title: Emiliania huxleyi endures N-limitation with an efficient metabolic budgeting and effective ATP synthesis.
abstract: BACKGROUND
  Global change will affect patterns of nutrient upwelling in marine environments, potentially becoming even stricter regulators of phytoplankton primary productivity. To better understand phytoplankton nutrient utilization on the subcellular basis, we assessed the transcriptomic responses of the life-cycle stages of the biogeochemically important microalgae Emiliania huxleyi to nitrogen-limitation. Cells grown in batch cultures were harvested at 'early' and 'full' nitrogen-limitation and were compared with non-limited cells. We applied microarray-based transcriptome profilings, covering ~10.000 known E. huxleyi gene models, and screened for expression patterns that indicate the subcellular responses.
  
  
  RESULTS
  The diploid life-cycle stage scavenges nitrogen from external organic sources and -like diatoms- uses the ornithine-urea cycle to rapidly turn over cellular nitrogen. The haploid stage reacts similarly, although nitrogen scavenging is less pronounced and lipid oxidation is more prominent. Generally, polyamines and proline appear to constitute major organic pools that back up cellular nitrogen. Both stages induce a malate:quinone-oxidoreductase that efficiently feeds electrons into the respiratory chain and drives ATP generation with reduced respiratory carbon throughput.
  
  
  CONCLUSIONS
  The use of the ornithine-urea cycle to budget the cellular nitrogen in situations of limitation resembles the responses observed earlier in diatoms. This suggests that underlying biochemical mechanisms are conserved among distant clades of marine phototrophic protists. The ornithine-urea cycle and proline oxidation appear to constitute a sensory-regulatory system that monitors and controls cellular nitrogen budgets under limitation. The similarity between the responses of the life-cycle stages, despite the usage of different genes, also indicates a strong functional consistency in the responses to nitrogen-limitation that appears to be owed to biochemical requirements. The malate:quinone-oxidoreductase is a genomic feature that appears to be absent from diatom genomes, and it is likely to strongly contribute to the uniquely high endurance of E. huxleyi under nutrient limitation.
authors.affiliation: Alfred Wegener Institute - Helmholtz-Centre for Polar- and Marine Research, Am Handelshafen 12, Bremerhaven 27570, Germany. Sebastian.Rokitta@awi.de.
authors.forename: Sebastian D
authors.initials: SD
authors.lastname: Rokitta
authors.forename: Peter
authors.initials: P
authors.lastname: Von Dassow
authors.forename: Björn
authors.initials: B
authors.lastname: Rost
authors.forename: Uwe
authors.initials: U
authors.lastname: John
mesh_terms: D000255:Adenosine Triphosphate / Q000096:biosynthesis*; D002244:Carbon / Q000378:metabolism; D004171:Diploidy; D004579:Electron Transport; D004734:Energy Metabolism; D020869:Gene Expression Profiling; D006238:Haploidy; D058087:Haptophyta / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D053858:Metabolic Networks and Pathways; D009584:Nitrogen / Q000378:metabolism*; D010788:Photosynthesis; D014176:Protein Biosynthesis; D059467:Transcriptome
chemical_list: D002244:Carbon; D000255:Adenosine Triphosphate; D009584:Nitrogen