson, Daniel A.; Hu, Hanhua; Fernie, Alisdair R.; Bowler, Chris; Nunes-Nesi, Adriano
Diatoms dominate the biomass of phytoplankton in nutrient-rich conditions and form the basis of some of the world’s most productive marine food webs1,2,3,4. The diatom nuclear genome contains genes with bacterial and plastid origins as well as genes of the secondary endosymbiotic host (the exosymbiont5)1,6,7,8,9,10, 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 proline12 and the precursors for long-chain polyamines required for silica precipitation during cell wall formation11. 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.
Leveraging metabolomics for functional investigations in sequenced marine diatoms
(Trends in Plant Science, 2012-07) Fernie, Alisdair R.; Obata, Toshihiro; Allen, Andrew E.; Araujo, Wagner L.; Bowler, Chris
Recent years have witnessed the genomic decoding of a wide range of photosynthetic organisms from the model plant Arabidopsis thaliana and the complex genomes of important crop species to single-celled marine phytoplankton. The comparative sequencing of green, red and brown algae has provided considerable insight into a number of important questions concerning their evolution, physiology and metabolism. The combinatorial application of metabolomics has further deepened our understanding both of the function of individual genes and of metabolic processes. Here we discuss the power of utilising metabolomics in conjunction with sequencing data to gain greater insight into the metabolic hierarchies underpinning the function of individual organisms, using unicellular marine diatoms as a case study to exemplify the advantages of this approach.